Rear Suspension - Part 2

The next step in the rear suspension upgrade was to replace everything except the lower control arm (e.g., the uprights, suspension brackets, upper control arms and rockers). The stock rockers are composed of six machined parts and six screws whereas the new rockers are machined from a single piece of 7075 aluminum. I’ve had the suspension apart many times and it was always a bit of a challenge to install the rod ends and misalignment spacers, but the tight tolerances and misalignment bushings that insert into the rod end rather than misalignment spacers that must be carefully aligned and often fall to the floor simplify the process. My contribution to the design was the addition of an integral mounting point for the anti-roll bar link. I had spent a lot of time fabricating a solution that bolted to the stock rockers, but this is more elegant and requires no additional parts.

From left to right; suspension mode adjuster, rocker, misalignment bushings, rear damper center bracket and camber adjustment shim

The mounting point for the sway bar link is located in the top left of rocker shown above

The stock suspension has a bit of a mismatch between the front 1.57:1 and rear 2:1 wheel:damper ratios. IMO, the crazy high rear spring rates that some people are running are due to this discrepancy and the need to have enough stiffness for the track. To address this, the new rocker has a Suspension Mode Adjuster that enables you to quickly change the ride height and motion ratio between Street Mode and Track Mode. The rocker’s Street Mode motion ratio is 1.5:1 which is close to the stock front 1.57:1 ratio. Track mode changes the rocker’s ratio to 1:1 which increases the wheel rate and reduces the ride height by 20mm (~0.8”). The suspension analysis software indicates a 500 lb. rear spring rate which, when combined with the semi-active dampers, should provide a comfortable ride on the street and aggressive handling on the track.

The Suspension Mode Adjuster captures the pushrod’s rod end. The parts are nicely machined with tight tolerances. I had to hit it with a polishing wheel to get everything to fit perfectly. Changing the suspension mode is easy; air jack the car, jack the lower control arm to remove the load on the push rod, remove the push rod’s bolt, pivot the adjuster and replace the bolt. Since all of the parts are captured by the adjuster and the rocker has built in stops there’s nothing to fall on the floor and there are zero alignment issues.

Unfortunately, the front suspension isn’t pushrod based, so Track Mode will be implemented by preloading the front anti roll bar. I’ll do another post about that mechanism when it’s finished.

Right rear upright viewed from the engine compartment side. The top piece is the upright bracket which allows shims to be slid between it and the upright to facilitate easy/predictable camber adjustment. The extension on the top right is for the toe link. This is cleaner that the stock approach and provides more room for big brakes. The bracket on the left is for motor-on-caliper parking brakes.

The rear roll center was 70 mm (2.8”) too high and the anti-squat was a little too aggressive. This was addressed via a combination of the previously posted custom uprights and CNC-machined brackets to adjust the locations of the lower control arm and toe link rod ends. The location of the bolts that mount to the billet chassis pieces couldn’t be moved and their heads were colliding with the rod ends. The solution was to use motorsport rod ends which increased clearance and to machine the low-profile hex-to-counter-sink adapters shown below to recess the bolt heads. Lowering the roll center reduces jacking forces and improves handling and the decreased anti-squat provides a slight improvement in traction and significantly more feel for what the rear is doing under acceleration.

The brackets for one side of the rear suspension; from left to right: forward lower control arm, rear lower control arm, toe link and toe link spacer. The toe link spacer improves bump steer. The bottom three are low-profile hex-to-counter-sink adapters.

In the stock suspension the pushrod binds on the upper control arm at full droop and the improved geometry exacerbates that issue. The binding isn’t that bad when you’re slowly raising the car via a car lift or hydraulic jack, but I winced every time I air jacked the car and they slammed together. The solution was to design a new upper control arm which, in addition to solving the binding issue, increased droop by approximately one inch.

To make all of this work National Aerospace Standard (NAS) nuts, bolts and washers were used. Their hex heads are dimpled and shorter than Grade 8 or AN bolts. This reduces weight, enables them to fit in tighter spaces and utilizes tools that are typically one size smaller. Their finish is also a lot nicer. The downside is that there’s a lot of confusing specification nomenclature, they aren’t cheap and I haven’t found a supplier that makes it easy for a garage builder to procure them.

The rocker is in Street Mode (the empty hole in the rocker is for Track Mode). Note that one camber shim is installed. You can see its removal tab protruding near the toe link.

The anti-roll bar link is the light gray tube below the spring

The TracTive dampers were too long to fit between the rocker and the rear chassis billet piece so they had to be cantilevered which required a three-piece bracket. In the photo above, the center bracket is machined from 7075, the top bracket is a 3D-printed prototype and the bottom bracket which isn’t visible is still being designed.

My brakes are much larger than the stock ones and I had to machine the stock toe spacer and arm to clear the banjo fitting and even then, everything was exceptionally tight. As can be seen above, the new toe link configuration is elegant and provides gobs of space.

While the front suspension was spot on, the rear suspension upgrade was a massive amount of work and I still have a few small things to finish. The benefits of the rear upgrade include:

• Active dampers; each corner’s rate is independently modulated with a 5ms response time

• Adjusted rear rocker motion ratio from 2:1 to 1.5:1 for street mode and 1:1 for track mode, giving much improved damping control

• Corrected rear roll center geometry, lowered by 70 mm (2.8”) which reduced jacking forces and improves handling

• Increased droop by approximately 25 mm (1”)

• Ability to quickly switch between street and track modes (i.e. change ride height and wheel rate)

• Decreased anti-squat provides a slight improvement in traction and significantly more feel for what the rear is doing under acceleration

• Shim-adjustable camber adjustment

• Integral anti-roll bar mounts

• Pushrod no longer binds on the upper control arm

• Improved rotor cooling (upright enables rotor to be cooled from the center and tapped holes are provided for carbon fiber brake ducts)

• Provision for wheel speed sensor wires to be routed in the upright

• Increased toe link clearance for large brake packages

• Rear toe link spacer to improve bump steer

• Tapped holes to mount parking brake brackets

• Motorsport-quality rod ends (high angle so they don't neck out at full droop of bump)

Cumulatively this results in a car that’s easier to adjust, is comfortable on the street and will perform better than many track-only SL-Cs.

