My SL-C will draw a lot of current for a home-built car. This is primarily due to lots of cooling fans (two for the radiator, one for the intercooler's heat exchanger, one for the engine oil cooler, and one for the transaxle cooler) plus lots of extras like the dynamic wing, linear actuators for the adjustable sway bars, etc.
I'm also planning on using a cutting-edge Lithium-Ion battery, the Antigravity ATX-20. I saw it in their booth at the SEMA show and picked it up. "This is a fake, right?", I asked. "Nope, that's the real thing," was the reply. !@%#! that's light. It has some has some really impressive specs:
- Weight: 4 lbs 11 oz (2.13 Kg)
- Size: 6.875" x 3.440" x 6.125" (mm: 175 x 87 x 156)
- Cranking Amps: 780
- Amp Hours (pb Eq): 25
Given that the engine is supercharged, the compression ratio is only 9:1 vs. the stock 11:1. Therefore the 780 cranking amps should be enough to start the car. However, the battery is only rated at 25 amp hours and, from what I can gather, an average car battery has around 70 amp hours. So, I need to have a really good alternator that generates lots of current at low RPMs.
An alternator is a generator that's driven off of a belt or serpentine connected to the crankshaft pulley. So why don't the just call it a generator? I think it's because cars run off of direct current (DC) and a generator creates alternating current (AC) which is then rectified into 12v DC.
Current plan... subject to change
Unfortunately my engine doesn't currently accommodate an alternator or an air conditioning compressor, so I've been pulling my hair about that. In the diagram to the right you can see where I'm thinking about placing these pieces which will require custom brackets!
Since it's an LS engine I wanted to go with an alternator with a General Motors (GM) bolt pattern. I chose the CS-130 style rather than the older and larger CS-144 style because space is at a huge premium in an SL-C – think Manhattan penthouse across the street from Central Park.
Once I had decided on the CS-130, I had to pick one of the hundreds if not thousands to choose from. My biggest concern is idling in traffic on a hot day which causes two issues from an electrical system perspective; (1) all of the power hungry fans will be running at full speed and (2) low RPMs typically means low power generation and even though the engine generates 1,000 HP it idles like a kitten at 650 RPM which is low. As I already mentioned the Antigravity battery has low amp hours, so a "normal" alternator isn't going to work for me.
After doing a lot of research, I found Mechman Alternators. They custom make alternators with really impressive low RPM output as demonstrated in this video.
Mechman's Elite & S Series have the following advantages:
- Six-phase hairpin stators: provide incredible efficiency resulting in extremely high output at low engine RPM, less energy wasted in the form of heat, and more horsepower to the wheels.
- Precision balanced low-mass rotors: allows shaft speeds of 20,000+ RPM.
- High Pole Count: results in less electromagnetic interference with electronics.
- Twin internal rectifier plates: most other high-performance alternators have only one rectifier with six diodes, theirs boast 12 press fit-diodes with 300% more surface area to dissipate heat. This results in cleaner power and better durability.
- Twin high efficiency cooling fans: excellent airflow.
Perhaps most surprising to me is that these are one-wire rather than three-wire alternators. One-wire alternators are easier to wire, but they typically have poor, and sometimes zero, power generation at low RPMs.
I went with model B8165170M with a 54 mm pulley. It has a billet aluminum case and is rated at 170 amps. However, what's really impressive is that it will generate 120 to 130 amps even at my low idle! I just got it today, and it's beautiful. I went with the "machined" finish because that's in keeping with all of the CNC'd suspension parts. Now I just need to figure out how to mount it.
