Michael Galimi
March 1, 2011

Our third modification was a different torque converter. The torque converter transfers the power from the engine to the transmission. It's a fluid coupler since it uses fluid to turn the converter's internals, which drives the transmission and ultimately turns the rear wheels. A looser converter essentially allows the engine to rev higher before the converter begins to lock up in order to drive the vehicle. This is beneficial in drag racing because a looser converter will let the engine race right into its torque band, rather than being bogged down.

According to Kevin Winstead of TCI Transmissions, the general rule of thumb is that every inch of diameter change in the torque converter works out to be roughly a 500-rpm change in stall speed. The most common diameters range from 7 to 13 inches. More slip equals more heat, which limits how loose you can go in a street application. Of course, there is more to it, such as the angle of the fins inside the converter, but we'll leave that for another day. Note that the smaller the diameter, the higher the stall speed (looser converter).

Broken down, the common spec when selecting a torque converter is stall speed, a term that is often misunderstood. "Stall speed is the maximum rpm that the engine flashes to as soon as the throttle reaches WOT at the starting line. A high-stall converter allows the engine to flash to a higher rpm than the stock torque converter, both at the starting line and on the gear changes as well. The stall speed should be optimized to allow the engine to reach the rpm range where it makes the most power. This allows the vehicle to accelerate much quicker than with the factory low-stall torque converter," said Winstead. He continued to dispel some other misconceptions with stall speed. "Many people think that a 3,500-stall torque converter means that the car will not move until the engine reaches 3,500 rpm. That isn't correct. The car will still begin moving just above an idle." When you jump on the loud pedal and go WOT, the converter will flash to the stall speed and get you rolling quicker-provided the proper stall speed was matched to your combination.

The dyno sheet for our test car shows some serious low-end grunt, as the torque is over 460 lb-ft at around 3,400 rpm and remains above that mark until about 5,000 rpm. We want to exploit the long torque curve to help the car get moving quickly.

The OEM torque converter stall speed ranges from 1,600 to 2,000 rpm-far too low for the torque curve of our test car. Ideally, a torque converter's stall speed should be within 200 rpm of peak torque. In our case, that number is a broad one thanks to the twin-screw's characteristics, and we settled on a 3,500-rpm stall speed. The general rule of thumb for torque converters in street cars is that anything over 3,800 rpm tends to be too loose, but it also comes down to personal preference. We settled on an off-the-shelf TCI 3,500-rpm stall speed unit (PN 456002). It retails for $995.95 from JPC Racing.

The most power our car made was 479 rwhp, but that was with 19 degrees of timing and 93 octane pump gas. That's what it made when we ran our best of 11.49 at 119 mph. After that test, we backed the timing down for everyday use. It was set at 16 degrees peak at 6,500 rpm (shift point). We wouldn't have to worry about getting some bad gas while tooling around town. The safer tune slowed the car down, and a month earlier it went 11.58 at 119.78 mph with the 4.10 gears. That was with peak timing of 16 degrees and Mickey Thompson ET-Street bias-ply tires.

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