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3 speed to Muncie 4 speed conversion

6K views 13 replies 8 participants last post by  DZAUTO 
#1 ·
Hi all,

I am doing a lot of research and I cannot find a lot of data on a 3 speed to 4 speed swap. Are these transmissions very similar and can I swap from a 3 speed to 4 speed pretty painlessly or what do I need to look out for when doing this sort of swap. also if any one has done this what rear gear would you recommend. I have a bone stock 327 on my 1968 Chevelle. Thanks!
 
#2 ·
If you have a 68 Chevelle with a factory 3 speed the transmissions are very similar. You should be able to remove the 3 speed and replace it with a 4 speed. Obviously you will need a 4 speed shifter.

In 1968 there was a regular and HD 3 speed. They had about the same first gear ratios as the 4 speed. The final drive ratios on all were 1:1. The only advantage you would pick up by changing to a 4 speed would be to keep the car in a more favorable operating rpm at certain speeds.
 
#4 ·
If your 4 speed is close ratio or 2:20 first, at least 3:73 or lower, higher numerically.

If M-20 wide ratio 2:54 first, at least 3:55 or lower.

Be sure to spring for Hurst Competition Plus shifter with metal rod bushings.

Get ready for Way More Fun. !


Edit: One of the best sleeper muscle cars ever was the 68 Chevelle, close ratio Muncie and L-79 327/350 option. It sent a lot of SS 396 owners home, very sad
 
#8 ·
Now, with all the above said, to be really trick and fool people, you COULD retain the 3sp column shifter. I would require a little bit of linkage fabrication. I did this MANY years ago on two cars, a 54 Chevy and a 55 Chevy. The reverse position on the column becomes the 1st gear position on the 4sp, 1st gear position becomes the 2nd gear position on the 4sp, then 2nd gear position for 3rd and 3rd gear position for 4th.
For shifting the 4sp into reverse, we used an Overdrive handle/cable from a 3spOD transmission (back in those days a 3spOD handle/cable was fairly common in salvage yards).

Other wise, your option is cutting a hole in the floor for a floor shifter. Hurst has an excellent shifter made specifically for the 68 Chevelle. From the factory, with a bench seat, Chevelles had a boot/bezel and hump for the shifter handle. These 3 parts are readily reproduction from the Chevelle vendors. Here is what it looks like. This is my son's 68 4dr that we converted from a Powerglide to a Muncie (of course, we had to locate and install clutch linkage).

Also, I would suggest placing a Wanted ad and checking the
For Sale adds here to see if anyone may have these parts for sale.
 

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#11 ·
“The front side marker lights had 307 in them. We replaced those marker lights with the plain marker light from a 6cyl car (no engine size).”

Hahaha, all the 60’s street racers I know did that ! Best was those early Chevy II ‘s with “I-6” emblems 😁

“Plain Jane”. All the better when messin’ with SS 396’s. They’re such bad losers. 😁
 
#12 ·
to make it easier, find a 70 or earlier 10/27 spline muncie trans so you can keep the same clutch, pressure plate, driveshaft and yoke. jim
 
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