Engine Swap - M20

There's no simpler swap from the possible choice of engines than the mighty M20. While the smallest version was 2.0 litres, its full-fat form is a 2.5 litre version, which is the only one worth having. Around town it gives all the smoothness you'd expect from a straight six, and there's plenty of grunt so you won't be chasing up and down the gears trying to find the right cruising speed. But let her rip, and she'll provide as much tail-happy sliding as you want.

Don't be put off by it "only" having 170hp. That much power in a tail-happy E30 has caught out more than one experienced driver being too enthusiastic in the wet. Which is why it was fitted as standard in the most powerful of factory E30s, the 325i.

For the daring, there's also the option of rebuilding your M20 first to a whopping 2.7 or 2.8 litres. Learn more about building a stroker engine.

Overview

The M20 can be fitted to any E30, no matter whether it started life as an M10, M40 or M42. With the right base vehicle a weekend is enough to get everything bolted in and ready to go. You can even use this guide replace your existing M20 with a bigger version, and in that case the engines can be swapped over in a matter of hours.

This swap is considered the easiest swap because all the parts are standard BMW. There's no need for custom components, and everything you need can be supplied by one donor vehicle. In terms of fabrication, you may need to mount (weld) a few brackets under the bonnet for engine ancillaries and modify the loom a little, but this is specific to the age of your E30. We'll explain that later.

Parts List

The parts list depends on which engine your car has already; the venerable M10, the stalwart M40 or simply a smaller M20 engine.

For all swaps, you will need:

• Full engine with all ancillaries (injectors, AFM, coil, alternator, engine mounts, starter motor and flywheel)
• Complete engine loom
• ECU
• Complete exhaust from manifold to back box
• M20 Cooling system of radiator, hoses and expansion tank
• M20 Coding plug for the Instrument cluster
• A mate on standby. An extra pair of hands can save your life.

If swapping from an M10 or M40, you will also need:

• M20 Gearbox.

• Differential

If swapping from an M20, you can re-use most of your ancillaries. The M20B25 specific components are the injectors, intake manifold and throttle body, ECU, AFM and exhaust.

The starter and flywheel are a matched pair. If you are upgrading an old (pre-1987) M20 to to a newer one, you won't be able to use your old starter with your new flywheel.

Engine

There are a number of M20 versions out there, ranging from 2.0 to 2.7 litres. However, we expressly recommend the 2.5 litre M20B25 since it produces the most power. All M20 engines are equally thirsty, so there are zero savings to be made by fitting a smaller capacity engine.

The 2.5 lump came in two versions; a High Compression and Low Compression version. The High Comp version was fitted to earlier cars and is considered more desirable, but was hampered by its rudimentary engine management. The Low Comp engine is far more common, and has the better Motronic 1.3 system. The best of both is to fit the High Comp engine, and convert it to Motronic 1.3.

Gearbox

While the standard M20B25 gearbox is a Getrag 260, it is not uncommon to fit the Getrag 240 gearbox from the 320i for its sportier ratios. Using this box also means its possible to retain your 318i or 316i propshaft. If you use the Getrag 260, you need the shorter propshaft that goes with it. It is possible to use an M40 gearbox, but it will sit at a significant angle, which necessitates customising the gearbox mount and shifter mechanism; not recommended.

Clutch

While parts suppliers do list differences between B20 and B25 engines, there's no practical difference. A B20 clutch will work fine on a B25 engine.

Prop

The prop needs to be matched to your gearbox. Since the available gearboxes are different lengths, you need to find a longer or shorter prop to compensate for that length. If using the original 325i gearbox, you'll need the prop that goes with it - no other one will fit.

Differential

While its possible to keep your original diff, you will find that first gear is now useless. All other E30 differentials have a higher ratio than the M20B25 ones, which is great for acceleration but terrible for motorways and wet-weather driving. We highly recommend fitting a medium-case differential designed for a 2.5 engine.

There are three practical ratios available for the M20B25 engine.

• 3.91 - Spirited driving, but quite high revs at motorway speeds which can be tiring and noisy
• 3.73 - Good all-rounder
• 3.64 - The cruising ratio, which helps claw back valuable fuel economy from the thirsty M20 engine.

Loom

The loom is specific to six-cylinder engines, since it contains plugs for six injectors. It comes as a two-piece unit, with the main part joining the ECU and all the ancillaries to the rest of the car via the C101 plug, while a small sub-loom connects the injectors and temp sensors via the C191 plug. This C191 plug is a weak point and can corrode over time, so make sure it's not crumbling or split on your new engine as this can lead to serious problems later.

The C101 plug on your loom

ECU

The ECU is a sealed box that contains the brain of your engine. Make sure you have the right one for your engine by comparing the part number against our list of ECUs. If you're struggling, you can fit an ECU from a 320i to your M20B25 engine; it'll just run weak. That can be fixed by fitting a chip, as long as you're running Motronic 1.3.

Removal

First off, you need to get your engine bay empty. Get a load of cardboard boxes, packing labels, plastic bags, zip ties and marker pens, and slowly dismantle everything in your cars engine bay. Start by disconnecting the battery and removing it from the car. Then drain the coolant system and dispose of the fluid responsibly. You can now start unbolting everything, but make sure you bag it and tag it as you go along, so you can refit anything later. Here's a handy checklist - cross them off as you disconnect them:

• propshaft
• gear shifter linkage
• rear light switch cable
• throttle cable
• engine earth strap on the engine mount
• coolant hoses to the internal heater matrix
• coil
• ECU

You'll also want to whip out the radiator and headlights so that they don't get damaged, and store them somewhere safe.

Now you need to get your engine out. The quickest way is out the top with the engine and gearbox as one unit. To do this, remove the bonnet and store it. Crack and loosen the nuts holding the engine mounts, and remove the nuts holding the mounts to their rubber bushes. Loosen the gearbox subframe nuts too.

At this point some people find it useful to jack up the rear of the car to ease the angle of the dangle. Raise it onto ramps or blocks, just make sure it's secure. Now use a jack to support the back end of the gearbox as you remove the gearbox subframe. You can now introduce your engine crane, complete with balancer if you can; it makes the job a lot easier. Hook the crane up to the engine, and slowly lift the engine up and out of the way. Dispose of it as necessary.

Upgrades

Since you've got the engine out, it's worth taking the time to upgrade these components.