4 speed to TH400?

[freakshow108]

I know this is a regularly discussed topic so i will make this a very specific thread. Has anyone done a swap from 4 speed to a TH400 on a 71 K20. I am contemplating this to achieve low rpms on the highway. Im not worried about towing and the 4wd will mainly be used for snow purposes. I have a TH400 out of a 2WD 71 and i hope to marry it up to a 205 transfer case. I dont mean to waste time on the forum thats why ive tried to make very specific. thanks for any help.

[Shyguy]

Is a TH400 and over-drive transmission? I don't thinks so but I don't know a lot about automatics since all my trucks have had the granny 4 speeds.

Changing from a granny 4 speed to a TH400 will not drop the rpms of the transmission.

I have found that manual transmissions seem to get better mpg than automatics.

The automatic should give you better control though when doing snow work I have heard.

Danny

[raycow]

The TH400 is 1:1 in high gear just like your 4 speed, so your engine rpm won't drop any for the same road speed. If anything, it will increase slightly because of converter slip.

If you still want to go automatic, look at an O/D transmission like a 700R4. If your only desire is to reduce engine speed, a 5 speed with O/D like a NV4500 will be less work to swap. Since neither transmission was offered in 71, you will need an adaptor to bolt it to your transfer case. Adaptors are available from outfits like Advance, Novak, and others.

You may also want to consider an axle swap as an alternative. This could be less work/money than a transmission swap if you find a later 4WD donor truck, as these would typically have taller gearing.

Ray