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. There is no fabrication or welding required, and just a few brackets need to bolted on under the bonnet for engine ancillaries. You may need to 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 inc. brackets, hoses and expansion tank
• M20 Coding plug for the Instrument cluster
• New rubber engine and gearbox mounts
• 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. You'll need the full kit of pressure plate, clutch plate and thrust bearing.

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 purpose of the C101 plug is that with a simple twist, you can unclip and disconnect the entire engine loom from the rest of the car. This makes removing and installing engines a breeze - you just unclip your old loom and plug in the new one. But there's one dilemma; the C101 changed its shape during the 1987 facelift. Therefore, if your car is from before 1987 and your engine is after, or vice versa, you're going to have to cut up some wires to get things to fit together. This can be done in four ways:

1. Cutting off both C101 connectors and joining the wires directly.
2. Cut the plug off your old engine loom and join it to your new one.
3. Get a new C101 plug for your car loom and fit that
4. Make an intermediary loom to fit between your car and engine looms, using new and old style plugs at either end

The first three of these options involves cutting and soldering wires; so does the last one, although you can buy these intermediary looms from other people ready-made if you really don't fancy doing the work yourself.

However you solve it, you need to wire your pins up this way round:

Rectangular C101	Round C101	Application	Image
1	1	Alternator charge light
2	2	Static Oil Level on Check Panel
3		Not Connected
4	4	Temperature Gauge
5		Not Connected
6	6	Oxygen Sensor relay
7	13	Fuel Pump relay
8	18	Ignition start (black/yellow)
9		Not Connected
10	7	Ignition switched power (green)
11		Not Connected
12		Not Connected
13		Not Connected
14		Not Connected
15	10	Dynamic Oil Level on Check Panel
16	5	Oil pressure light
17	11	Reset for Service Lights
18		Not Connected
19		Not Connected

There is also the small issue of the C104 plug; a small connector next to the ECU plug that carries the tachometer and fuel econometer. If you're fitting a newer engine to an older loom then you're going to have to cut two of the wires on your engine loom so that they can connect to the existing C104 socket.

The wires you need come out of the ECU plug are pin 6 (Black) and pin 32 (White/black). Cut the C104 off your old engine loom, and connect pin 1 to the black wire and pin 3 to the white/black wire. Pin 2 is not used.

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. If you find that you're running a Jetronic or Motronic 1.0 ECU, we highly recommend that you acquire the parts to upgrade to Motronic 1.3.

Cooling

The coolant system for all M20 engines is pretty much the same, with radiators varying slightly in size but not having any real effect on the quality of the cooling. The one significant difference is the expansion tank. For engines before 1987, this was bolted to the right-hand suspension turret, just in front of the battery. For the 1987 facelift it was relocated to the LH inner wing, just behind the headlight, and the hoses re-arranged accordingly. This difference is clearly visible with the water pumps available for the engines, whose outlets change to suit the plumbing layout.

Either of these radiators will need to have a bracket fitted to hold it. If you don't want to do any welding and you aren't happy drilling holes in your car to use ordinary bolts, there is a third solution; a 318iS radiator, which has an expansion tank on the side. You'll just need to get creative with your cooling hoses to make sure it's all plumbed in correctly.

Finally, your radiator is held in place on plastic mounts which clip into holes on the chassis rails. These mounts will be different to your existing radiator mounts, and need to be sourced. They are two different sizes; one short, one long. There have been reports that various radiators don't fit these mounts despite being sold as Genuine BMW radiators, so prepare to be creative with your mounting arrangement, or source another radiator.

Exhaust

The Exhaust for your engine is simple, but it's essential that you fit the right one for your engine. Everything from your manifold to your tail pipe counts as a system, and you can't mix and match between systems. There are three types of manifold (three-stud, and small and large four-stud), and these need to be matched to the downpipes. Then, depending on whether you have a two- or three-piece system, you need to match your downpipes to your centre and back boxes.

The positive thing in all this is that they all perform exactly the same; the pipes have the same diameter, so the only issue is parts availability. Based on which system you choose, you'll also need seals, flanges, hangers and rubber mounts for your system.

Brackets

There are three mounting points that need fettling in order to mount all of your M20 parts. These are

• Relay bracket - 41141904837
• Expansion tank mount - 41141871979
• AFM mount - 41141913049

All of these brackets are on the inner wing. They can be cut off a donor vehicle, or bought new from a BMW dealer using the above part numbers. If you don't want to (or can't) weld, then these brackets can simply be bolted in place as needed.

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
• clutch slave
• rear light switch cable
• throttle cable
• engine earth strap on the engine mount
• coolant hoses to the internal heater matrix
• Power steering hoses, if fitted
• 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.

Fitting

With your engine bay empty, take the time to clean and degrease everything, and deal with any Rust in the area.

Upgrades

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