Thursday 12 July 2012

Does an early '80s E21 316 with an M10 run on unleaded fuel?

The answers before I bought the car pointed to yes but it turns out that the four-cylinders might not after all. Ignition timing is fine, apparently, down to as little as 92-octane unleaded fuel, but valve wearing is still a problem and an additive is recommended. It's about £8 a per tank so could be expensive over time. I don't reckon the previous owner had been putting any in regularly or they would have given me half a bottle with the car, so I think I'll be taking my chances.

The other thing is regular 95-octane unleaded or 98-octane super. The engines are designed for 100-octane leaded petrol so regular unleaded might be a fair drop performance wise, but does it make a difference to the valve wear?

As we parted company the previous owner said to fill it with super to get the best out of the engine, but said it would run fine on regular and that was all that was available to me at the filling station so it had to do. The car charged home without a grumble from the engine and feels very lively indeed, though I'll be filling it with super next and see if it makes any difference.

UPDATE: It turns out my engine has had the hardened valve treatment so it can run any unleaded without excessive top-end wear, so I haven't needed any costly, annoying additives and that's a relief. The engine seems happier starting up and a bit livelier running on 97/98-octane petrol, which is what I've been using for the infrequent use the car gets. Obviously the power gain isn't vast on a 100bhp engine and super-unleaded is dear stuff, so now I'm starting to use the car a lot more I'll be switching back to 95 RON for the sake of economy.

Fuel Consumption:

I've worked out from averages that my late 316 M10 1.8L engine, running a Weber DMTL carb. and the 5-speed 'sport' gearbox is giving me ~26mpg, which isn't bad at all really. Using the lower octane 95 RON fuel may give better mileage, or may make it worse, either way I would imagine the difference will be negligible given the amount I drive the car.

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