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.
