M20 engine swap into E21 320i

Goal: swap in an M20B25 engine into a US E21 320i body

Performed on: '83 320i with an engine from an '89 325i

Applies to: '80+ E21s, maybe all E21s... swapping in other M20s (M20B27, etc) should be similar

Time needed: who knows... took me about 3 months working every weekend

Disclaimer: I do not assume any responsibility for the correctness of these instructions or their applicability to the specified models. Use at your own risk! Also, use commons sense, don't do stupid things.


As we all know, the stock US 320i engine is weak by today's standards, so I got the idea that it would be really fun to put a more powerful engine in that light E21 chassis. Why M20B25? Like all M20s, it's easy to fit physically since there was an E21 that came from the factory with an M20, and the M20B25 is the most powerful of the M20 family and has the highest potential for upgrades. It has relatively modern electronic engine management which is very self-contained and thus easy to swap in (unlike newer BMW engines, whose management is integrated with other chassis systems, which makes them more difficult to swap into an old car). M50/M52/M54 motors may also be reasonable choices, but will involve additional difficulties, specifically related to the transmission and driveshaft... and possibly electronics too. But they do have a more modern design (24V, variable valve timing, etc), so they should be considered too (and some of the info here will apply to them as well).

Ok, enough small talk, now here's how to do it. The procedure is OVERLY detailed in some cases... it may seem like I'm stating the obvious, but I think it's a good idea to do that anyway, just so there's no confusion, and so that people know what to expect before even starting the swap.

UPDATE (May 2015): I've put up all the pictures I took during the whole process here.



Read this first

Read this entire procedure in detail before even starting to work on your car. This is important.

Preparing the E21 320i

Taking stuff from the E30 325i

Putting the M20 engine into the E21

Cooling system

Fuel system

Shifter linkage

Drive line


Other miscelaneous stuff


I'd like to say a big "Thank you" to the E21 legion members on bimmerforums.com who gave me all the necessary info for this swap. Here's a link to my thread there: http://forums.bimmerforums.com/forum/showthread.php?t=848310

I also found these links helpful:
M20B25 into Euro 323i
M20 into US 320i
E30 tranny into Euro 323i
E30 Motronic vs. E21 323i wiring diagram


Copyright© 2008-2009 by Todor Kalaydjiev
Please do NOT copy-paste any information from here; use a link to this page instead. Thanks.