Another reason we chose the standard-grade exhaust is its modular construction allows us to delete the cat for race gas and track usage, which we'll test at a later date. The exhaust fit perfectly, which made for a smooth and easy bolt-on installation. The exhaust gave a smooth, deep throaty sound that wasn't too loud nor tinny.
Back at XS, we were again frustrated by the ECU's reaction to our fuel. The exhausts reduced backpressure and the added power it allowed gave the ECU fits, greatly increasing the power variation. We'll be conservative and say the gain was more than 8-peak hp to 240.6 hp, but had maximum gains of 12-15 hp in most other places in the powerband. On some runs, if we wanted to cherry-pick, the car belted out 30-more-peak hp than stock! Damn gas. Despite this, the exhaust wins our bang-for-the-buck award.
Next up was AEM's cold-air intake, which is usually a proven power adder. But consistent readings again were thwarted by the 91 octane. The AEM intake is a lightweight aluminum short-ram design with an open-element cone-type filter that uses an aluminum shield to block hot air wash from entering the engine compartment. It's hard to beat the stock EVO system because it has a large filter element, big airbox and a cold-air ram.
AEM dyno-tunes the length and diameter of each application for best power. We registered a 4-hp gain up to 244.5 hp, with general gains of up to 7 hp over a wide part of the powerband, from 3300 rpm on up. The AEM intake also produced even more dyno variation.
Our theory is that these mods were improving cylinder filling and thus tickling the knock sensor more. Due to run-to-run power variation, it was difficult to confirm this gain as a solid one, but this was a conservative non-cherry-picked run. The AEM intake made a throaty roar when the throttle was applied and you could really hear the compressor bypass valve at work, popping and hissing like an open blow-off valve.
Our next mod was Unorthodox Racing's Ultra S underdrive pulley set. The EVO application is a three-piece set consisting of a crank, alternator and water pump pulleys. The Unorthodox pulleys gain power two ways. First, they underdrive the crank-driven accessories with a lower drive ratio, thereby cutting parasitic loss. They're much lighter than stock, reducing the amount of power needed to spin them up to speed.
The Unorthodox pulleys cut nearly 5 pounds of rotating weight off of the car's reciprocating mass. Other tuners have confirmed these pulleys usually net a 6-whp gain, but we couldn't honestly pick out a gain in the mass of variability in our dyno runs.
The car did rev faster and felt faster. We've always experienced good gains with Unorthodox's pulleys before so we'll have to ignore our data and take other tuner's word for it.
We blame it all on the gas and the hyperactive knock sensor. For 4G63 builders, removing the front damper and replacing it with an underdrive pulley is controversial. Many respected 4G63 builders don't recommend this. Since the 4G63 is an internally balanced engine and we've never had a problem with any Unorthodox pulley, including very high-horsepower and racing engines, we're not worried. We'll keep a close eye on bearing life when we tear into this engine later.
We installed a pair of Unorthodox Ultra Series adjustable timing sprockets, which we found to be the lightest on the market. After a round of dyno tuning, we gained 1-peak hp up to 245.7 hp, but as much as 11-hp more after the peak, effectively flattening the powerband and giving excellent top-end pull by retarding both the intake and exhaust cams by 4 degrees. Of course, this was a conservative run and we were still plagued by considerable run-to-run power variation.