We hate California 91-octane premium fuel. Apparently so does Project EVO. California gas is to fuel as Bud Light is to beer-less octane, more detonation. Although we used some of the best bolt-on parts available for the EVO, we were thwarted at every turn by California's urine-like fuel. Our results are disappointing compared to what tuners in other parts of the country are getting. Typically California gas strips 30-50 hp from a turbocharged car with modern fuel injection. Project EVO was no exception. We hate excuses even more than California gas, so we pledge to continue to develop the car until we get some more respectable numbers out of it.
In part one of our series, we squeezed more stick and balance in the corners of an already fine-tuned cornering machine without reducing its refinement as a daily driver. We also improved the stamina of its powerful brakes in anticipation of hard track use and more power from the motor. In part two, we attempt to squeeze more power out of the EVO's tried-and-true 4G63 engine.
From our baseline dyno runs, we knew we'd have a hard time with Project EVO. Strapped to XS Engineering's 4WD dyno in stock trim, we experienced a lot of variability when attempting to get a solid baseline stock power run. The EVO's knock sensor has a lot of authority over spark, mixture and boost, and with California's poor fuel, we were getting trace knock. The ECU tried to eliminate the knock, thus the boost and spark advance were erratic. So was the engine's power curve.
This is why. Ask any Californian with a turbocharged car about how bad our gas is. Our 91 octane usually means 2-4 psi less boost and/or 4-6 degrees less advance than other states' 93-octane fuel. We suspect the volatility rating of California gas makes it even more knock sensitive than the 91 suggests. Our 91 acts like what old 89 octane used to in terms of detonation threshold and EGT.
After many dyno runs, we picked 232 hp as our baseline. This number was approximately between our recorded high- and low-horsepower thresholds. Our run-to-run variation was as extreme as plus/minus 5 hp. First, we bolted on a Vishnu Performance Systems standard dump-back exhaust system.
We chose this system over Vishnu's more expensive turbo back and signature-series systems because the EVO has a pretty cool divorced wastegate turbine housing and outlet elbow from the factory. The wastegate discharge exhaust flow, which can interfere with the flow coming out of the turbine, is kept separate until it's smoothly merged with the turbine flow downstream. This minor detail is usually good for up to 5-10 hp and a few hundred rpm earlier spool so we didn't mess with this excellent bit of stock engineering. The more expensive Vishnu systems have a fabricated elbow/dump tube, which doesn't do much for power over the stock cast piece and costs more.
The Vishnu exhaust was nicely crafted from polished 3-inch TIG-welded 304 stainless tubing and came with everything we needed for a clean install, including spacers to lower the chassis cross brace to make room for the fat exhaust pipes. The exhaust features a free-flowing perforated core main and sub muffler. A high-flow cat with provisions for a rear-mounted OBD-II O2 sensor for no MIL light operation is also part of the exhaust package.