BMW M30 and M102 Six Cylinder Engines

BMW M30 engine

M30 in an E24 635CSi.


The BMW M30 "big six" isn't so much an engine as an experience. Driving one with a manual gearbox you feel that the engine IS the car and that the body panels are just along for the ride. The accelerator pedal is used to reign in the power of the engine rather than to ask for more. It never feels at home when at idle, in fact I've heard it described as agricultural at idle. Unlike it's rivals of the time this engine was designed to rev and had a camshaft designed for that purpose alone. It is a beast.

The 25 year journey started in 1968. BMW was doing very well for itself with the "Neu Klasse" saloon and it's derivatives but it was time to go up market in a booming German economy. And yet developing an engine was a costly business even then, so BMW leveraged as much of the superb four cylinder M10's design and technology as they could. In many regards the M30 is an M10 with another two cylinders. In no way is that a criticism.

In 1968 to celebrate the year I was born BMW fitted the first 2.5 and 2.8 engines to their E3 large saloon and the 2.8 into the E9 coupe. At this stage they were carburettor based. In fact the 2.5 did eventually make it's way into an E9 2500CS for 1974 due to the fuel crisis.

In 1971 the M30 grew to 3.0 for both the E3 3.0 and glorious E9 3.0CS. For the 3.0CS a whole host of racing versions were based on this up to 3.3 in size. A Bosch Jetronic-D injection system found it's way onto the 3.0CSi and early E3s.

The E12 five series, the first five, launched in 1974 and the M30 was a natural fit in both 2.5 and 2.8 capacities. Fuel injection was made available for the 528i, I suspect this was one of the first applications of Jetronic-L rather than the D used by the 3.0CSi. E12 530 and 530i versions were never sold in europe.

When the E24 six series" launched in 1976 the M30 was the only option in either 3.0 carb or injected 3.3 versions. The year after the E23 seven series came out and was to use almost every M30 variant during it's ten years in production. Worthy of mention is the E23 732i which was BMW's first car with micro-processor controlled Motronic fuel injection.

But it's the 3.5 (actually nearer a 3.4) that the M30 is best known and loved for. The first 3.5s were sold in 1979 for the E12 M535i, 635CSi and 735i and utilised Jetronic-L injection. These used a different block compared to later cars as their bottom end was the testbed for the M88 24 valve engine. For 1981 the M30 3.5s were upgraded to Motronic and not a great deal changed until the M30's replacement by the M60 V8 in 1993. The engine went on to be the star of the show in the E34 five series and E32 seven series.

A little known M30 derivative is the M102 turbo, initially a 3.3 but later revised to a 3.4. I believe the 3.4 may actually have been called an M106, can anyone confirm this? This was used for the E23 745i whose name arose from the theory that the engine made the same power that could be expected from a 4.5. It was never sold in right hand drive markets, which lead to South Africa having a 745i with the M1's M88 24 valve engine.

BMW M30 engine

M30 with early style injection manifold in a 1980 635CSi.


It's M10 origins brought along much of that engine's proven technology. So throughout the production span we have manually adjusted tappets, durable chain drive and two valves per cylinder with an overhead camshaft. There was the usual iron block, aluminium head and strong forged crankshaft which ran in seven bearings to ensure smooth power delivery. Like most BMW engines it was included at 30 degrees towards the exhaust side to lower it's installation height.

Early carburettor fed engines used a pair of Solex Zenith 35/40s but this was an engine crying out to be fuel injected. Bosch's fledgling Jetronic-D system was fitted to the 3.0Si and 3.0CSi. This was a common fuel rail system in which banks of injectors were pulsed together rather than individually and the amount of fuel delivered was based on manifold vacuum pressure. It was advanced for it's time.

In the mid-70s more widespread adoption of fuel injection in other models saw the arrival of Jetronic-K continuous fuel injection which in some ways is less advance than D but doubtless cheaper. This more mechanical system was good for power but not so good for economy. BMW soon switched to the electronically controlled Jetronic-L which fired each injector sequentially and is more like the systems in use today. This could be adapted better for fuel economy, low emissions and use with a catalytic converter. VW/Audi and Porsche still ran Jetronic-K into the 1990s on some models, BMW were always a generation ahead in their take up of fuel injection technology.

The last leap for the M30 was to Motronic injection starting with the E23 732i in 1981. The system was developed with this car as the testbed by Bosch. Motronic was micro-processor controlled and used a lookup table to determine injector opening times and duration, timing advance, etc. There was no vacuum advance on the distributor, instead the rotor arm covered a large angle for each cylinder and the coil was fired by the ECU at the correct moment. Cars could be chipped by third parties changing the contents of the lookup table and inserting a new socketed chip (it wasn't soldered). Motronic cars are easy to identify as the distributor is driven directly from the end of the crankshaft and sits at the front end of the head, on other engines it's at the side of the engine with a separate drive shaft. It's an excellent system and is one reason why large BMWs were so much more economical than Jaguars and other competitors.


BMW M30 engine

M30 with EML electronic throttle in an E32 735i.

