I bought a 4L80E automatic trans out of a late model pickup to put in my ’66 Bel Air. We swapped a big block Chevy into the car a few years ago with a TH-400 transmission and now I’d like to add an overdrive transmission. What will it take to convert this new trans into my car?

G.F.

Jeff Smith: The physical part of bolting this transmission in the car will be easy. The 4L80E is just a longer, heavier, overdrive version of the TH-400. It uses the same first three gear ratios but adds a 0.75:1 overdrive. This means that with a 3.55:1 rear gear, the overdrive drops the effective rear gear ratio to 2.66:1.

This makes the first question more about torque converter selection. Yes, you could use the stock torque converter, but I wouldn’t recommend it unless you will merely use the car as a cruiser and never do any performance driving. But since your car already has a big block in it, it’s obvious you will be doing some spirited driving. Assuming that’s the case, the stock converter is not only very tight in terms of stall speed, but it also is extremely heavy. We weighed a stock delivery truck 12-inch diameter unit and it pushed the scale to an amazing 52 pounds. The problem is that you have to accelerate that big chuck of steel every time you step on the throttle. A much smaller, 11- or 10-inch converter will offer a much improved stall speed, it will be lighter, and still allow you to use a lockup for highway cruising.

I have some personal experience with a TCI 10-inch single disc lockup converter bolted to a 4L80E behind a carbureted 496-cubic-inch Rat motor. The engine makes just shy of 600 horsepower and is packaged in a ’70 Nova that is plenty impressive. We’ve never accurately tested the stall speed, but it’s roughly around 3,200 rpm. This is not a budget converter by any means, but it is extremely responsive to throttle. In other words it’s not mushy despite its small size and high stall speed for street driving. Plus, with the lockup clutch, it works very well once the trans shifts into high gear.

Of course, this conversion will also require a new driveshaft as the 4L80E is longer than the typical short tailshaft TH-400 that is currently in your car. There are many good driveshaft companies that can build a made-to-order shaft and ship it to you. Companies that we’ve worked with in the past include Denny’s Driveshafts, DynaTech, and others. Each company has specific measurement techniques but the most important point is that the measurements must be taken with the car at ride height – not hanging from a frame lift or up on jack stands. Triple-check your measurements and there will be no drama.

With the trans in the car, the next critical element is a stand-alone transmission controller as all these electronically controlled transmissions demand control. There are at least five different systems that we know about but in the interest of brevity, we’ll outline four. Before we get into the electronics, it’s important to mention that these transmissions also require a throttle position sensor (TPS) that converts the linear throttle movement into a voltage signal to the computer so it knows how far the throttle is opening. This is critical information. If you have a Holley carburetor on your engine, Holley does make a kit that adapts a standard TPS sensor over the top of the electric choke mechanism.

The Chevrolet Performance SuperMatic controller is fully computer controlled and will require a laptop to configure your transmission. I like this controller because it offers some amazingly detailed control strategies, but if you are not into computers, it might be intimidating. One thing we really like is that the SuperMatic is the only controller that offers data logging. This system was easy to load and almost as easy to tune so don’t let its laptop requirement scare you.

Painless offers a trans controller called the TORC does not require a computer and is completely self-contained in the handheld unit that will display information as well as allow changes. The digital display did require some interpretation until we understood how the letters were displayed, but it certainly was easy to modify shift points and shift quality.

ASK-04-03

This is TCI’s EZ-TCU controller. Here, we are inputting the maximum shift rpm. One great thing about these units is if you change gear ratio or tire size, you can easily update the unit with a few simple keystrokes and have an accurate speedometer display.

The third in our overview is TCI’s EZ-TCU controller. This unit uses a separate hand-held unit from the actual controller. One point we should make about all these controllers is that it’s very important that the power and ground both be connected directly to the battery. We didn’t do that on one of the first units we tested and had all kinds of difficulties with electronic noise.  The TCI unit is very easy to use and employs the same kind of self-learning strategies as the FAST EZ-EFI self-learning EFI units. This makes adapting this system to a hot rod very simplistic and it’s also one of the least expensive units. The handheld uses a monochromatic screen, but if you are more interested in what it tells you than how it tells you, the main display will feed you full of all kinds of useful information including vehicle speed, engine rpm, what gear you’re in, and whether the converter is locked up or not.

As you can see, there’s quite a bit of technology wrapped up in this electronic trans conversion, but then you’re also getting some 21st century driving advantages beyond just adding an overdrive gear. If you spend some time to tune the trans to your application, it will dramatically improve the entire driving experience. The price is a bit steep when you consider everything but over the long haul, it’s worth it.

Keywords
Ask Away, Jeff Smith, transmission conversions
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.