Shown here is the C4 installed versus the five-speed. The C4 versus five-speed swap is a direct bolt-in using the same transmission mount and crossmember. You can even use the same driveshaft. If you don't already have an adjustable double-hump crossmember from FRPP, you can grind the welds off your fixed unit so that the mount can be slid forward to accommodate the automatic, or rearward for the Tremec.
Without a doubt, the C4 is easier to live with. In all the time we ran that transmission, whether in competition or test and tune, we never did anything except set the pressure in the tires and the airbag, and make sure the fluid levels were right. It wasn't necessary to give the car a second thought. While it left hard with the transbrake, it always hooked consistently. You don't always have that luxury with the stick. Unless you over-tire the car, or use a tune-up that guarantees adequate traction, you'll be far more susceptible to changes in track conditions. This is the smart approach for racing consistency, but it will leave some e.t. on the table. An adjustable suspension and a logbook full of good data will go a long way toward success with a five-speed.
Another issue to consider is driver consis-tency. It's easy to pull gears with an automatic. Repeatability is critical with a stick if you want to be consistent.
Now, if you want to talk about the fun factor, the stick wins hands down. The satisfaction of winning a race driving a stick car is far greater than driving an automatic. It's common knowl-edge that dropping the clutch and banging gears is all too much fun.
We have a couple more thoughts on the dyno results. The C4 made 302 hp at the wheels versus 290 for the stick. But the torque was 300 lb-ft for the stick versus 273 for the C4. Naturally, the five-speed is going to show a broader powerband on the dyno, and the increased torque output coupled with the gear ratios in the manual are what accelerate the car down the track quicker. But it still bugged us that the car appeared to suffer more driveline loss with the five-speed.
Ultimately, we don't think it boils down to the transmissions themselves. Other than this test, we're not aware of anyone ever trying a stick and an automatic in the same car on the same dyno. So, we really have nothing similar to compare our results to, and in the spirit of letting the results tell the story, we've come up with the following theory. On the rollers there was a dramatic difference in the way the engine accelerated up to 6,200 once the converter locked up over 4,700 rpm. With the five-speed, the engine accelerated slower in high gear between 4,000 and 6,200 rpm. We attribute this to the extra weight of the billet-steel flywheel and clutch versus the lighter chromoly flexplate and converter combo. Frankly, we have no idea of the actual rotational weights of each transmissions internals. Sure, the five-speed has roller bearings riding on a mainshaft compared to the non-rollerized planetaries in the C4, but the Tremec is a large, heavy box. Who's to say it's actually lighter internally? It sure was heavier when we hoisted it into the car on our backs.
One of the few changes necessary to install a five-speed in your Mustang when replacing an automatic is the installation of a new roller pilot bearing to support the input shaft of the trans. Be sure to wipe out the excess grease before installing your flywheel. Natur-ally, if you're swapping from stick to automatic, you'll have to remove this bearing from the crank so the pilot on your converter will fit.
What we're thinking is this: On the track the engine works less to accelerate the car with the five-speed, but on the dyno it works harder to spin the engine from 4,000-6,200. Conversely, the engine works harder to spin the C4 down the track, but has an easier time of it on the dyno.
Where we truly see the most dramatic difference with the five-speed on the track is in high gear. The exact same engine pulls much harder in high gear with the stick. That was evident on the very first pass.
Ultimately, does any of this theory matter? Not really, but since we got ourselves into this mess by adding the aspect of before and after dyno tests, we felt we needed to try to explain it. The bottom line is that the car is quicker with the five-speed, and we're lovin' it.