I started fabricating the cat-back exhaust system. The plan is to fabricate stainless steel sections for the catalytic converters (cats) that connect to the merge collectors and titanium mufflers via v-bands. My engine builder recommended an Air-Fuel Ratio (AFR) of 10.5-11 which is crazy rich and will quickly burn out any cat. I’m going to try running cats with a leaner AFR. To provide the best chance of success, I purchased G-Sport catalytic converters which are EPA compliant and support up to 850 HP each. Worst case, I’ll have to replace these sections with resonators. Time will tell.
I purchased 321 stainless steel transition cones from SPD to adapt the 3” merge collector outlets to the 4-1/2” cat ODs. I used a hydraulic press, a die, and 3-1/2” 321 stainless steel tube to fabricate transitions from 4-1/2” cat OD to the 3-1/2” down stream exhaust. Abe welded the transitions and tacked an elbow section and a flange.
Catalytic converter welded to transitions, elbow and flange are tacked; customized Vice Grip with v-band jaws
Mr. Miyagi would have found automotive fabrication great training — you know the whole Wax On, Wax Off thing. Parts come on and of the car many times and it makes sense to find ways to make that process easier. For example, I swapped all of the suspension nylocs with plain nuts for the build phase. Abe modified a Vice Grip to have v-band jaws as shown in the picture above. The cats have already been in-and-out a dozen times (times two because there are two sides) and it’s already saved a lot of time and we’re no where near done.
Left catalytic converter
Right catalytic converter
I’m going to install cutouts before the cats and I purchased several to figure out which would work best. I selected 2.5” cutouts from DPW. Although the exhaust is 3.5” there is no need to have the cutouts be that large because the exhaust will take the path of least resistance. The worst case is that some exhaust flows through mufflers and the car isn’t quite as loud. DPW provides an inlet tube and a turn-down tube, neither of which worked for my situation. No problem, I’ll make my own. The challenge was that DPW uses proprietary v-band flanges. They’re very lightweight which makes sense because they only need to support a fraction of the weight of a standard v-band. I tried for a couple of weeks to determine if they could provide standalone flanges to no avail. That left me with two options: salvage them or fabricate new ones from scratch.
Inlet tube (left), cutout valve (middle), turn-down tube (right), v-band clamps (bottom)
I went for option one which is a lot less work and requires no additional material. The first step was to cut the flanges from the tube with an abrasive cutoff wheel. Once that was done I chucked the flange in the lathe by grabbing the v-band (as opposed to the tube). Machining relatively thin-walled tube on the lathe is always a little nerve racking so I took a bunch of shallow passes with a boring bar. The flange from the turn-down tube was trickier than the one from inlet tube because the curvature of its tube projected inside the flange. This resulted in a 50% interrupted cut (i.e. cut for 180 degrees and then skip for 180 degrees ). While the sounds made me wince, everything worked fine.
Flange being bored on the lathe, the tube has already been removed
Flange cut from tube with an abrasive cutoff wheel (left) and flange with weld bead and tube removed with lathe (right)
I only purchased one cutout to ensure that everything worked. I need to purchase a second one and repeat the process twice.