Pure Stock Prep School Part 2- Pontiac Tech

Four-Speed
There isn't much to be done with four-speed cars as Pontiac only offered two choices, at most, for any given model. For most of the musclecar era, the Division used primarily a wide-ratio M20 transmission or the close-ratio M21 or M22. We've had more success with the M21, in general, at the dragstrip. This only holds true for 400-and-under cubic-inch models with 3.90 or steeper gears. The 455 cars can use the M21 with 3.73 and steeper gears. Its weakness, however, is in the 2.20:1 First gear ratio. This inhibits quick 60-foot times in some combinations. The benefit is down the track as the shifts are tightly spaced and the M21 lugs the motor less than the M20 from shift to shift thanks to its nice, tight rpm drops.

Conversely, the M20 has a very large rpm drop between the third and fourth shift, which really lugs the motor down and hampers trap speeds. But the M20 is preferable in taller-geared combos, such as cars running 3.23 or 3.55 gears, as it allows them to get moving much quicker early in the race with its first three gear ratios that are much lower than the M21's. This partially covers for the lack of a short rear gear at the start and the initial lug that is created in Fourth gear.

After you've chosen either the M20 or M21, you need to make sure the trans is operating properly, and the shift linkage is well-adjusted.

Regarding the clutch, the best one we've found to make the motor and trans work together has been the Centerforce Dual Friction. It's very durable and communicates well with the driver. We balance the clutch and flywheel just as we do the motor.

Rearend
Setting up the rearend for the race is pretty simple. If you like your gear ratio and have a Posi, you're already done. For the Pontiacs we build, the gear ratio is based on the owner's requirements, or we'll pick one based on engine dyno results. To build in durability and-more importantly-safety, we change over to stronger axleshafts. At this time, we also put in fresh bearings and check the condition of the factory Posi. We like the factory four-pinion units, as they're durable and effective. If the original Posi unit needs to be replaced, we've had the best luck with Eaton Posi units. Again, they're durable and relatively affordable. If your Pontiac has a factory two-pinion Posi and you'd like to upgrade, the Eaton provides additional performance over the lighter-duty GM units.

Gear choice must be balanced with the primary use of the vehicle. If it's a PS racer first and foremost, then the choice will be much easier. If you drive to local shows and can't stand to see the tach hovering around 3,000 rpm, then you need to moderate your desire for speed. One additional thing to keep in mind is that an automatic car with torque converter slippage runs approximately 200 rpm above a four-speed car.

Pure Stock cars like to come through the traps around their peak horsepower, or slightly above it. An example of how a fairly modest gear change can really add performance is the 455 H.O. Scott Tiemann drives at the PS races. It's the fastest four-speed H.O. to date, but with a change from the factory 3.42 rear gear with an M22 to a 3.73 gear, the car gained four-tenths of a second, in e.t. The modest gear change also improved the 60-foot time from a consistent 2.00 seconds to 1.92s.

If performance is the final goal, here are some generic gearing suggestions: