The M10

BMW’s original production 4 Cylinder engine has a lot of fans all over the world, due to its reliability and quality of design. An ever-present in various BMW models for over 20 years from the late 1960s, the construction and flexibility of application still stands up to scrutiny today.

Due to the quality of components used within the M10 assembly, the engine is very tolerant to fairly high levels of modification, without many side-effects. That being said, we must also acknowledge that all M10 engines are not the same. Some are more tuneable than others, mainly due to cylinder head design variations and compression ratio.

The best cylinder head designs are the 2002Tii (latterly marked E12 after 1973) and the E28/E30 1.8i unit. Both offer large valves and inlet ports, which greatly benefit induction flow, resulting in power and torque increases over other versions.

The M10 engine had a lot of different piston crown designs over the years, so before fitting any revised cylinder head to your engine, it is essential that you make sure the combustion chambers are compatible with the pistons you are using – failure to do so will result wasted time, gaskets and in serious cases, expensive damage to your engine.

If you do have an M10 fitted to your vehicle, and you wish to improve it, in terms of its power and torque output, it is essential to know whether it is a worth-while venture or you are money which would be of greater benefit spent in other directions – such as an engine conversion for example. A lot of the M10 derivatives were designed as low output ‘work horses’, which will cost you a lot of money to improve.

These are the ‘White Elephants’ to avoid playing with, expressed by BMW vehicle model:-
2002 (standard 8.8:1 CR)
E21 315 and 316
E30 316

These engines were designed in the main, as low specification, base model engines, with an eye on economy rather than power output and as a result have low compression ratios and restrictive cylinder head designs.

On the flip side, there are a few incarnations that react very well to a bit of fettling:-
E21 320 and 320i (up to 1978)
E30 318i
E28 518i
E12 518 and 518i

All these units have quite sensible compression ratios and cylinder head designs, meaning they react well to revised camshafts, exhaust systems and induction modifications. The pick of the bunch is the 2002 Tii engine with its forged crankshaft, 10:1 CR pistons and large valved and ported cylinder head. All m10s on the hole are good quality with very few drawbacks, but there are a few components that BMW fitted which will cause you problems if you aren’t careful:-

Thermostat – Clearly a bit of an ‘after-thought’ on BMW’s part, these units can open too early, open too late or fail altogether in the shut position, causing overheating and in severe cases head gasket and cylinder head failure. We recommend replacing the unit for an original equipment item every 30k miles.

M10 Carburettors – Whatever your car is fitted with from the factory, rest assured it will be faulty and causing you problems by the time the engine gets to 90K miles, resulting in intermittent running issues and poor fuel economy – they really are dreadful. We recommend replacement with a Weber 32/34 DMTL conversion – it will be the best money you have ever spent on your car.

The only other regular faults we see here at Fritz, are the radial cracking of head bolt ports in the block. Whenever you have a chance to inspect your engine block, especially when replacing the head gasket, take time to have a good look around the areas of the block face where the head bolts screw into. It is common, unfortunately these days to find ‘hair-line’ cracks emanating from these areas into adjacent water and oil ways. Usually this is due to a failure to properly evacuate the head bolt holes of liquid contaminants when conducting a head gasket or cylinder head change. The resulting hydraulic action of winding the head bolts in on top of the liquid, cracks the block. It also can be symptomatic of excessive overheating, but either way, if you find these cracks, the block is scrap, and you will need a replacement.

The M20

BMW’s small capacity 6 cylinder engine arrived in 1977, and was first fitted to the E12 and E21 ranges of vehicle in 2 and 2.3 litre varieties. Renowned at the time for their smoothness and effortless power delivery, road testers of the time waxed lyrical, although time soon showed that not everything in the garden was rosy.

BMW’s efforts were unfortunately a bit off the mark, especially the 2 litre version. It’s 1990cc capacity, spread across 6 cylinders, resulted in a very short stroke engine, which sounded lovely when abused, but delivered precious little torque or power, resulting in a sharp pull to the kerb, every time an enthusiastic driver passed a fuel station. The 2.3 litre version a bit better, but let down by Bosch’s pitiful K-Jetronic system and practically useless 200 casting cylinder head.

In 1983, the range was updated, with better injection systems (LE-Jetronic) and the revised 731 casting cylinder head for the launch of the E28 and E30 model ranges. Both versions of the M20 were marginally improved, but the 2 litre remained a bit of a waste of time, and the shortcomings of the 2.3’s power output was highlighted when placed in the new, heavier E30 323i. In the same year, BMW also released their ETA engine. The slower revving, long stroke 2.7 litre version offered large amounts of low-end torque and surprising fuel economy for the displacement.

In 1985, BMW released the 2.5 litre version of the M20, as an acknowledgement that the 2.3 wasn’t really up to the job, in the E30 shell, as a flagship model. Using the same bore as the 2.7 ETA engine with a shorter stroke, and the much revised 885 casting cylinder head. The engine soon became a firm favourite of tuners and racers, and was greatly aided by the use of Bosch’s Motronic 1.2 injection system, which allowed electronic based tuning for the first time.

