Up to 1977 BMW used 4-cylinder engines for its original 320 and 520 models, evolving from the trusty M10 used in the 2002. After 1977 they began to use freer-revving 6-cylinder engines for all the 2 litre models, the M60 and after 1981 the M20, both still using the same 1990cc, the latter of which has evolved into all the modern BMW straight sixes we know today. The 2 litre four pot is still a very sought after power-plant though, with its light weight and torquey character making for a more responsive front-end, particularly in the smaller chassis models.
I have wanted to compare the specs of these two engines side by side for a while now to see how the bore/stroke and power outputs stack up, so here goes:
CARBURETTOR:
| M10 4-cyl. | | M60/M20 6-cyl. |
| Bore | 89.0mm (3.504") | | 80.0mm (3.15") |
| Stroke | 80.0mm (3.15") | | 66.0mm (2.60") |
| Capacity | 1990cc (121.4 cu in) | | 1990cc (121.4 cu in) |
| Comp. ratio | 8.1 : 1 | | 9.2 : 1 |
| Power (BHP) | 109 @ 5800rpm | | 120 @ 6000rpm |
| Power (PS) | 110.5 @ 5800rpm | | 122.4 @ 6000rpm |
| Power (KW) | 82.5 @ 5800rpm | | 90 @ 6000rpm |
| Torque (Lbs ft) | 115.7 @ 3700rpm | | 118 @ 4000rpm |
| Torque (Nm) | 157 @ 3700rpm | | 160 @ 4000rpm |
The 6-cylinder looks to be the faster motor, making a good 10 bhp more, with the shorter stroke and higher compression ratio no doubt making for a peppier drive. However, with its longer stroke and bigger pistons, the 4-cylinder still makes very similar torque, so unless you are screaming it all the time, the lighter engine might still make a more balanced drive under normal use.
INJECTION:
| M10 4-cyl. | | M60/M20 6-cyl. |
| Bore | 89.0mm (3.504") | | 80.0mm (3.15") |
| Stroke | 80.0mm (3.15") | | 66.0mm (2.60") |
| Capacity | 1990cc (121.4 cu in) | | 1990cc (121.4 cu in) |
| Comp. ratio | 9.3 : 1 | | 9.8 : 1 |
| Power (BHP) | 125 @ 5700rpm | | 123 @ 5800rpm |
| Power (PS) | 126.8 @ 5700rpm | | 125.1 @ 5800rpm |
| Power (KW) | 94.6 @ 5700rpm | | 92 @ 5800rpm |
| Torque (Lbs ft) | 126.6 @ 4350rpm | | 122 @ 4500rpm |
| Torque (Nm) | 172 @ 4350rpm | | 165 @ 4500rpm |
The injected engines are a different story, with the compression ratio of the M10 not too far behind the 6-cylinder and both making the same power, though I would say the M20 is going to pull harder and smoother as you get up the revs and, ultimately have a higher redline. On the other hand, the 4-cylinder actually seems to be giving a bit more torque this time, obviously the higher compression and bigger bore contribute to this and it is all produced lower down the rev range. This could be why I have heard M10 enthusiasts saying it is more torquey, but either way it stands to reason that the lighter, more responsive handling and low-end grunt makes the 4-cylinder a better choice for round-town driving or even short-track racing.
I actually owned one of the rare 1990cc 4-cylinder M10 engines in 2012, having been given it with the '83 E21 316 I bought. The previous owner had intended to build it up and drop it into the E21, but never got round to the project and I decided not to pursue it given that the M10b18 1.8 litre unit was plenty quick enough for the car fitted with a twin-choke Weber carb, so I sold the 2 litre lump in bits, missing suitable con-rods, for £350!
More 1977 engine changes included the 1.6 litre engine used in the E21 316 now badged as the 315. The 316 now received a restricted-power version of the 1.8 litre M10 engine and a new model, the 318, was given a hotter version of the same engine. The 316 dropped back to 1.6 litres for economies sake with the newer M4x series of engines in the late E30 and E36.