Wiring: Vibration Isolation

I’ve been spending a massive amount of time working on the electrical system. I’ve retained a high-end German firm to design and manufacture a motorsport harness that will subsume the pile of sub harnesses that I have. More about them later.

Stefan was able to squeeze a one-week trip to Boston between the Rolex 24 at Daytona and testing at Sebring. We figured out where to mount all of the devices and he took measurements for the harness. He was adamant that all critical electronics must have vibration isolation. Apparently the flat-plane cranks in the LMP cars and certain tracks are really tough on even motorsport electrical components.

What’s better than a shop dog? Two shop dogs. Stefan hard at work.

The bracket below utilizes three vibration isolators to mount the dual Lambda to CAN module. Note that the automotive connector on the middle wire has been snipped off. It will be replaced with a bulkhead motorsport connector that mounts to the vertical tab on the bracket.

MoTeC LTCD and stainless steel bracket with three rubber vibration-dampening sandwich mounts

Three small tabs were welded to the chassis cross brace. They’re small and located on curve of the tube making it difficult to align them perfectly and prevent them from moving during welding. To solve that challenge I made a jig composed of a fixture plate and six aluminum spacers. The fixturing plate had to be wide enough to span the apex of the tubes. Once it was clamped to the bottom of the brace it held everything in place.

Welding jig: fixture plate, six spacers, bracket and three weld tabs

There’s a lot of electronics in the footbox; engine ECU, three PDMs, active damper ECU, two dual H-Bridges, EPAS ECU, parking brake ECU, etc., all of which need to be isolated.

Given how tight things are in the footbox, I designed two stainless steel brackets that utilize four vibration-isolating sandwich mounts. The left one mounts two PDMs and the right mounts the engine ECU and a PDM. Considering that those four devices run about $11k, it’s cheap insurance. In addition, the isolators have studs sticking out which makes it easy to mount the plate and they raise the devices 1/2” above the mounting surface which makes it easier to attach / detach the connectors.

Devices mounted to the right-side plate, empty left-side pate and a vibration-dampening sandwich mount

The plates are mounted to the side of the footbox behind the side impact bar sub frame. They are easily accessible and will eventually be hidden by a carpeted closeout panel

Fleet Week 2024

During my last trip to the Bay Area I got a ride in Mesa’s SL-C. It’s a unique and outstanding build which gave me a case of tire envy — his 405s make my 345s look emaciated.

During my most recent visit I invited him to watch Fleet Week from our deck. As you’d expect, the Blue Angels were outstanding. If you ever get a chance to watch it from a roof deck there, it’s worth it. In years past they’ve flown even closer to the buildings, but it’s still a visceral experience because it’s loud enough to feel. The vibrations were setting car alarms off, some of which you can hear at the end of the video.

The pictures and video below were taken on an iPhone and with all of the glare on the screen it was impossible to determine if the planes were in the frame. There were lots of flybys, but most of my video was of open sky… so the following is the best we managed to capture.

Rear Suspension Upgrade - Part 1

A couple of people have asked my why I upgraded the Penske dampers with ones from TracTive. It’s pretty simple, they improve braking, acceleration and cornering while also providing a comfortable ride on the street. That’s pretty much the Holy Grail for a street/track SL-C. Here’s what Johannes van Overbeek said about TracTive.

Having spent over 20 years in professional sports car racing and having the good fortune to work with some of the best teams and winning races like the 24 hours of Daytona, 12 hours of Sebring (2x) and the Petit Le Mans (2x) I have driven on nearly every damper you could name.

As I learned about TracTive it was clear they have a completely different technology than anybody else in the market. Driving it for the first time was a revelation. You really could, finally, have the best of both worlds. A car that was compliant on the road and an animal on the track.

If there is such a thing as horsepower for the corners, this is it.

I finally have the vast majority of the CNC parts for the rear suspension upgrade. The only stock part is the lower control arm, everything else has been completely replaced. Improvements include semi-active dampers, shim-adjustable camber, improved bell crank ratio, lowered roll center, improved anti-dive, improved rotor cooling, no binding of the push rod and upper control arm, and the ability to easily switch the bell crank ratio between Street and Track mode which changes both the ride height and wheel rate. All of he parts are machined from 7075 which is closer in strength to mild steel than 6065.

Right rear upright viewed from the engine compartment side. The top piece is the upright bracket which allows shims to be slid between it and the upright to facilitate easy/predictable camber adjustment. The extension on the top right is for the toe link. This is cleaner that the stock approach and provides more room for big brakes. The bracket on the left is for motor-on-caliper parking brakes.

The seam at the top with the horizontal bolts is where the camber shims are inserted. There are none shown.

Right rear upright viewed from the tire side. The four tapped holes on the outer edge are for a carbon fiber brake duct cover.

Semi-Active Dampers with Integral Nose Lift

The holy grail for many SL-C builders is a comfortable car on the street which can also tear up the track. While it’s impossible to optimize for both, technology can help transition between the two use cases. To that end, I’ve added cockpit-adjustable anti sway bars, driver tunable electric power-assisted steering, an active wing, and multiple engine, traction control and shift tunes. These are collectively modified via the manettino rotary switch on the steering wheel. Once a given mode is selected, the driver can dynamically tweak any of the settings.

Over the last couple of years I’ve been trying to upgrade my Penske dampers with TracTive’s semi-active dampers which are apparently used by Pagani, Donkervoort and a variety of track/race cars. Similar to the banned F1 active suspension technology the damping rates are dynamically modified by an ECU based on settings and multiple accelerometers. For example, going through the same corner with the same speed, once on the brakes and once on power, you would see totally different damper settings for all individual wheels. However, unlike the F1 tech the ride height is not changed, hence the semi-active moniker.

