Even though the preliminary fuel-pressure test checked out, we'd need to monitor it during a full-throttle pull through the gears to make sure the pump could handle maximum draw. In anticipation of the upcoming chassis dyno session, we ordered a fuel-pressure regulator from BBK as a means of tuning the wide-open throttle air/fuel ratio. The BBK kit includes the billet aluminum housing and a fresh set of gaskets, since the upper intake plenum has to come off for the install.
The converter was followed up with a set of 3.73:1 gears to replace the stock 3.23s, a standard upgrade that should have helped get it all down the track a bit faster. Instead, we gained more 60-foot (1.88) only to see e.t. stay almost exactly the same. Glad's seat-of-the-pants feedback was that the car leaped hard off the line but was laboring to reach the traps by half track. During this time, we'd also added a ported upper plenum from corvetteplenum.com as well as an Edelbrock manifold base, yet the gains in naturally aspirated form just weren't showing up. We specify naturally aspirated because Glad had also begun experimenting with a Nitrous Works plate system by that time. It pushed the Firebird into the 12s (corrected), but in a sense, only added to the overall confusion of our combination. Where had we gone wrong?
To the Dyno
We needed to answer some questions about the car's setup before moving on, and while further track testing might have gotten us there, a trip to the dyno would involve less trial and error. With a 150,000-mile engine running hypereutectic pistons and nitrous, guessing might not be the best plan. To this point, we didn't know how the engine's air/fuel mix was holding out under full-throttle, and we wondered how much additional ignition
lead was helping under naturally aspirated runs.
The instructions advise removing the upper plenum bolts and loosening the bolts holding one set of runners to the base. That looked like a hassle, so we gambled that the upper plenum would come off without messing with the runners. We were right. The BBK regulator replaces the factory regulator housing but retains the stock diaphragm. Fortunately, the kit includes the tamper-proof Torx bit needed to make the swap. The bolt in the center of the new housing will provide adjustment.
The first runs on Westech's Superflow chassis dyno revealed some oddities. Most notable, the horsepower curve seemed to go flat once the engine hit 4,300 rpm-it didn't drop off, but hung right around 224 hp, wavering up and down slightly all the way to about 5,200 rpm before declining. The air/fuel was only slightly lean, and we used the new BBK adjustable fuel-pressure regulator to sort that out. Using an Actron fuel-pressure test-gauge, we dialed the regulator to increase fuel pressure with the vacuum line off from 43 psi to 48 psi, taking the wide-open throttle (WOT) air/fuel readings to 12.1:1 from 13.3:1. Despite this, we saw no change in power.
Charting the ignition advance curve during a power pull also revealed that with the stock setting of 6 degrees initial, total timing came in right around 34 degrees, which is what we'd found these engines to favor in past testing. It did, however, point out that we'd need to run about 4 degrees less if we opted to spray nitrous again.
So, with what seemed to be proper fuel delivery and ignition advance, we still couldn't get our power curve to take on a proper bell shape. After some discussion, it was decided that the valvesprings should probably be changed. The hydraulic roller lifters in this 350 are notoriously heavy, and with 157,000 miles, it was easy to believe we were on the verge of valve float as a result. With that mileage, even if the valvesprings didn't cure the problem, changing them would still be a good idea.