Engine & transmission transplant tips

December 1, 2015 | By Staff


I have a 1978 Buick Skyhawk with a V-6 engine and a four-speed manual transmission on the floor. I also have a 1980 Pontiac Sunbird with a fourcylinder 151 engine with an automatic. The Buick has good running gear but the frame is bad. The Pontiac has a bad engine but a good tranny and the body is excellent.

My question is: Since these two cars have similar body styles, can I change engines and trannies without doing major mechanical changes? I want the V-6 in the Pontiac, and if possible, I’d like the manual transmission in there too. Any help would be appreciated.


Both of the cars you listed were built on the General Motors H platform and were derivatives of the Chevy Vega, so the engine switch itself doesn’t sound like it would be too terribly difficult as engine swaps go. However, before you head out to the garage to get started, there are a number of things that you must take into consideration.

To begin with, I would imagine that the Buick V-6 needs a bigger radiator than the four-cylinder engine. Maybe you can just swap them. Also, the fan on the new engine needs to be the correct distance from the radiator to be effective. And that means you would need to check the lengths of the two engines and how they are mounted to the subframe in order to get the gap right. Some parts, such as motor mounts and radiator core supports may interchange between cars. Study and measure carefully in those respects.

Another factor to consider is the suspension. The V-6 very likely weighs more than the four, so the front springs on your recipient car may be too light for the new engine. It is possible that the springs can be interchanged between cars, but you will need to check the specs in your service manuals to know for sure.

As for the transmission swap, going from an automatic to a manual is actually more complicated than the other way around. You will need to hang a clutch pedal and hook up its linkage, and you will need to locate the shifter on the transmission tunnel of the car and cut a hole for it. Also, you will need to determine whether the transmissions are the same length, and mounted in the same place. Otherwise you may have to deal with a difference in driveshaft length. It is a good bet that the driveshaft from the donor car will fit though.

The differential in the four-cylinder car is most likely of a higher numerical ratio than the one in the V-6 in order to provide better acceleration from a standing start, but it will limit the top speed of the car without over-revving it. You may be able to switch out the two differentials, but then you may not. Also, the differential in the four-cylinder car may not be stout enough for the V-6 engine’s increased power.

I don’t know what the air pollution laws are in Washington, but in California you cannot put an earlier engine in a later car unless it meets the smog laws for that year of car. I would say check that aspect before you proceed.

The final factors I would consider aren’t technical, but I believe worth thinking about. If you enjoy working on cars, doing the switch could be a fun challenge. But it is a big job, and could cost quite a lot if you have to pay somebody to do it properly. Shops these days get $80-$100 an hour.

Also, I don’t know the current value of a 1980 Sunbird because my book doesn’t go back that far, but a well-equipped, well-cared-for 1992 model with 50,000 miles on the clock is worth about $2700 according to the Kelley Blue Book, so I would imagine that the value of your car could easily be exceeded while making all the modifications necessary to produce the results you want.

A cheaper alternative would be to sell both cars and look for a 1980 Sunbird in good condition, equipped with the options you prefer.

I might attempt to make the switches outlined here because I love working on old cars and I love doing creative things, but I would certainly go into it knowing that financially it’s not a good bet, and that there would be perhaps months of work ahead of me as a hobbyist restorer.