BMW M20 Six Cylinder Engine

BMW M20 engine

1982 E28 520i with Jetronic-K, very rare.


BMW had a problem in the early 1970s. The M10 four cylinder engine had done an amazing job for the company in its smaller cars and even in the E12 520i. But the next step up was the M30 "big six", it's smallest version was a 2.5 but it was meant for greater things and was heavy - there was no way it could be made smaller. So there was a gap, an opportunity for a smooth mid-sized six cylinder engine. And let's not forget the history of the wonderful pre-war six cylinder 2.0 as used in the original 328.

Low production costs were a priority on the new design, it should be kept as close as possible to the costs of the M10B20 it would replace. Low weight was also vital.

The prototype was christened the M60 (not be confused with the later 1990s V8). In 1980 BMW changed their engine numbering system and the M60 became the M20 we know and love today.

The new engine started it's life in the 1977 E21 320 (carb), 323i and E12 520i. The E12 525i retained the M30B25 as did the later E28.

As time passed a 2.5 was developed for the September 1985 E30 325i but the M20 didn't appear in a five series until the launch of the E34 525i in 1988.

The largest M20 was the 1981 2.7 Eta used in the E28 525e (325e and 528e in the US, same engine size). This was designed as a low revving economy engine and worked well for the role it was intended in an era before turbo diesels.

BMW M20 engine

M20 in an E30 325iX automatic.


Early on in the design studies a cylinder spacing of 91mm was decided upon and set the pattern for BMWs engine to this day. To keep costs down it used a timing belt. The cam and crankshafts had seven bearings to ensure smooth running and durability. An aluminium block was investigated but not taken forward until the advent of the M52 20 years later.

At this stage fuel injection wasn't on the table, it was to have a single Solex four-barrel carburetor. Another costs saving feature was a cast rather than forged crankshaft. A planned 1.8 was never built.

Production started with a 2.0 carb and a 2.3 with Jetronic-K injection. Later the 2.0 would be available with the same injection system and from 1983 with Jetronic-L. When the 2.3 got Jetronic-L power went up from 139 to 150bhp.

Jetronic-L allowed efficient use of an oxygen sensor in a catalytic converter as feedback to the engine management system and in 1986 a cat version of the 520i was made available. Cat's didn't become law until the early '90s in the UK so were never needed on UK M20s.

The 2.5 is the most famous version of the M20 for it's starring role in the E30 325i. This engine was always Motronic, the distributor was placed at the end of the head and there was no need for vacuum advance or other fiddly hardware - the computer handed all that. And the term computer is well chosen for Motronic was a micro-processor powered unit unlike older Bosch Jetronic systems which were simply electronics modules. This is why a 325i can be "chipped" for more power and a 320i can't.

Because the cylinder bore was so large there were no water passages between the cylinders on the 2.5 or the 2.7. But the engine coped just fine. The M20 had initially been designed to grow only as far as a 2.3.

The "Eta"

A unique concept was tried on the M20, the "eta" engine. It was an idea born during the 1974 fuel crisis and the abandoned M61 stratified charge prototype engine. This was to be a unit optimised for low rpm torque at the expense of high rpm power. The aim was much improved economy. An eta didn't like to rev over 4500 and made the same power as a two litre M20, but it was a lovely engine to drive. I had an automatic 525e (they were all autos in the UK) and it was simply lovely. Just effortless to drive and always power on tap. It was no boy racers car but was a lovely commuter or long distance ride.

To achieve this the eta had Motronic injection, lower piston ring tension and fewer bearings on the camshaft and crankshaft to reduce friction. To the same end it had softer valve springs which was fine for a lower revving engine. A noticeable identification feature are the much longer runners on the intake manifold to increase mid-range torque.

The eta was launched in the US in 1981 and europe in 1983. The final year of the eta was the so called "super-eta" with an extra 8bhp, I think due to a higher compression ratio. This may have been a US only modification due to limits on their earlier engines. A planned M30 3.5 eta was never built.

Various conversions of the 2.7 eta block are possible using a 325i head to make a "stroker" engine. These tend to yield around 200bhp making used eta blocks quite desirable. Worth noting is that the eta pumped a lower volume of oil so you need a 325i oil pump setup.

Automatic Motronic cars such as the Eta have an electronically controlled kick down, the gearbox ECU lives next to the engine's DME under the dashboard and they share information. Other cars have a mechanical kick down cable attached to the throttle body.

BMW M20 engine

325e Eta, intake manifold has much longer runners.


Code Size Power
Made Bore x
Timing Weight
VANOS Used in

(little six)






121 @ 6000

129 @ 6000

137 / 150 @ 5300

170 @ 5800

125 @ 4250

118 @ 4000

120 @ 4000

151 @ 4000

167 @ 4000

177 @ 3250











77 - 83

81 - 93

77 - 85

85 - 93

81 - 88






Belt 117 No E12 520, E21 320

E21 320i/6, E30, E34

E21/30 323i

325i, E34 525i

325e, 525e, 528e


BMW M20 engine

Nicely detailed M20B25 in an E30.


Any engine with a cam belt needs a new one now and then. It's not just the miles but they can crack and age over time too, certainly five years is too much. It was a collision engine so if it snapped it was new engine time. Because it was so long access was tight and it was a good idea to remove the radiator to do the job.

Bad running on Motronic or Jetronic-L engines could be caused by a worn air flow meter. But it was often made a scape goat for all maladies and generally worked well. Vacuum leaks due to cracked hoses were often the real culprit.

These have manually adjustable valves which need to be tinkered with every 30k miles or so, but not too tightly. A mechanic friend of mine (hi Kevin) once said, "I like to hear my M20s".

These can crack water passages in the head if overheated, it's very long head after all so can warp. If it's off the car have it pressure tested.

Early versions could have problems with head bolts snapping but it's rare. Later bolts have Torx heads for identification. The bolts should never be reused as they're stretched past their elastic limit when installed.

It's not that uncommon to get a slight oil leak from the low side of the valve cover gasket. Be careful not to strip the threads when putting the new one on, it's only aluminium.

Idle control valves can stick a bit. Soak them in carb cleaner for a while and snap them back and forth a few times.

If you get odd electrical problems check the woven wire earth strap hasn't returned to nature.

If left low on oil the front camshaft lobes can wear because the engine is higher at the front when installed. So check the oil level regularly (same on M30). Fitting a new camshaft without removing the head first is murder because there's no clearance to get it out (same on M30).

On Motronic cars a good distributor cap is vital as it's the only way for the computer to control the timing. I've seen a new one make a lot more power and gain 5mpg.

Motronic cars have a timing pin on the flywheel to sense top dead centre (TDC) as well as a speed sensor on the flywheel teeth. These live on the gearbox bell housing near the bottom. If either goes bad or has a dirty connection to the main wiring harness the engine won't start.

If looked after it'll go on forever and is a great engine.

M20s can suffer from a crack in the water passages at the top of the head below the camshaft. The 2.5 seems to be more prone than smaller M20s.