The TracTive ACE dampers can transition to any desired stiffness setting in 6 milliseconds. According to the manual:

To dampen the wheel and body movements, the motion of the vehicle is monitored. The following axis are included:

• Vehicle Vertical Axis (Z-Axis)

• Vehicle Lateral Axis (X-Axis)

• Vehicle Longitudinal Axis (Y-Axis)

The following signals are used for monitoring:

• LATERAL acceleration

• LONGITUDAL acceleration

The following signals are used to calculate the current driving situation:

Control of the damping action for the vertical movement of the vehicle can be subdivided into comfort and safety. The ACE system is set to use a small amount low-speed (body) damping to enhance comfort. At the same time, the wheels may not loose contact with the road-surface. Depending on the situation, the TCU control algorithm should shift towards optimized vertical force transmission.

Lateral G-force
The ACE system is able to detect steering input by monitoring lateral G-force. (e.g. transition from driving in a straight line and into a corner) When a rapid enlargement of the lateral G-force is detected, the TCU concludes that cornering has begun. As a result, the dampers can be adjusted accordingly. This means that the ACE system can be used to reduce the rolling motion of the vehicle. This “anti-roll” behavior is programmed inside the TCU and is specific to each application.

Longitudinal G-force
Using the longitudinal G-force, the ACE system is able to determine whether the vehicle is accelerating, braking or moving at constant velocity. During (excessive) braking, the vehicle would normally see a pitching motion. While the acting G-forces are detected by the TCU, this pitching motion can be counteracted by increasing front damper stiffness. Based on the G-force-rate and/or the absolute G-force, it is determined how much increase or decrease in damping is required for optimum wheel and body control. The same goes for sudden accelerations, depending on the application, either a stiffer or softer damper is required to gain maximum grip on the driven wheels. This is done by using the longitudinal G-force together with the control algorithm in the TCU.

Combined G-forces
Combining both longitudinal and lateral G-force, actual driving situations can be extracted from the sensor data. Different damper behavior can be set for all kinds of combinations. Going through the same corner with the same speed, once on the brakes and once on power, you would see totally different damper settings for all individual wheels. This is how the ACE system increases safety, more grip, more control and better predictability.

The control panel shown above enables the driver to dynamically control the following four settings:

• FRONT damping adjustments gives you control over the damper stiffness over the whole axle and not the individual dampers.

• REAR damping adjustments gives you control over the damper stiffness over the whole axle and not the individual dampers.

• ROLL support allows you to adjust the percentage of damping that is added when G-force is detected in the LATERAL motion to support the chassis thru the dampers while entering and moving thru the corners.

• PITCH support allows you to adjust the percentage of damping that is added when G-force is detected in LONGITUDAL motion. This can be braking and or accelerating of the vehicle.

These settings can be be stored in five different tunes which can be activated via CAN bus messages.

The primary challenge to the upgrade is that it’s more complex than just getting the dampers to fit. The valving, spring rates and bump stops need to be engineered. I subsequently learned that Henry Nickless with Chiron World Motorsports is building two high-spec Lambo V10 SL-Cs with TracTive dampers. He has extensive experience building and racing LMP cars and setting up suspensions on a wide-range of cars, so I’ve been working with him.

Starting with first principles, the chassis and suspension were scanned and a 17-page vehicle dynamics report. The good news was that the front suspension geometry was spot on and there was no need for any changes (the rear didn’t fare as well, more about that in a later post). We opted for a hydraulic nose lift that’s seamlessly integrated into the top of the damper rather than the typical cups that go around the shock body. This approach has several advantages:

• The springs are standard length which provides increased suspension travel, especially after a bump stop and bump springs are added.

• It’s more compact which results in less weight.

• The hydraulic ram is located at the top of the damper, so it’s weight is sprung. The cups have a larger OD which often collides with the upper control arm, resulting in the shock being inverted and the weight becoming un-sprung.

The downside is higher cost and that the shocks are longer.

Penske front damper with with RamLiftPro cup (left), TracTive front damper with integral lift (middle), TracTive rear damper (right). The two silver plugs at the top of the middle damper are M10 1.0 mm hydraulic ports for the nose lift.

Initial observations:

• The TracTive dampers appear to have the build quality as the Penske dampers.

• Other than the thin wire exiting the bottom of the TracTive dampers, one wouldn’t know that they were active.

• The springs on the TracTive have a smaller OD whereas the Penske’s.

• The TracTive reservoir hoses have a larger OD.

• The TracTive lift has two hydraulic lines per damper whereas the RamLiftPro has one.

To accommodate the longer damper a new top bracket was CNC’d from 7075 aluminum. It uses the lower stock hole and requires two new upper holes to be drilled. There is small bump on an the backside of the damper to accommodate the hydraulic piston and a small hole must be cut into the monocoque. I laser cut a plywood jig to cut the hole with a router. That was easy. However, I had nut plates integrated into the removable side-impact bars which required a fair amount of rework.

Top front shock brackets, flanged bushings and indexing cone washers (all shown parts are custom)

Installing/removing the stock dampers is a pain-in-the-ass. You need remove one side and loosen the other side of the upper control arm and then you have four stacks of flat and cone washers for the that need to be aligned and held in place while you push the bolt through the assembly. Inevitably you wind up dropping one or more stacks on the floor… I’m constantly amazed how far stuff can roll to a find creative hiding place.

With this version there is no need to touch the upper control arm. Each stack for the top rod end is replaced with a single flanged bushing that indexes the bracket and each stack on the bottom is replaced with a single cone washer that indexes the rod end. Since everything is custom machined it slides together like butter and the parts hold them in place while being installed.

In the pictures below, the shock has been installed. You will note that it projects above the monocoque which requires the notch in the spyder’s fiberglass on to be extended. However, there is no effect on the nose. Also note that the hydraulic lines haven’t been installed yet and that reservoir hose is routed under the control arm, but the plan is to route it above the control arm.

The hydraulic nose lift system is contained in a compact stainless steel box considering that it includes the hydraulic pump, reservoir, valves, and a motor controller. The RamLiftPro system includes a rocker switch to reverse motor direction which runs power to the cockpit. The TracTive system takes decentralized approach wiring approach. Power and ground are wired directly to the unit and a control wire with a supplied button controls everything. This reduces weight and keeps power out of the cockpit.

