We dyno'd the car with each transmission at PRA Solutions using their eddy-current dyno. Who would have thought the C4 would record more rear-wheel horsepower than a five-speed? Well, it did-by 12 hp. But since horsepower is a calculated expression of work over a period of time, and we observed the engine accelerated in high gear more quickly with the automatic, that result shouldn't be all that surprising. What matters most is the change in torque output, and there the five-speed shined.
We tried launching harder than 4,700 on our first outing, but it didn't improve anything, and the car appeared to hook (see comparison chart). We've disregarded that run as an anomaly. On the last run of our first outing, we managed to run an 11.92, and anyone who witnessed that run must have thought the driver's right arm and left leg had fallen off. Thanks to some well-worn lower control arm bushings, the left rear shock broke in two almost at the exact moment of the 1-2 shift. As the clutch got kicked, the shock was already separating, which induced a nosedive of embarrassing proportions. As the shift was completed, the car attempted to pull another wheelie as the rear bumper made an attempt at scraping the ground. It's fun to watch on video, but yours truly felt like an idiot behind the wheel. At least we got a decent timeslip for our troubles.
Back home, we installed new bushings in the LCAs, as well as Strange adjustable shocks. We started out with the rear shocks set on position 6 and tightened the front struts another 11/42 turn for our next outing. We also added 0.5 psi in the slicks. The changes apparently worked as the car ran an 11.93 right off the trailer at a 2,400-foot corrected elevation. We followed that up with an 11.88 thanks to a 5,500-rpm launch, but after that the track was inconsistent.
While we found no surprises when we took the five-speed to the track, we were surprised by a portion of the results on the dyno. Naturally, we were going to be able to record torque numbers at a lower rpm because we didn't have to contend with a slipping torque converter. And we expected to see higher overall torque output with the stick. But we didn't expect to see less rwhp (see graph).
Dyno pull No. 013, in red, represents the C4 compared to the broader power band of the five-speed, in blue. That fat torque curve (27 lb-ft more), when coupled with the higher gear ratios of the five-speed, resulted in almost 3-tenths quicker e.t.'s and a 4-mph increase in trap speed.
It was particularly surprising because RIPP uses an eddy-current dyno from Dyno Dynamics. These units have a reputation for delivering consistent results. We had made no changes to the car other than the stick. The dyno was loaded the same way it was for the C4 tests, and the air was even a little better at the start. Just to be sure we weren't crazy, we double-checked everything, ran it with and without fans blowing over the engine, and played with the tune-up. The same timing and jetting that made the best power with the C4 also made the best power with the five-speed. The results were certainly consistent, we just saw different numbers than we expected. But as we like to say around the MM&FF offices, dynos-like gauges and weather stations-are just reference tools. It's best to consult them to observe changes and let the real-world results speak for themselves.
So we did. The five-speed has proven to be between 2 and 3 tenths quicker and 4 mph faster than the C4 using the same launch and shift rpm. We suspect there's a bit more e.t. available-if we can tune the suspension to handle a harder launch on the 26x10 slick. If you're wondering why we didn't try bigger tires, a 28x10.5 slick would lower our trap rpm by 500, not to mention slow the engine's acceleration in every gear. This would probably slow the car. Even if we gained better 60-foot times with consistent 6,000-rpm launches, at best we might break even. Our opinion is that PTM would run quicker using a little more gear and the small tire. And it would certainly need a gear change to accommodate a taller tire. But those changes don't fit into our immediate plans. For now we'll stick with this gear and tire combo.
Even more impressive is the five-speed has turned in numbers at 2,000-2,400 feet that the C4 only produced once in mineshaft air (500-700 feet below sea level). Using the formula that every change in Density Altitude is worth 0.01 in e.t., the C4 combo was running exactly what it should have in the heat prior to the trans swap. It would have run 12-flat to mid-11.90s at sea level or below. Using this same formula, we expect the five-speed is capable of running 11.70s in better air.
We called in a few favors and swapped the transmissions at home in the driveway. The entire job can be handled with hand tools and a floor jack, but it helps to have an extra pair of hands. The Tremec was installed along with an FRPP billet-steel flywheel, a heavy-duty clutch, and a Lakewood blowproof bellhousing.
Another thing to consider is that there would be an even greater difference if we were compar-ing a five-speed to a C4 with a 10 or 11-inch converter. With a stall-speed range of 3,000-3,500 (10-inch converter) or worse, 2,000-2,500 which would be typical in a street/strip car, a Mustang outfitted with a mild engine like ours just couldn't accelerate as hard as with the 8-inch race converter we used. You can expect to see similar results if using a T5 or T45 in comparison to a three-speed automatic. An AOD should be compared to these results only if you are using a unit that has been modified to bypass the lockup function and it has increased stall speed. An OEM AOD that releases the converter in high gear is an unfair comparison to a C4.
You shouldn't expect a Pro-Shifted five-speed to be noticeably quicker than a synchronized box. Its main benefit is reliability. It would be hard to miss a gear with a Pro-Shifted trans, but they're not suited to street driving.