In late 1987, with the introduction of the mandatory application of catalytic converters in Europe, the M20, 2 and 2.5 litre engines received Bosch’s new Motronic 1.3 system, and a small drop in compression ratio, in order to comply with the new emission laws. The ETA engine came to the end of its production life, with the last 525e and 325e models rolling off the line in early 1988.

The M20 generally, is a tough and fairly tuneable unit. As said previously, the 2 litre is a bit of a loss – you can spend a lot of money and get very little useable increases in torque and power. However, the application of the longer stroke 2.3 crankshaft into a late Motronic 1.3 governed 2 litre, can give good increases with the correct map – 180bhp + is not difficult to obtain.

The 2.5 version is held back by the size of its AFM, a standard engine with a revised ‘stand alone’ management system will yield just over 200bhp in good condition, and as much as 230bhp when a decent exhaust, inlet and camshaft are added to the equation.

Using the 2.7 ETA crank and rods, with the 2.5’s 885 casting cylinder head is a well trodden path and will add to the torque and power output, but be careful of the ETA crank – it is only a cast item. The balance weights are a bit bigger than the shorter stroke 2.5 version and due to the reciprocating mass increase and longer stroke, they do tend to snap at high revs when abused. Alpina utilised a lightened version of the M21 2.4 forged diesel crank in the creation of their C2 2.7 E30, to eliminate this danger.

In RHD guise, the 2.5’s 885 casting cylinder head can be very fragile due to the standard factory cast iron ‘log’ exhaust manifold. The manifold holds residual heat against the head after hard running, resulting in a large temperature differential between the inlet and exhaust sides of the cylinder head – as a result it is common for the head to crack longitudinally through the cam journals. The application of a 6-branch tubular exhaust manifold reduces this risk – and gives you a power and torque increase.

The M30

Possibly the best engine BMW have ever produced in a lot of respects. With experience gained building small capacity Aero engines in the 1930s and 1940s, the M30 was first fitted to the E3 2500 in August 1968. 25 years later, the last M30 rolled of the production line as a low compression 3.5 litre unit, fitted to the E34 535i as late as April 1993.

In the intervening period, the M30 powered all of BMW’s medium and large vehicles, in many different displacements and formats, until emissions laws finally and sadly killed it off. Overall, there aren’t many ‘turkeys’ in the M30 range, although the 2.5 litre and early 3.5 litre (1977-1981) are possibly the only two versions that demand criticism.

The 2.5 litre version suffers from similar issues to the M20 2 litre – the stroke is too short, resulting in a low torque, rev happy unit. Frustrating to drive and damaging to your fuel budget, these are best stayed away from.

The early 3.5 litre unit (3453cc) was introduced in the Phase 1, E12 based 635csi in December 1977, as an enlargement of the already successful 3.3 litre unit. Instead of using the 86mm throw crankshaft from the 3.3, BMW instead opted for the 84mm item first used in the M73 24 valve race engine of the mid-1970s, meaning that the bore of the new engine had to be increased massively from the 89mm set-up in the 3.3, to 93.4mm, to achieve the displacement required. As a result, the head gasket ‘land’ between the bores was reduced to around just 5mm, making head gasket failures subsequently common place on more abused road units.

In 1982, BMW accepted the error and revised the engine’s internals – 86mm stroke vs. 92mm bore, reducing the displacement slightly to 3430cc, but increasing torque and reliability.

Towards the end of its production life (1988-1993), the 3.5 litre M30 engine was modified to comply with Europe’s new emission laws. The compression ratio was reduced from 10:1 to 9.3:1and a revised cylinder head casting produced, with larger valves and ports, in an effort to maintain a similar power output to the previous 3.5 litre incarnations. As a result, tune ability and cylinder head reliability were reduced. If you are planning build an M30 engine for competition use, DON’T start with this engine as a foundation.

The jewel of the whole range, is the 3430cc version built between 1982 and late 1987. It offers a 10:1 CR and bullet-proof build quality – over the years we have used this engine for track racing, beach racing, drag racing and off-road ‘Safari’ racing without any major failures or issues.

The M30 has very few drawbacks overall, but it does have a couple of maintenance issues that need to be acknowledged to keep the unit reliable. The oil spray bar that supplies lubrication to the cam lobes, does have a tendency to ‘sludge’ up if service intervals are not properly followed. The reduced oiling to the cam results in lobe wear, which ruins the camshaft and increases the likelihood of the cast aluminium followers to snap at high RPM. Additionally, the use of modern synthetic and semi-synthetic oils in these engines can have a very negative effect on internal engine wear. The engineering tolerances of the components within the engine are not designed to cope with such light oils, nor is the oil pump – so keep it sensible – a good quality 15/40w oil is perfect for most applications of this engine.

In truth, the M30 is the toughest and most forgiving engine we have ever had the pleasure of working on. We still see cars in our workshops with over 300k miles on the clock, and as long as you keep oil and coolant in them, they seem to go on and on.

Many tuners and DIY enthusiasts in the past have fitted them into the smaller 3 series models, resulting in ‘rocket ship’ straight line performance, but handling bordering on the dangerous due to its near 150kg mass. Today, this conversion has been superseded by the advent of BMW’s more modern multi-valve engines, with their comparable power outputs and smaller masses. If your car didn’t have an M30 fitted to it from the factory in its model range – DON’T fit one, there are better options available.