Most fuel injected M30s have a cold start injector (more of a valve really) that allows fuel into the upper inlet manifold. It's controlled by a thermo-time switch that lives on the thermostat housing. If you're M30 won't start easily this is one thing to check. Around early 1988 when the E34 came out BMW stopped fitting this and just increased the duty cycle on the injectors for a cold engine, much better idea. The newer engines have a plastic airbox at the side of the engine instead of a metal one, the valve cover is slightly different as there are no mountings for the air flow meter.

A brief mention must be made of the USA E12 530i version, oh dear. In order to comply with tough new emissions laws in the 70s BMW fitted thermal reactors to the engine and an air pump. The engine also had to cope with low octane American petrol. Not only was it down on power as a result but economy fell to 12mpg (US gallons) or less. Worse, the thermal reactors cracked in service and the cylinders coked up to the point where they had to be blasted with walnut shells and compressed air through the spark plug hole. It should be noted that BMW weren't the only manufacturer to have tears before bedtime thanks to the new laws. I'm sure the experience hastened their drive to advanced fuel injection systems.

And so to the turbo charged M102 used in the E23 745i and fitted privately to a few fives and sixes. Initially this was a 3.3 with a lowered 7:1 compression ration, a KKK turbo running 9psi of boost and developing 252bhp. It was controlled by Jetronic-L and used an intercooler to reduce the charge temperature by 40C. The later 3.4 variant used a lower boost pressure but had almost identical performance thanks to the upgraded Motronic injection. Alpina made a turbo M30 for their 1989 E34 B10, it was capable of 188mph.

An interesting addition to some of the later M30s was an EML electronically controlled throttle. This allowed for traction control to be fitted and used one of the electric stepper motors from the M70 V12. I had this on a 1989 Alpina B10 3.5 E34 and it worked wonderfully in the wet. I thought this was purely an Alpina thing but I've since seen it on late E32 735i's. This technology didn't reach mainstream BMWs until the 2001 M54.

BMW M30 engine

Alpina M30B35 in an E34 B10, note newer plastic airbox.


Code Size Power
Manufactured Bore x
Timing Weight
VANOS Used in

2.5 carb


2.8 carb


3.0 early

3.0 late



150 @ 6000

150 @ 5500


184 @ 5800

185-208 @ ?

188@ 5800

200-206 (euro)

218@ 5200

156 @ 3700

159 @ 4000


177 @ 4000

188 @ ?

192 @ 4000

209 @ ?

229 @ 4000























early 80s

77/8 - 93









Chain 143 No E12 525

E28 525i, E23 725i

E12 528, 2800/CS

E28 528i, E23 728i


6 cyl 530i & 730i

3.3Li/633, 732i/733i


M102 3.3
252 @ 5200 279 @ 2200 74.1 82.1 84- 86 ? Chain 145 No E23 745i


BMW M30 engine

M30 in an E28 //M535i.


A worn rotor arm (9 quid) can cause a loss of high end power and even a non-start situation. It can also have a big effect on economy. Well worth replacing along with new single electrode Bosch spark plugs.

One of the biggest causes of intermittent stalling on old BMWs is dying relays or dirty wiring. For peace of mind replace the aging main, fuel pump and unloader relays for 10 quid a time. Also a good idea to unplug and clean all wiring connections and body grounds.

BMW approve unleaded only in post 6/87 cars, in practice most people run unleaded on older cars too and I've never heard of pinking or valve seat problems. Expect almost 30mpg on the motorway if you have a light foot. This drops to 22-24 in fast country roads and 17 in heavy traffic. Tank holds 16 imperial gallons.

The high pressure (3.0 bar) fuel lines don't last forever. Ten years of heat and vibration will have caused cracking inside. They should be replaced with the correct BMW hose to avoid a fire hazard, at the very least trim 10mm off the end. Don't forget the biggest culprit for fuel smells on pre-88 cars, the 6" line from the fuel rail to the cold start injector. Best changed by someone with small fingers and a little Vaseline on the ends of the new hose.

Poor idling can have two causes. A sticking idle control valve can be cured with white spirit and a toothbrush. Vacuum leaks are harder to find but even the small hoses, such as the one to the top of the fuel pressure regulator, can cause problems.

M20s and M30s have a reputation for eating camshafts. This isn't the case unless oil changes have been neglected or the "banjo" bolts on the oil spray bar have worked lose. The head must be removed to change a bad camshaft so make sure any ticking noise isn't major expense.

These engines need a valve adjustment ever 25,000 miles.

Pre 1980 engines have larger waterways in the cooling system than later models. This can sometimes cause the head to crack.

Overheating can cause the alloy head to warp. This may be caused by a bad viscous coupling leading to overheating in traffic. An ancient radiator is often the real culprit though. The water pump is really cheap and easy to change. US spec cars with air conditioning use a two speed electric fan in front of the radiator, this can be a further cause of overheating.

If the engine seems tilted forwards or out of line with the air flow meter housing suspect failing front engine mounts, not expensive.

Alternator rubber mounts fail and cause the alternator to tilt inwards. Costs pennies and takes an hour to fix. Circlip pliers are handy.

The M30 is bullet proof if looked after and should see at least 300k miles before needing major work. I know of some with over double that.