I installed a 2004 8.1L big block Chevy into my 1978 3/4 ton 4wd pickup. It has the original TH400 three-speed trans. I’m not racing this truck, just a daily driver that I use to run around on the sand dunes. I’m trying to get the rpm down at cruising speed.

We figured out the rear end was a 4.10:1 ratio but the only matching gear ratio was 3.73:1. So we put in the 3.73:1 gears with posi carriers front and rear. This only gained me a five mph difference from the 4.10:1 gears.

Is there a way to upgrade the trans to an automatic four-speed with lockup torque converter? Thanks

D.L.

The quick answer to your question would be to install a 4L80E late model four-speed automatic overdrive transmission. Think of this transmission as a TH400 with an overdrive- which is exactly what it is. The overdrive ratio for the 4L80E is 0.75:1 which means in overdrive this will reduce your engine rpm at cruise by 25 percent.

So let’s say that your cruise rpm at 70 mph is 3,000 rpm with the current TH400 trans. With the overdrive, this will reduce your cruise rpm by 25 percent or 750 rpm which will put your cruise engine speed at 2,250 rpm. If you choose to use a near-stock converter for a lower stall speed that includes a clutch lockup, this should reduce the rpm by another roughly 150 to 200 rpm with the converter clutch locked up. This eliminates the converter slippage.

The practical application of this idea requires some work but with a large truck the swap should fit without having to perform any floor pan surgery. The transmission will be slightly longer so relocating the crossmember will also be required. Of course this will also demand a different, shorter driveshaft. We’d also recommend an efficient trans cooler.

Because this 4L80E is an electronically-controlled transmission, the big issue will be having to invest in a stand-alone controller to allow you to set shift points, shift firmness, as well as when the converter should lock up among other variables. There are several companies offering transmission controllers. We’ve listed them in the chart below. The prices range from around $450 to well over $1,600 for the Chevrolet Performance system so you have a broad spectrum of choices—we’ll include a parts list at the bottom of this article.

A Holley Sniper Transmission Control system
This is Holley’s Sniper stand alone GM electronic trans controller that is part of the Sniper family. It’s relatively affordable and this kit is designed to be used without the use of a Sniper EFI throttle body system. There are also kits that are even more affordable that will work in conjunction with the Sniper system. (Image/Holley)

All of these systems will demand certain inputs. The most important are engine speed and input from a throttle position sensor (TPS). This is a critical input since this indicates load to the transmission which is used to establish line pressure, shift firmness, and many other control variables.

You did not mention if your engine is electronically fuel injected or running a carburetor. We will assume you are running a carburetor. With EFI, you can piggy back the TFS signals from the throttle body but a carb doesn’t use this sensor. A company called HGM Electronics makes a very nice bolt-on TPS sensor kit that will adapt to a Holley, Q-jet or an Edelbrock style carburetor which should cover the bases for your carb choices. HGM also makes a very nice controller you might want to look at as another option.

Holley has recently introduced a Sniper stand alone controller for carbureted engines. We’ve not experimented with this but our experience with the Sniper EFI has been really good so this might be another controller to investigate.

4L80E Controllers & Accessories Available at Summit Racing

  • TCI EX-TCU – TCI-302820
  • HalTech Nexus Ti4L Transmission Interface Kit – HTH-HT-196001
  • DIYAutoTune MicroSquirt CAN Transmission Controller – DIY-S0030
  • TCI Wireless EZ-TCU Controller – TCI-302600
  • TCI TCU 2.0 Automatic Transmission Controller – TCI-303820
  • Chevrolet Performance Automatic Transmission Controller – NAL-19302410
  • FAST TCU 2.0 Automatic Transmission Controller – FST-30382-KIT
  • Edelbrock TC Transmission Controller – EDL-36213
  • Holley Sniper Automatic Transmission Controller – SNE-551-102
Keywords
Ask Away! , Transmission Control , Transmission controllers
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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.