There isn’t any information on the website or documentation for the lift kit. To my understanding it’s currently used by a couple of OEMs (I think Pagani and Donkervoort) and I only learned about when asking them for specs to design/machine a similar system. I’ll post more information one I get it plumbed up and tested.

The hydraulic system

It Has a Name

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I’ve been thinking for a while about a name for the car. It’s going to be white with black accents. I’ve though of it as a Stormtrooper color scheme, but I didn’t want to go the Star Wars route so I went a different direction. When I was around eight my grandparents took me to Sea World and I was picked out of the audience. They had me stand at the edge of the pool and the Orca came half out of the water and gave me kiss — my guess is that they don’t do that anymore. Given that it didn’t chomp me, maybe it was an omen. They’re bad ass, so I’ve named the car Orca and created the badge shown below.

Many people are more interested in the result than the journey, so I have started a parallel site OrcaAutsports.com which has summary-level details and some professional photos (more to come). The coolest page is the one that allows the headers to be interactively rotated 360 degrees. Click the button below to check it out.

Fuel Fill Tube

Over four and a half years ago I modified the fuel filler neck to clear the throttle body and the induction tube (post here). The mod isn’t difficult, but you need someone with good welding skills because the neck is made from thin spun aluminum.

Stock (left) and modified (right)

The modification avoids the throttle body, but it guides things into a tight space between the 2”x 2” chassis rail and the oil reservoir. I used a flexible tube to connect the filler neck to the fuel tank. That worked, but it looked like ass and it rubbed both the chassis and the top of oil reservoir.

I recently pulled the engine and wanted to see if I could come up with a better solution. I started playing around with some 2” OD aluminum mandrel bends to determine if hard tube would work. After making a couple of cuts for mocking purposes it occurred to me that I had a whole set of 2” icengineworks blocks that I had used to design the headers. DUH! Fortunately I figured this out before wasting any material.

While designing the headers I found it difficult to conceptualize the shape when adding one block at a time. However, I found it intuitive to pre-construct J-bends and U-bends, hold them in place and then snap them apart in the desired location and, if needed, add or remove additional blocks. I was able to get a perfect fit with three mandrel bends clocked at different angles.

Perfect match to the prototype. Everything welded and hose beads rolled into each end.

At first I was concerned that the angle of the fill neck might make refueling difficult. However, with the stock setup the nozzle is typically pointed directly into a 90-degree bend. Whereas with the modified angle the nozzle is pointed straight down with no impediments.

It’s really tight, but even with body deflection nothing is going to rub. The horizontal hardlines are for the heater and the coolant bypass. I still need to add a cover plate to seal the firewall around the tube. The two bolt heads visible in that opening are for the rollover flapper valve.

I like where the fuel tank is located. However, it’s clear in this picture that fuel filler neck and fuel door are mounted to the fiberglass body and if the car rolled the fuel door could be easily torn off resulting in a 2” opening pouring vapor and gas into the engine compartment in close proximity to the headers. People worry about putting a rollover valve on the 6AN vent line, but ignore the fill tube which is 28 times larger. I replaced the stock fuel tank with a FIA-compliant fuel cell which, amongst other safety features, has a rollover flapper valve. It would be straightforward to retrofit the stock fuel tank with one.

Fuel Injectors

When I made the initial decision to run E85 (see post here) I was aware that everything in the fuel system needed to be E85 compatible and that I needed to add an ethanol content sensor. I subsequentially learned that E85 contains 40% less BTUs than gas which means that the system must flow more. Since E85 is 85% ethanol and 15% gas that equates to (0.85*1.40) + (0.15*1.00) = 1.34 or 34% more volume. E85 also increases power, so the Wide-Open-Throttle (WOT) demand is even higher. Based on this information, I upgraded the low-pressure pump and chose an appropriate high-pressure pump. Problem solved, right? Nope.

What I failed to intuit is that what goes in must come out through the fuel injectors so I need to upgrade them. Everyone that I spoke with recommended the Injector Dynamics ID1700x for my application. The tuners love them because they supply accurate detailed data which simplifies their job and results in excellent tunes. Apparently one supplier stole their data, fudged it and presented it as their own (thread here).

While other suppliers modify stock Bosch injectors, Injector Dynamics is the only aftermarket supplier to have earned Bosch’s Authorized Technical Partner status under which Bosch Motorsport manufactures Injector Dynamics’ proprietary design. Injector Dynamics then breaks them in for a few hours and groups them into matched sets.

They’re very trick, but at $300+ a pop a set costs almost a quarter of a fully-dressed LS3 crate engine. That said, the upgrade was simple. I just removed the fuel rails, lubricated the O-rings with WD40 and popped them in.

OEM fuel injector (left) and Injector Dynamics ID1700x (right)

I’m not crazy about the purple aluminum bodies, but unless you shine a light on them they aren’t all that noticeable

Zircotec ThermoHold Ceramic Coating

The equal-length 180-degree exhaust crosses under the dry sump and results in a lot of tube stuffed next to the engine block. To mitigate heat issues, I decided to go with Zircotec's patented ThermoHold ceramic coating. The technology was created for nuclear reactors and then trickled down through F1, motorsports and then to us hobbyists. Most ceramic coatings consist of ceramic chips suspended in a polymer. They are applied with a spray gun and then baked (Zircotec also offers a similar lower-performance coating). ThermoHold is more akin to welding than painting. Zirconia ceramics are plasma sprayed at temperatures in excess of 10,000°C (18,000°F) travelling at nearly twice the speed using a water-cooled gun. Similar to welding, the part is shieled with argon gas. The resulting coat is 0.3mm thick layer and contains lots of small air pockets which further reduces heat transfer. Note that aerogel, the best known insulator, achieves its amazing insulation properties because it’s mostly composed of air pockets making it the lightest known solid. Unlike the smooth polymer-based finishes, ThermoHold has a texture similar to 220 grit sandpaper and can only be removed by grinding it off.

Robotic arm plasma spraying ThermoHold

Zircotec has significant intellectual property regarding the process which is highly sensitive to power fluctuations leading them to having their own electrical substation.

