Racing With A TH400 Transmission In A 1970 SS Chevelle

Posted by on Monday, July 21st, 2014

The Th400 is a heavy duty three-speed automatic transmission designed and built by General Motors. It comes from a family of transmissions that include the THM400, the THM375, the 3L80 and the 3L80HD. The TH400 was first put into production back in 1964 and was last used in President Reagan’s limousine in 1984. This transmission came with a 2.48:1 ratio first gear and a 1:1 ratio final 3rd gear. Although strong, it was not great for gas mileage. The TH400 could be found in many performance vehicles including the El Camino, the Corvette, the Monte Carlo SS, the Trans Am and the Oldsmobile 442. It could also be found in the 1970 Chevelle SS.


The 1970 Chevelle SS came with the TH400 automatic transmission


The 1970 Chevelle SS (Super Sport) was a very high performance vehicle and could be ordered with a high horsepower 454 LS6 7.4 liter big block engine. This motor was rated at 450 horsepower, but many believe it was underrated by General motors to help keep insurance costs down. These cars could be ordered with 4:10 ratio rear ends and cowl induction hoods, and were capable of turning 12 second quarter mile times straight from the factory.

Over the following decade, these cars became nothing more than old used cars and often exchanged hands for very little money. Because of their performance potential, many were turned into full out race cars. Owners modified the motors to make even more power, but often did little to prevent the transmission from breaking. If you’re lucky enough to own one of these cars and plan to race it, here are a few things you can do to make it stronger.

Upgrade the input and output shafts. Both are weak points on the TH400, and can easily be twisted and completely broken in half.  Today, replacement shafts made from 300M are available. 300M is a very strong steel that can easily handle the output of most high horsepower engines.

Upgrade the bands and the clutches. The friction material found on the stock bands and clutches simply cannot transmit a lot of torque, and will end up slipping and failing very quickly. If your Chevelle is a dual purpose street and strip vehicle, I advise that you use the “blue” bands and clutches made by Raybestos. If you plan to purely race your vehicle, you might want to consider using carbon fiber bands and clutches.

Install a transmission brake. A transmission brake works by applying the forward and reverse clutch packs at the same time, simply locking up the transmission. This allows cars with high rpm stall converters to launch at very high rpms in a zone where the motor is reaching its maximum power. Transmission brakes eliminate all the automatic features of an automatic transmission. Because of their violent shifting characteristics, they are not recommended for street use.

Install an aftermarket transmission cooler. You want to buy the largest transmission cooler that will fit your Chevelle SS.  It is recommended that you mount it in the front of the vehicle where it will see adequate air flow. If given the choice, choose a stacked plate design over the more commonly found tube and fin designs.

Sooner or later, due to high mileage or abuse, you will be faced with a decision to replace the TH400 transmission in your SS Chevelle.  I highly recommend doing so with a re-manufactured transmission over one that was simply rebuilt. A re-manufactured transmission will include major internal modifications and upgrades designed to make it both stronger and more durable. A rebuilt transmission will not include this feature. A re-manufactured transmission also comes with a much better warranty; three years compared to an average 90 days for a rebuilt transmission. And in the rare event that you need to contact the company that re-manufactures transmissions, they are just a quick phone call or email away!

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