Next up was the exhaust system. In our Supra exhaust shootout a few months ago (Turbo, June 2003), the Thermal Research & Development system did extraordinarily well, increasing that stock Supra's peak power by 11.4 hp and 20.4 lb-ft of torque, with a peak gain of 28.1 hp at and 31.7 lb-ft at 4650 rpm-right in the middle of that second turbo's spool-up. Although it was a little louder than the average aftermarket exhaust for the Supra, the extra power was what we were after, and the 6-pound weight loss didn't hurt, either.
We procured a system for Project Supra, but this time we tested it with a turbo downpipe system from Stillen. By replacing the stock cats, which are the biggest bottleneck in the exhaust system, massive backpressure is eliminated for better flow. It's also 15 pounds lighter than the factory cats. It should be noted, however, that any cat-less downpipe is intended for racing purposes only, and shouldn't be used on a public road (but that didn't mean we couldn't test it on the dyno).
On the dyno, we weren't surprised by the new numbers. With the HKS filter, horsepower shot from 283.5 to 311.7, but with a much smoother curve to boot. The total weight loss over the factory exhaust was 21 pounds.
For just over $1,200 in mods plus a couple of hours worth of installation, the car netted substantial gains from 5000 rpm up-basically when the second turbo is at full bore, with power increases ranging from 10 to nearly 50 hp. Keep in mind this is using the 275-hp run as our actual baseline, otherwise our actual gains on this particular car, including pulling the fuses to reset the ecu, were more like 60 hp near redline. At least in the case of Project Supra, that was the change in power we saw from this morning to this afternoon.
Next time we'll shoot to unleash a few more ponies, so we bumped the spark a bit more to avoid any misfires the next time the car is tested. The HKS Twin Power DLI system incorporates both the CDI and transistor method ignition to provide optimum spark duration and maximum voltage output. According to HKS, this results in a smoother throttle response and powerband.
The CDI method (Capacity Discharge Ignition) provides maximum voltage by transferring all stored energy in the capacitor to the ignition coil so that the ignition current can build up quickly, thus making the Twin Power more effective in the higher revs where a normal spark would tend to diminish. The combination of the CDI and the transistor method incorporates two distinct systems into one high-performance unit, hence the designation of Twin Power. HKS reports the Twin Power produces a spark output that is typically 1.5 to 2.5 times stronger than that of the stock ignition system and consumes about 35 percent less power input than other ignition amplifiers. The DLI also has a CARB number, making it 50-state legal for public roads, and has an MSRP of $625.
The DLI unit was tested against the previous runs, but showed no changes in power, indicating that at least at this power level the stock ignition system with the Iridium spark plugs is holding up. Jason assured us that from his experience in testing these Supras, the DLI system will come in handy when we decide to turn up the boost.
Stay tuned, we'll be seeing just how much better this car does with a stand-alone engine management system and some other toys.