“There are a large number of parameters that influence the interaction of the plasma-spray feedstock with the plasma jet and the substrate, and these parameters can result in very wide variations in the final product (e.g. feedstock type and composition, feed-rate, plasma gas composition and flow rate, energy input, torch geometry, nozzle design, nozzle offset distance and substrate cooling). The Zircotec process ensures that these parameters are tightly controlled within pre-defined set points, thereby providing control over the quality of the final product.”

— Zircotec

Preparation steps include cleaning with a strong detergent, masking the areas not to be coated and applying a metallic-based bond coat. A coat of ThermoHold Performance White which is good to 1,400 °C (2,552 °F) is then applied. An optional ThermoHold color coat which is good to 900 °C (1,652 °F) can then be applied.

The primary disadvantages are cost, the parts must be sent to England and that coating can’t be removed (this also makes it durable).

Given how long it took to fabricate the headers, I was terrified that they might get lost or damaged shipping them from Boston to the UK. Fortunately, I was able to freeload a ride and prearrange a quicker-than-normal turnaround. I dropped them off on Monday and pick them up on Friday.

I was leaning towards Performance White on the headers, but I decided to go with Performance Graphite. It matches my color scheme better and it will be less likely to show wear.

Zirocotec ThermoHold Performance Graphite

The catalytic converter assemblies will generate a lot of heat so I had Zircotec cover them with ZircoFlex SHIELD. The assemblies were; (1) plasma-sprayed with ThermoHold Performance White, (2) wrapped in fiber insulation and (3) enclosed in a hand-formed and welded 0.1 mm thick shield. This is the same process that they use on F1 cars and the workmanship, particularly on such a thin material, is outstanding. Unlike other dimpled materials that I’ve tried, you can easily handle the assemblies without denting the covering. All of this comes in at a price. Covering the small catalytic convertor assemblies cost more than double the cost to coat he headers and merge collectors.

ThermoHold Performance Graphite on the headers and merge collectors; ThermoHold Performance White, fiber blanket and ZircoFlex SHIELD on the catalytic converter assemblies; and ThermoHold Performance Yellow Gold on the heatshield

Given that the engine is out, the next step is to fabricate a heat shield between the dry sump and the tubes that cross-under it.

Merge Collector Fasteners and Saddle-Style Weld Bungs

I’m finishing that last couple of items on the headers and catalytic converter assemblies before I have them ceramic coated. The headers and merge collectors have double-slip connections which are typically secured with springs similar to the one shown below.

Instead I used these cast stainless steel mounts. Note that the bottom flange is radiused to better fit the exhaust tube’s outside diameter.

The mounts are connected via a 1/4” bolt. After the coating is finished, the nuts will be replaced with jet nuts tightened just enough to prevent rattling. While they will allow less expansion than the springs, double slips don’t move much and retention is more for safety than anything else.

The O2 sensors were added to the catalytic converter assemblies. These saddle-style cast stainless steel weld bungs are a lot nicer than the standard ones because the flange is curved to match the OD of the exhaust tube. They are available from Vibrant Performance.

The short boss on the back side matches the size of a standard hole saw which results in a clean, self-jigging fitment

The saddle-style flange fits the OD of the cone well.

The next step is to have the headers, cutout tubes and catalytic converter assemblies ceramic coated.

Half-Inch Tube Connectors

To improve the serviceability of the tail subframe I sectioned the 1/2” tube in two places. However, the smallest tube connector I could find was for 1” tube. I considered machining them, but I need four and each requires multiple lathe and mill / rotary table operations. Given that my standards are much higher than my machining skills, I decided to have them CNC’d. This allowed me to add some nice features; (1) a chamfer for the weld, (2) boring of the section that inserts into the tube to reduce weight, and (3) tolerances that capture the nut which removes the need for a wrench.

Two symmetrical tube connectors

I’ve used Hubs to CNC parts in the past so I uploaded the CAD model and selected 4140 alloy, “as machined” finish, and economy offshore (23 business day delivery). `The prices below are for a single part (i.e., half of a set). As you can see, the unit economics for CNC machining is radically different than 3D printing or laser cutting.

Quantity Unit Price:
1 $107.21
2 $71.73
10 $22.35
1,000 $1.15

This is due to the need to generate the G-code for the first part and potentially the need to fabricate custom fixturing, which this part didn’t require. These unit economics influenced the design. I had considered an asymmetric design in which the half with the socket head cap screw was different than the half with the nut, but that would result in two different parts and half the quantity. Since there was only a small aesthetic difference for a hidden part, I went with a symmetrical design.

The screen shot below shows the pricing difference for 10 pieces (i.e., 5 pairs) for on/off shore and different lead times.

I’m very pleased with the quality

Chassis tube sectioned and tube connectors ready for welding.

The part geometry captures the nut so no tools are required on the back side

Header Heat Shields

The heat shields that protect the chassis from the headers are finished. They were complex because the stock 2”x 2” chassis rails aren’t parallel nor co-planar and, complicating matters, I scalloped four of the chassis tubes to accommodate the headers. In addition, there were several hoses in the way. Fortunately, the guys at Back Bay Customs are excellent fabricators. The left side is a single piece and the right side is two pieces to accommodate the dry sump filter and lines. Everything is removeable with the engine and headers in place.

The chassis tubes were scalloped in previous post to accommodate the headers.

Paper and tape were used to make templates

The heat shield conforms to the scalloped 2” x 2” chassis tube. 1/8” shims were welded to the back side of the heat shield to provide an air gap between it and the chassis tubes. There is a small heat shield mounted to the engine block to protect the back of the alternator.

Induction Tube Heat Shield

The heat shield for the induction tube was fabricated from 0.062” 5052 aluminum sheet and two pieces of aluminum tube. I’ll add reflective foil to the hot side in the future. I’m halfway through the heat shields, six completed and six more to go.

Radiator Inlet Duct

To optimize cooling, it’s imperative that all air that flows into the radiator inlet in the body is forced through the condenser and radiator. This is best accomplished via an air-tight, diverging duct that provides a smooth transition from the opening in the body to the outer edges of the radiator core. Specifically, it’s preferable that the air isn’t allowed to contact the underside of the nose, the vertical panels that support the radiator nor the radiator’s side tanks. 

