I have a 1965 Chevelle with a strong 383ci small block. I want to upgrade from my TH400 transmission to an overdrive automatic. It looks like the 4L80E would be the best, but I’ve heard that it may not fit under my Chevelle’s stock floor pan. How much work would it be to convert to the 4L80E? I know about the 4L60E but I’ve heard they are unreliable.

T.L.

The 4L80E is very similar to your TH400. The first three gear ratios are the same for both, but the 4L80E adds a 0.75:1 overdrive gear. The problem is that the TH400 is already a tight fit under the small transmission tunnel in the early Chevelles and the 4L80E case is substantially larger. This means you have to remove the tunnel and either create your own out of sheetmetal and or invest in a prefabricated tunnel. We have two prefab tunnels that fit 1964-72 GM A-body cars in the parts list below.

Regardless of which path you choose, there is substantial cutting and welding required. At a minimum you will need sheetmetal cutting tools, a small MIG welder, and the skills to use them. You could also farm this work out to a local shop, but that will add to the overall cost.

The 4L80E is also longer than the TH400, which means you will also need a different or modified crossmember as well as a shorter driveshaft. The good news is that once installed, the 4L80E is a beast fully capable of handling a large amount of torque and horsepower.

We would advise you reconsider using a 4L60E. Unless your engine is blessed with a power-adder and makes in excess of 700 horsepower, a 4L60E will be easier to install and cost significantly less overall once all the options are considered. Just something to think about before you take the plunge.

We have installed 4L60Es in a 1964 El Camino and a 1967 Chevelle. Like your ’65, they have tight transmission tunnels. The only sheetmetal work we had to do was dimpling the floor pan on the passenger side of the tunnel to accommodate the transmission’s servo cover housing. The cooler lines fit tight to the floor pan, but this is an easy work-around.

Since the 4L60E’s transmission case is smaller than the 4L80E’s, you can tuck the transmission closer to the floor to create the proper driveline angle. Some installers shortcut this and lower the transmission tailshaft for more clearance. This creates a severe driveshaft angle that causes driveline vibrations.

Parts List
Tin Man Fabrication Transmission Tunnel TMF-100311                       
American Powertrain Transmission Tunnel AWR-SMGM10001

Tin Man Fabrication Transmission Tunnel for 1964-72 GM A-Body
This Tin Man Fabrication transmission tunnel will make fitting a 4L80E, 6L90, 8L90, and other transmissions in a 1964-72 GM A Body much easier. The tunnel is made as compact as possible to allow the use of a factory center console. Follow this link to see the installation videos. (Image/Summit Racing)

Keywords
GM A-Body , 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.