1991 Pontiac Firebird Formula - Dyno Diagnosis, TPI Style

Stock initial ignition timing on a TPI 350 is 6 degrees BTDC, and that's where the Firebird was when we checked it, though it had been retarded to that point to accommodate nitrous. Charting the advance curve on computer-controlled cars is difficult, since many won't advance properly in Neutral. Instead, we made another pull while Tom Habrzyk kept the dial-back light on the timing tab. With 6 degrees initial, total timing seemed to be 34 degrees, a bit aggressive for nitrous pulls. Good thing we checked...after running the juice.

Changing the valvesprings with the heads still attached to the engine and the engine in the car isn't a whole lot of fun, but it can be done. It takes an air-hose fitting that fits in the spark-plug hole to pump compressed air into the cylinder so the valves stay in place when the retainers are removed. We used one from a leakdown kit. You'll also need the special "on-car" valvespring compressor; we got ours from Comp Cams.

While we had the valve covers off, we opted to install a set of Comp Cams roller rockers with 1.6:1 ratio, rather than the stock 1.5:1 ratio. This will increase the valve lift and hopefully add a little more power. With late-model, small-block Chevy engines, self-guided rockers are required. The Comp rollers we're using (PN 1318-16) have the required guides that straddle the valve-tip roller.

The flat line of the power curve persisted, and there weren't many variables left untested. We wondered if after 157,000 miles, the original valvesprings might be giving up. Not hard to believe, especially considering factory hydraulic roller lifters are notoriously heavy. That puts more stress on the valvesprings, and now that we're trying to rev past the factory power peak, the springs might not be able to control the valve action. It was decided that, even if this wasn't the real problem, a fresh set of valvesprings was definitely a decent idea. We opted to upgrade with Comp Cams beehive springs, which offer improved valve control while also providing additional retainer-to-guide clearance and resistance to coil-bind.

Despite our efforts with the springs and rockers, the power peak was only 228 hp, but at least now it was flowing in a proper curve. It was particularly frustrating, considering the extra valve lift provided by the roller rockers should have done something. However, during this round of testing, Westech's Tom Habrzyk could sense that the torque converter was slipping even above its reported 3,000-rpm stall speed. Using a programmable feature of the Superflow chassis dyno, he tested his suspicions by accelerating the engine while limiting wheel speed. We managed to get just shy of 4,000 rpm before the engine was being held back by the driveline-our converter was flawed. We'll be removing it for further testing with B&M soon to see what went wrong.

Another rocker-arm requirement when dealing with center-bolt valve covers, as used on most '87-and-later Chevy small-blocks, is the special rockers needed to clear the stanchions for the valve-cover bolts. Even with Comp's Pro Magnum rockers, the stanchions have to be trimmed.