There’s a fair amount going on inside of the duct; condenser lines, nose hinges, splitter support rods and a tow hook, so it took a while to figure out the best approach. The first step was the top of the duct. The triangular section of the nose subframe was designed to pitch downwards from the top of the radiator to the top of the inlet in the body. It stiffens the subframe while providing a robust mount for the top of the inlet duct and the tow hook (the car be jacked on the car on tow hook). The top of the duct was made from four parts:

• Two triangular inserts.

• A cover between the nose subframe and the radiator.

• A cover that is attached to and pivots with the nose. When closed, it seals the body to subframe.

The video below shows prototype parts and the pivoting motion.

The triangular inserts sit on flanges covered with thin strips of rubber (not shown)

The triangular inserts were formed using custom dies. They were cut from 1/4” steel plate on a CNC plasma table, the edges were dressed with files and a pencil belt sander, the panels were annealed in the areas to be shaped and a hydraulic press was used to emboss the shape.

A closeout panel was fabricated to seal between the nose subframe crossbar and the top edge of the radiator. It also seals the top of the condenser.

It was tempting to use the vertical panels that support the radiator as the sides of the duct, but they’re not well suited for that purpose. They’re several inches wider than the both the inlet and the radiator core which will result in the air expanding beyond the radiator core, colliding with the nose hinge standoffs and hitting the radiator side tanks head on. This makes it more difficult to seal the sides of the radiator, increases drag and I assume has a deleterious effect on mass airflow through the radiator core. Since the pivoting half of the nose hinge is bolted to the side of inlet in the body, the ideal location for the duct side is between the fixed and pivoting halves of the hinge. The issue with that approach is that the hinge would bind. The solution was to manually machine a 0.062” pocket into the fixed half of the hinge to accommodate the side panel.

0.062” pocket machined into the interior face of the stationary part of the hinge. Note that the leading and top edges were left full thickness which facilitates air tightness and maintains aesthetics. It would have been a lot easier to design this pocket in CAD and have it CNC’d, but it came out great.

The splitter support rod tabs that had been welded to the subframe last year interfered with the top of the duct. Fortunately, they were only tacked into place so it was easy to cut/grind them off and fabricate a new set that cleared the duct.

Unlike the stock radiator which has very little in terms of vibration isolation, the custom radiator is supported via two rubber sandwich isolators per side. To maintain this level of isolation foam was used to seal the top and bottom of the duct and rubber flanges were used to seal the sides.

The stock vibration isolators (left/black) are small grommets and are next to useless. The custom radiator uses sandwich isolators (right/red) so it’s important to ensure that duct is properly isolated from the radiator and condenser.

An adjustable rubber flange (black) is notched around the condenser lines and is used to seal the sides of the duct to the face of the radiator while keeping the two isolated. The top and bottom are sealed with foam strips.

CV Joint Heat Shield

The heat shields for the CV boots are finished. I designed and laser cut the flanges from 1/4” 6061 and the guys from Back Bay Customs formed the shields from 0.062” 5052 and welded them up. So that’s heat shield #4 and #5 finished… only six more to go.

Upper Firewall

I was looking for a local shop to form some panels and discovered Back Bay Customs which is about 60 miles north of Boston in Portsmouth NH. After seeing one of their cars in Boston, I spoke with Adam, the owner, and decided to haul my car to the shop. I’m very picky about my car and I was pleased to see a large tidy shop, lots of metal forming equipment, CNC plasma table, paint booth, and multiple fabricators. I’ve been very pleased with their creativity, fabrication skills and communication.

The first project was the upper firewall. Sealing this area for heat, sound and vapors is critical and the stock firewall left a fair number of gaps, so we decided to replace it. Adam noticed that the firewall wouldn’t sit flat on the traverse 2” x 6” chassis tube. WTF? 

The issue was that the two mounting brackets for the rear hoop legs weren’t coplanar with the 2” x 6.” One was 1/8” forward and the other was in excess of 3/8” forward which resulted in the top of the firewall tilting towards the cockpit while also imparting a left-to-right twist. This looked like ass and made it impossible to get a good seal. The solution was to cut off the brackets, grind the hoop smooth, fabricate two different brackets to account for the variance in offsets and weld everything.

Replacement brackets welded in place. Notice that the left bracket projects further than the right bracket

The next step was to fabricate a blister to accommodate the induction tube which projects into the firewall. To fabricate the curved corners, Adam made a paper pattern, cut and annealed 1/8” 5052 sheet, and then used a soft mallet and a slapper to stretch it over a post dolly. During the shaping process he had to anneal the sheet several more times to keep it soft. He then trimmed the shaped piece to match the pattern, tweaked the middle section and welded it together.

From left to right; soft mallet, slapper, profile gauge, and post dolly

Blister mocked in place

Once all of that was done the firewall still had too much flex which was remedied by welding tabs to rear hoop’s upper radii. This is particularly important because most SL-Cs, including mine, mount a large coolant expansion tank to the upper firewall.

Firewall support tab tacked to the rear hoop

The cage is nicely fabricated and serves its primary structural/safety purpose well. However, it falls short in multiple areas which result in either unnecessary compromises or a lot of work for the builder:

• The front hoop doesn’t fit the body well which impairs vision and results in massive A-pillars. I fixed that issue in this post. Given the large number of SL-Cs that have been produced, RCR should have started CNC forming the front hoop long ago.

• Perhaps it’s just my car, but there’s no excuse for the issues with the rear hoop leg brackets. It was downright sloppy. There should be a jig to ensure alignment and a quality check before the car ships. 

• The small tabs in the upper corners should be stock. While this is a trivial fix, not all builders have easy access to welding equipment and it’s a shame to wreck the nice power coat finish that comes from the factory.

