GM Transmission - Getting In To Overdrive

Both the 4L60 and the 200-4R are more performance-minded options and both are designed to work with lockup torque converters, which are very important for the longevity and durability of these units, not only for rpm reduction while in Overdrive, but also for cooling and lubrication of the transmission. Either a manually controlled pressure switch (activated by the throttle pressure cable) or an electronic computer will control the lockup feature when the transmission hits cruising speed in Fourth gear, putting the engine and transmission in a direct one-to-one relationship.

Dyno testing is an imperative and very essential part of a performance transmission. Gear Star utilizes an AXIS Line Dynamometer that places tremendous load and drag on the transmissions and converters. Gear Star uses this machine to test all of their transmissions and converters as a package. They check pressures and temperatures in all forward gears and reverse and run the transmission and converter to 130 mph, repeatedly listening for any noise or vibrations.

Gear Star recommends an external transmission cooler for any performance transmission since the high stall converters generate a tremendous amount of heat. A radiator-enclosed cooler may not fully meet the needs of your transmission and Gear Star suggests a low-profile cooler that mounts in front of the radiator. If space is an issue, Gear Star recommends a frame-mounted cooler with an integral fan, such as ones offered from Flex-a-lite. These coolers keep the transmission fluid temperature at 160 degrees versus 190-200 degrees when using the radiator cooler.

Properly set up, an overdrive transmission will make your hot rod or custom much more enjoyable to drive. In most cases, the First gear ratio is lower (not the case with the 4L80E), which will help get the car rolling from the stoplight quicker and also help make up for some of the rear axle ratio lost when running tall tires. These lower First gears enable you to run a little higher rear axle gear that will also help with the overdrive.

Final Drive RatioSome thought needs to go into the transmission, rear axle, and tire selection to find a combo that will work the best for you. Cruising at too low of an rpm can be just as bad as too much rpm. Ideally, a small-block should be spinning between 1,800-2,600 rpm at 65 mph. Here's a formula to help you calculate your hot rod's ratio:

mph x final drive ratio x 336 / tire diameter = rpm65 mph x (0.67 overdrive x 3.50 rear axle) x 336 / 28 = 1,829 rpm