yeah..comptech. Really. I never noticed it before. Does everyone else have this w/ comptech exhaust. Maybe I just got too nervous cause I noticed it right after my check engine lite started to flash.

Crap gas
Dyno results, however, were strange. Our car baselined at 225 hp on HKS's all-wheel-drive Dynojet. With the exhaust, output went up, but by a different amount and in a different part of the powerband with every pass. Our shitty, knock-prone Southern California gas was screwing with us, forcing the ECU to constantly dodge the dreaded ping. As you'll see in a moment, this should not be typical.

The next step, as usual, was a trip to Vishnu Performance. We relied on Vishnu for tuning on our WRX project car, and the following ordeal should explain why we've come back with the EVO.

The Vishnu EVO we tested back in the September '03 issue showed huge gains from retarding both cams 5 degrees, so we sent our car to Vishnu with a set of AEM cam gears in the trunk. Its mission: tune the car as is, with the stock cam timing and the RMR exhaust, then adjust the cam timing and tune it again. We wanted to see just how much power was available with each step.

That request should've been simple, as Shiv Pathak, Vishnu president, hydrocarbon-sniffing tuner, and occasional SCC contributor, had already tuned dozens of EVOs and, at that point, already had the maps pretty much dialed. Problem is, his EVO tuning is done with the ChipTorque Exede, a very sophisticated piggyback ECU, and a piggyback is limited to making adjustments relative to the stock ECU. Our ECU, for some reason, was different, so the offsets that worked on every other EVO were an utter failure on ours.

Pathak's only choice now was to custom-tune our car, an exhaustive iterative process he tackled with obsessive-compulsive thoroughness. Dialing in our two settings (stock cam timing and 5-degrees retarded) took him 189 dyno pulls, most of a day, and a tank and a half of gas.


Then the real work began. We returned to HKS, wanting all our testing to be done on the same dyno, and performance was back to its old, erratic self. Those 189 tuning runs were on crappy, 91-octane Northern California gas, but now we were on crappy, 91-octane Southern California gas. Despite having the same octane rating, the two fuels are clearly not the same. Luckily, we had anticipated just such a problem, and Pathak was already there, computer in hand. It took four hours and another 100 dyno pulls to get things dialed on our uniquely shitty petrol, but in the end, with pupils throbbing in a hybrdocarbon-poisoned haze, Pathak declared victory. The RMR exhaust, AEM cam gears, and Vishnu's exhaustive tuning took output from 233 to 273 hp with driveability and gas mileage improvements to boot.


Your results should vary
All this gasoline drama and our screwy ECU mean our results are a worst-case scenario. Our ECU was tuned more aggressively, with slightly leaner mixtures and more advanced timing than most. Strangely, our baseline dyno results are a close match for other stock EVOs, but the more aggressive stock tuning may explain why our car took the highway to knock sensor hell as soon as we put a free-flowing exhaust on it. This shouldn't happen on anyone else's EVO, and it certainly shouldn't happen in an area with 93-octane gas.


The exhaustive retuning drama should also be unique to our situation. Pathak has been making dozens of trips to various parts of the country to tune cars. With at least 10 cars gathered at each location, he's seen a comprehensive picture of regional fuel differences and is able to ship his Exede piggyback ECUs with different maps depending on where you live.

Just how big is the difference in fuel? The week before our second tuning session, Pathak was in Texas, where he was tuning on the same kind of Dynojet in similar weather conditions (hot and humid). Several Texan cars with the same modifications that made 273 hp on our car, made an even 300 hp at the wheels on 93-octane Texas gas. Color us jealous.