Fuel Cell: Part 2

In a previous post I modified the stock fuel tank to convert it to a FIA-compliant fuel cell. The challenge is that to replace the bladder in a SL-C with a tub you must; remove the windshield, center body, cage, seats, tub and fuel tank close out panel. That’s a lot of work, especially given the amount of sealing that needs to be redone. For this reason, I designed the fuel cell so that the bladder can be removed from the bottom of the car. As can be seen in the picture below, this leaves the access plate and center section of the bladder unsupported.

The bladder is removed via a hole in the bottom of the car which requires the bladder to have removeable support structure

To support and protect the bladder I fabricated a bolt-in structure from rectangular tube and 1/4” plate. To get the fit perfect I took careful measurements and ordered a chipboard prototype from SendCutSend. Even though the piece was large, roughly 24” x 10”, it only cost $6.29. After laying it on the fuel cell I tweaked a few measurements and ordered the final aluminum part.

The center oval plate is lower that the rectangular plate to accommodate the thickness of the access panel and gasket. The oval ring provides clearance for the bolt heads on the access panel.

Bladder support structure installed. Note that only a few of the bolts are installed in the access panel.

The next step to add a thin cover plate to the bottom of car. I couldn’t figure out a good way to use quarter-turn fasteners, so I went with screws because the panel should only be removed every 5-7 years. Drilling and taping 22 holes on the underside of car was tedious.

Cover panel installed with Celco fasteners. The next step was to tap the holes.

While I was finishing up the fuel cell, I considered using Holley’s recently released LiDAR-based fuel level sender, but it can’t see through foam. That would have required me to cut a full-height hole in the foam and insert a porous tube to ensure that the foam didn’t obstruct the device. While this would have worked, the primary purpose of the foam is to prevent a flame front from forming. I assume that’s a small risk, but it felt antithetical to one of the key safety features, so I replaced the stock fuel-level sender with a high-quality one commonly used in aviation.

Although the bladder is tough and the fuel cell is very-well protected in a SL-C, there are two simple precautions to reduce the chances of the fuel-level sender puncturing the blader; (1) a bend was added to the tube in a location indicated by the manual and (2) a nylon foot was machined on the lathe and press fitted on the tip of the sender.

Starter Heat Shield

Having finished the X-pipe and oil inlet heatshields, the starter heatshield was next. However, when I went to install the starter, it collided with the catalytic converter.

How the F did that happen? Not the first time that something was overlooked because I didn’t have all of the relevant parts in place when making decisions. In a previous post I used a rotary table to machine the starter’s mounting flange to clock the starter to its maximum, so I thought I was screwed. The only solution would be to machine the starter’s gear reduction housing which, if possible, would result in the top bolt being captured between the mounting flange and the housing. Not ideal, but workable. So, I popped the starter apart and determined that I was in luck. There are isolated cavities on the left and right of the cavity that contains the gears. This allowed me to machine a notch for the captured bolt (the problem at hand) as well as drill and tap a hole to support the heat shield. Once that was done, I repeated the process of machining the mounting flange to further clock the starter as described in this post.

The cavity on the right will be notched to accommodate the head of the captured bolt and the cavity on the left will be drilled and tapped to provide support for the heat shield

While the starter/catalytic converter collision was a very unpleasant surprise, the reality is that I would have wound up in the same place even if I had noticed the issue before fabricating the cat-back system. That said, you really want to know that you can get out of a hole BEFORE you fall into it.

As is typical in a SL-C, everything was really tight and it took me a while to figure out how to design the heat shield. Part of the challenge is that the mounting flange is cantilevered at a 15-degree angle resulting in the assembly constantly falling over while I was that trying to take measurements. After much frustration, it occurred to me to 3D print a jig to hold it at the correct angle… yeah, that made a big difference.

Starter mounted to a 3D-printed jig (top), miscellaneous scrap shims (left), feeler gauge (center), digital angle finder (right) and mock 3D-printed bend profile (top right). Note the modifications to the starter; a notch has been machined into the top to accommodate the captured bolt and a hole has been tapped on the right side to support the heat shield.

I have found the following useful when measuring awkard items. Scrap shims of various thicknesses combined with a feeler gauge. Just make sure that you debur the edges of the shims so that they sit flat. A digital angle finder is also very useful — Abe has a manual one, but he borrows my digital one a lot. It measures to one tenth of a degree and there are buttons to zero it and to display the reverse angle which means that you don’t need to do any math. It’s well worth the $16. I also 3D printed narrow sections of the bend profiles to fine tune clearances. For example, the thin black piece in the upper right of the picture above is the profile of the main shield and a welded mounting arm.

Like the other heat shields, this one required me to learn a few more sheet metal tricks:

Tutorial:
I spent a little over two hours watching a tutorial on Solidworks sheet metal features; 80% I knew, 10% showed me how to do certain things better, 5% was completely new and 5% was irrelevant to my use case. The high-end CAD packages have a lot of advanced features and it’s worth spending a little time and money to access quality online training.

Unfold and Fold Features:
For the last two heat shields I was able to use Solidwork’s Corner Relief feature to ensure that bending wouldn’t deform the corners. However, no matter what I tried, SendCutSend rejected several of the bends because it didn’t like several of the corner reliefs. The solution was to use the Unfold feature to flatten the problem flanges, add extruded cuts to the problem corners, and then refold the flanges via the Fold feature. Problem solved.

Closed Corner Feature:
Once the bend radius and K-factor are set for the type and thickness of material, the software automatically calculates the bend allowances which is great. However, I had several areas where after making several bends the material bent back onto itself and I wanted to ensure that the gap was tight enough to be welded. Fortunately, Solidworks has a Closed Corner feature that does exactly that. You simply click on the two edges that you’d like to “close,” specify the desired distance between those edges, and then choose one of three options; (1) butted, (2) edge A overlapping edge B or (3) edge B overlapping edge A. SendCutSend specifies 15 thousands as the minimum tolerance which worked out great.

In the picture below, the feature is closing the corner between the two blue edges. The yellow part showing how one of the flanges is being extended. The second flange remains unchanged because I had specifically trimmed it in a previous step, but in many cases both sides are extended. You can also see the left most (i.e., butted) of the three options is selected and that the distance is 15 thousands of an inch.

