Getting Into Overdrive - Technical Article - Rod & Custom Magazine

In life everyone wishes for the best of both worlds, and that wish definitely extends to the world of hot rodding. Not only do rodders want maximum performance from their rides, but with gas prices rising over the last couple of years, rodders are striving for better mileage as well. For GM-minded rodders, the choices have only gotten better and more plentiful as time has marched on.Only a short while ago the Turbo 350 and 400 were the main choices when it came to backing up a GM mill with an automatic. As late-models became equipped with overdrive automatics, so did many Chevy-powered hot rods. We wanted to look more deeply into these choices so we called up Gear Star Performance Transmission to get some answers and suggestions.

The first GM automatic overdrive transmissions to see widespread use were the 700-R4s, which were fairly simple, only requiring a 12-volt live wire activated by ignition when in the on position. But the wheels of progress at GM did not stop with the 700-R4. In this article we'll focus on GM's beefier replacement for the 700-R4--the 4L60 and 4L60E as well as the 200-4R (a natural replacement for the Turbo 350).

Besides the many advantages of the 200-4R over the tried and true Turbo 350, like greater performance potential and the all-important overdrive function, the 200-4R--with its multi-use capabilities--will mount to any standard bellhousing-equipped GM engine (Chevrolet, Buick, Pontiac, Oldsmobile) and is an extremely efficient transmission when it comes to putting the power to rear wheels. The 200-4R features less internal rotating mass and a lower inertia level that will not rob your high-performance engine of precious horsepower. It's also the most direct-fitting overdrive automatic to replace a Turbo 350 (short tail shaft) because of its nearly identical exterior dimensions; even the same driveshaft can be used after swapping in the 200-4R, although the trans crossmember will have to be moved back.

The 200-4R uses a 27-spline input shaft and utilizes a B15 12-inch converter that can be stalled from 1,300-2,200 rpm. The 200-4R is a fairly easy transmission to build to handle high horsepower and the most affordable transmission available in the series of GM overdrive automatics. This transmission does not require an extra computer and is controlled by a simple-to-install and easy-to-adjust throttle pressure cable.

Next up in the line of GM automatic overdrives are the 4L60 and the 4L60E. The 4L60 and 4L60E will only mount to Chevrolet engines and require an adapter plate when used with any other engines in the GM line. The main advantage of this unit is that it has bigger gears than the 200-4R with more rotating mass and a higher inertia level that is important for longevity in heavier cars and towing applications that need to move more weight than a lightweight hot rod under 3,000 lbs. The 4L60/4L60E utilizes a 30-spline input shaft and a B29 12-inch converter that can be stalled from 1,300-2,200 rpm.

The 4L60 requires a 12-volt live wire to utilize its lockup converter. The same specs apply for the 4L60E, except for the fact that it's an electronically controlled transmission and requires a standalone computer. Lockup occurs with a direct one-to-one connection between the engine and the transmission at highway speeds when not under acceleration and keeps the transmission clutch packs from slipping. During acceleration, when not in lockup, the necessary "slipping" created as the transmission smoothly goes through the gears causes all the heat in an automatic transmission. Without lockup, a transmission would quickly burn itself up no matter how many coolers are attached to it. At speed, once the accelerator is re-applied, the transmission disengages lockup and the necessary slipping of internal clutches occurs, allowing you to smoothly accelerate your vehicle. The lockup function is either controlled by a manual throttle pressure cable or electronically by programmed settings inside the computer (in computer controlled transmissions). Computers come preprogrammed, but can be tuned by the owner to maximize the powerband or to adjust "shift feel" to the owner's preference.

Another option in the line of GM automatic overdrive is the 4L80E. The 4L80E is a more expensive option because it's only available as an electronic unit, thus requiring a standalone computer to run. This transmission can be built to handle heavy towing or very high horsepower, but the 4L80E is not the most efficient transmission for putting the power to the rear wheels, as it is heavy and has a rather high inertia level. This transmission is more suited to a cruising car than a strip/streetcar.

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.