M20 Conversion Basics

Could there be an easier engine swap than an E30 325i M20 into a E21 323i? Actually, yes: an S52 into an E36 is pretty plug and play. But this almost as easy. There are a few really good guides online, including the ones covering 320i swaps. They didn’t always apply to my project so here’s what I learned along the way.

First, here are the best resources I found:

Todor’s: http://www.todor.info/repair/e21swap/

bf.com thread: http://www.bimmerforums.com/forum/showthread.php?1045649-How-to-swap-M20s-E21-323i-to-Motronic-E30-325i

The swap is easy enough that you don’t need a lot of how-to. This is what you will need:

Complete M20 engine with wiring and ECU (get it as complete as you can)

E30 air flow meter

E30 cone filter intake (the E30 air box might fit but I didn’t try)

E30 throttle cable

New rubber fuel lines from the frame rail to the E30 fuel rail.

E21 fuse box plug and pins

E21 fuse box plug

E30 C101 connector. The top should be on the engine wiring but get the body-side for a factory look.

If you have a 1988+ M20 you will need the 1987 cooling system parts (water pump and hoses). Some of the hoses can come from the E21 system.

An extra E30 front exhaust manifold. The E30 exhaust layout has the manifold outlets next to each other. The E21 has them spread apart. This might vary from car to car, however. I could have used the E21 manifolds but they looked too rusted on the studs. On my car I could not use the E30 rear manifold and had to source a front one. Everything bolted up from that point.

E30 oxygen sensor and bung. And realize that the bung cannot be welded to a rusty exhaust pipe.

You will probably want to relocate the battery to the trunk. That required a battery tray, battery cable, junction box for the engine bay, and wiring from the junction to the starter and alternator. And you will need to cut several holes in the interior sheet metal.

You also need to cut a hole in the firewall for the engine ECU so it can mount under the dash, a la E30.

The E21 flywheel and clutch will bolt on to the E30 M20. Some of the parts are actually the same anyway.

Make sure the engine is complete and running. If you buy a non-running engine upfront you will be chasing those issues later instead of driving it. I chased a no-start issue for weeks because the engine I bought needed basic maintenance. Here is what tripped me up once the engine was in the car:

  • fuel pump relay
  • short in the fuel rail
  • alternator voltage regulator
  • alternator ground
  • TPS adjustment
  • throttle stop screw

They’re not expensive so make sure this stuff is taken care of.

You will need the motor mount arms from the E21 (which you won’t get until the engine is out of the car).

As for the actual swap itself, I did it in my driveway with a Harbor Freight engine hoist. You have to pull the hood off and the latch mechanism and rods need two people to safely remove.

I pulled the engine with the transmission attached but it was SCARY. I won’t be doing that again. It came out pretty well but the two pieces together made it a struggle to reinstall.

Here is the wiring that needs to be done:

oil pressure light in instrument cluster
fuel pump power
alternator charge light in cluster
ignition coil power
ECU power
power to starter
coolant housing temp to cluster
RPM signal to tachometer. This was the only connection that did not work. I ended up running a dedicated wire from the coil to the C101 plug (bypassing the E30 ECU).

Here is my wiring inventory:


I was very pleased with the results and how straight-forward this was. It was a great learning experience and I’m enjoying the fruits of my labor.

in it’s new home


Here are the links to my engine build and pics from the swap: http://douging.smugmug.com/Cars/BMW-M20-Engine-Restoration

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