Close corner feature

It would be extremely difficult to figure out all of the bend allowances and closed corners without CAD. The curved line on the right is the final shape, but that flange was extended to provide a parallel edge to the bending line allowing it to be pushed in the CNC back stop. If you look closely, there are small bridges on the curved portion to facilitate removal post bending.

Multi-Body Part:
I have been aware of the multi-body feature for years, but I never used it. My workflow was to design separate parts and combine them into an assembly to ensure that everything fit. This works, but you have to update the relevant dimensions in multiple files and you have to mate everything in the assembly. I have used external global variable files before, but that takes a little effort.

With a multi-body part, you basically design other parts in the same file. This allows those parts to be parametrically driven by the first part. There is no need to update the dimensions in multiple files, no need for an external global variable file, and no need to mate the parts in an assembly. I wouldn’t do this with all of my parts, but in the right situation it really simplifies workflow.

Part too Small:
One of the parts was rejected by SendCutSend because it didn’t meet the minimum size for bending. The solution was to extend the part to meet the minimum size and to add bridges to make it easy to remove the excess post bending.

The heatshield involved four laser-cut pieces, one hand-cut piece, ten CNC bends and two spacers fabricated on the lathe. The bends were perfect, but when I went to install it I didn’t have as much clearance as I wanted, so I cut a 45-degree corner into the bend with the red line below and then welded a piece into the gap.

The heatshield is solidly mounted at three points; the arm that connects to the lower flange bolt, the tapped screw in the front face of the starter’s casting, and the tab (purple) that was added to the long bolt in the back of the starter. It is solid enough that the assembly can be lifted by the heat shield.

The starter was easy to take apart, but I couldn’t reassemble because it needs to be compressed. So, I took it to a repair shop. When, I decided to add a mounting tab to the rear bolt the starter’s halves separated on that side and I couldn’t get it back together. So, I made a second trip to the repair shop. If you’re going to remove that bolt, ensure that you tightly clamp the starter in a vice before doing so.

Heatshield (left) and starter (right). The arrow points to the rear mounting tab and spacer. You need to clamp the starter when loosening the long bolt to install the rear tab or you’ll be visiting the repair shop to get things put back together.

Three heat shields down, nine more to go.

Bellhousing Cover and Oil Supply Heat Shield

The left merge collector is where the stock oil filter goes, but even with a dry sump pan I had to modify the oil inlet fitting as described in a previous post. After all of that work, things are still danger close. The only route to the inlet is to cross the oil line in front of the bellhousing. Fortunately, the shallow dry sump pan provides lots of room. The first step was to fabricate a cover for the bellhousing that also provides a mount for the oil inlet line and a heat shield. The cover was laser cut from 0.100” stainless steel and the three mounting tabs for the heat shield were cut from 0.187” stainless steel.

Bellhousing cover (gray) with welded mounting tabs for the heat shield (green)

To ensure optimal line routing AN fittings were welded to a mandrel-bent stainless tube which was mounted to the bellhousing cover with P-clamps. Thermal sleaves will be applied to the stainless and flex tubes.

The oil inlet fitting is danger close to the left merge collector (show above) and the oil line also runs parallel to the right merge collector and the exhaust tubes that run under the oil pan, all of which requires heat sheilding.

The heat shield was fabricated from three laser-cut parts, nine CNC bends, one hand-formed curve and one manual bend. The CNC bend lines are shown as faint lines with the large gray piece have some unwieldy bends.

The heatshield is welded and ready to install.

The heatshield is installed via three button head screws

Everything fits like a glove. No way could I have achieved that doing manual bends!

The hand-formed curve is visible here. Note the gap between the heat shield and bell housing cover plate. The mounting holes are slotted and I forgot to push the shield flush before I took the picture.

The heat shield wraps around the bellhousing to protect the stainless steel and flex tubes. The bend on the curved piece had to be done after the curve was formed, so it was done manually in a brake.

I still need to Cerakote the shield and figure out what heat mitigation materials I’ll attach to the shield. Only eight more heat shields to go.

X-Pipe Heat Shield

Other than the exhaust tips and ceramic coating, the exhaust system is done. I now need to fabricate eight heat shields to solve all of the self-inflicted heat management challenges. The first heat shield protects the transaxle from the X-Pipe and exhaust cut outs.

The heat shield was fabricated from 0.048” stainless steel. I chose it over aluminum because it would warp less during welding, it has significantly lower thermal conductivity and it allowed a thinner material to be used.

It was fairly involved to get right:

• 11 laser-cut pieces

• 20 CNC bends

• 7 heat/vibration isolators (purple parts)

• A fair amount of welding (green pieces to the gray piece

These parts required me to learn a few new sheet metal tricks. The recessed box required five corner bend reliefs. Fortunately, SolidWorks has a specific feature which makes that easy. The two sloped sides required me to modify their flanges. Specifically, the CNC brake operator pushes the edge of the part into a back gauge to ensure that the depth and angle of the bend is correct. This requires that edge to be parallel to the bend line — which it wasn’t.

As can be seen in the picture below, the solution was to extend the sloped sides to create an edge parallel to the bend line. The extension is attached via small tabs to facilitate removal. After the part arrived, I cut the extension off and sanded the edge smooth. The tabs required more effort than I anticipated to remove. I took a closer look at SCS’s guidelines and it recommends that the tab width is 50% the material thickness which seems right (mine were twice that wide). They also recommend that each tab is spaced out by 1x the material thickness which seems a seems a bit excessive. I had the corners precut so I only needed to sand the straight edge with the tabs.

The sloped side has been extended to create an edge that’s parallel to the bend line. The tabs facilitate the removal of the extension.

Welding has been completed. Small stitch welds were used to reduce warping.

I 3D printed sections of bend profiles to check fitment and SCS’s bends were spot on. Note that the holes in the stainless have 0.74” doughnuts welded to the outside to provide the correct thickness for the isolators.

The dropped box on the right side accommodates the pneumatic shift servo

Front view

The fit was excellent. Once I finish the remaining seven shields, I’ll have them ceramic coated with Cerakote and I’ll apply one of the ZircoFlex or ZiroForm products to the underside.