I have a 1981 Chevy C30 dually truck with a 454 big block and a 3.73:1 rear gear. I’d like to remove the TH400 and replace it with a 4L80E. Is this possible?

R.M.

This is a great application for an overdrive transmission for several reasons.

First, the overdrive will work very well when combined with your rear gear ratio. The 4L80E is essentially a TH400 with a 0.75 overdrive ratio in fourth gear. This effectively cuts cruise rpm by 25 percent. Plus, the 4L80E is a very durable transmission so it is designed to handle the torque of your big block.

Let’s look at the some specifics as to the difference in cruise rpm with the overdrive as opposed to your current situation.

With the 3.73:1 rear gears, we will assume you are running pretty close to the stock rear tire diameter of 30 inches. We plugged 70 mph into our rpm equation to get a cruise rpm at 70 mph with a TH400 trans.

The equation looks like this:

RPM = MPH x Gear Ratio x 336 / Tire Diameter

This gives us a cruise rpm of 2,925 at 70 mph, but does not take into account slippage from a normal, non-lockup converter. We’ll add another 200 rpm to this which gives us a cruise rpm at 70 mph of 3,125 rpm.

A 4L80E reduces engine speed by 25 percent which takes 3,125 down to 2,242 rpm. But the 4L80E also uses a lockup torque converter (this is an option for the 4L80E but it’s a good idea especially in your case) so we can now reduce that number by another 200 rpm down to approximately 2,040 rpm. So you can see that this will reduce your engine speed dramatically, which should improve fuel mileage by at least 10 percent.

That should bring a smile to your face.

4L80E transmission sitting on shop floor
The 4L80E is really not much more than the TH400 with an overdrive added on. The case is longer than the TH400 but it should fit no problem. There are early and late models of the 4L80E but our sources tell us that except for the placement of the cooler lines, the transmissions are essentially the same.  This is an early model 4L80E with both cooler fittings located near the bellhousing on the passenger side. (Image/Jeff Smith)

Some Other Considerations

There are some caveats to this swap. While we don’t have any experience with those big 2500 series trucks, the transmission should fit under the stock floor with no trouble. The cross member will probably have to be relocated but that too should not present any problems. The 4L80E is around four inches longer than a typical TH400 depending on the length of the stock extension housing which means you will need a shortened driveshaft.

The biggest overall conversion factor will be that the 4L80E will need an aftermarket stand alone transmission controller to operate properly. There are several companies offering a stand-alone controller including FAST, Holley, HGM Electronics, MSD, Painless, DIY Autotune and several more. We’ve got their part numbers in the parts list below.

All of these controllers work well, but we’ve had really good success with the controller from HGM Electronics. One aspect of their new Sport version is that you can now set up and run the controller with a simple Bluetooth wireless connection to your smart phone. This improvement to this controller has not only reduced its overall price slightly but is really handy, assuming you use either an iPhone or a Android phone.

Side of a Holley 4150 carburetor
This is HGM’s slick stainless steel throttle position sensor bracket. It’s adjustable and very nice and works great. I’d use this bracket anytime I would need to use TPS with a carburetor. (Image/Jeff Smith)

Don’t Overlook the TPS, Torque Converter, or Transmission Cooler

Assuming that your engine is carbureted, another point is that you will need to install a throttle position sensor (TPS) sensor on your carburetor. Again, HGM offers a very nice and compact stainless steel TPS mount with specific brackets for a Holley, Edelbrock, or Q-Jet carburetors. The TPS sensor is required for any of the aftermarket controllers but I would choose HGM’s TPS overall the other bracket—it is really simple and solid.

Another important choice will be a torque converter. We’re going to assume that you will be doing some heavy duty towing with your dually, so a converter is a critical choice since a little bit of stall speed will help get the truck and trailer moving—but then a lockup converter is also a good idea for when you are driving without the trailer. We generally don’t tow heavy loads in overdrive since it puts a greater load on the trans and the engine. If the lockup clutch is continually unlocking because of load, then it might be a better idea to tow in third gear rather than in overdrive fourth.

A larger or additional transmission cooler is also a good idea if you intend to do a of heavy towing. Ideally, monitoring the transmission temperature is a great idea. I’m pretty sure the 4L80E has an internal temperature sensor that can be monitored through the smart phone connection at least with the HGM controller. It’s best to keep ATF temperatures below 225 degrees F to ensure long transmission life. That’s true for any automatic transmission along with using a synthetic fluid that can handle higher temps without losing lubricity. Good luck with your swap!

TH400-to-4L80E Transmission Swap Parts List

Keywords
4L80E, Ask Away!, Jeff Smith, TH400
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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.