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1991 Pontiac Firebird Formula - Dyno Diagnosis, TPI Style

What do you do when the strip numbers don't jibe with the hardware?

Photography by Terry McGean

We went back to the shop and ordered a set of Comp Cams' new beehive springs as an upgrade. The conical shape of the springs combined with the ovate wire they are made from allow the springs to compress further before experiencing coil-bind; the smaller diameter of the top coils and the retainer also make them lighter. All of this adds up to a drop-in spring that will add lift capacity and improve valve control without placing undue stress on our stock hydraulic lifters. While we were at it, we installed a set of Comp's Pro Magnum roller rockers with 1.6:1 ratio for a little extra valve lift. These rockers are self-guided, like the stockers for '88 and later Chevy small-blocks, and after a little trimming, they fit under the stock valve covers.

Converter in Question
Back at Westech with our new springs and rockers, we were bummed to see that the output had only increased by 3 hp. We were now making 228 hp at 4,800, but at least the power curve was indeed a curve, not a flat line. Were we on the verge of valve float before? Not known, but obviously the real problem remained. Tom Habrzyk was doing the driving on the dyno, and after a couple of pulls, he wanted to try a different test. To him, the converter felt unusually loose, particularly for a 3,000-rpm stall rating behind a fairly tame engine. He started to accelerate very slowly, holding the trans in Second gear while he programmed the Superflow to maintain 30-mph wheel speed. Next, he proceeded to roll into the throttle gradually while watching the rpm on the dyno's readout. Confirming his suspicions, the engine reached just shy of 4,000 rpm before it started to bog due to the limited wheel speed-our converter was amiss.

If our torque converter isn't stalling until 4,000 rpm, which is well past our torque peak and very close to the horsepower peak, it's no wonder the car wasn't improving its quarter-mile performance. The off-the-line hit of the high stall was working to improve 60-foot times, but excessive slippage was shaving off the gains before the big end.

That leaves us to move to the next level of diagnosis: The B&M torque converter dyno. We'll pull it back out and have it tested to see what went wrong, then correct the situation and retest. Maybe we're already in the 13s. Stay tuned.

On The Rollers
Test One is the best of our naturally aspirated runs with the original valvesprings, all of which were similar. This one had the best air/fuel mix after tuning fuel pressure and 34 degrees total timing (6 initial). Test Two is the best with new valvesprings and 1.6:1 roller rockers. These pulls begin at higher rpm.

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