Run 89 or better octane


I was talking to an experienced engine builder recently re. damaged pistons. He’d also talked to Metric Mechanic on the issue. The consensus that emerged is that it’s prob a mistake to run 87 octane fuel on a newish engine.

Sure, the masses perceive, accurately, that our engine was designed for low octane unleaded fuel. But the early M20’s were all higher compression and designed for leaded fuel. I had a 323 when I was in Germany in the '90s and leaded fuel was readily available. Lead, for the youngsters out there, was the primary anti-knock compound.

What the engine builders are telling us, in the context of damaged pistons, is that the idea that our 8.8 compression ratio is always going to be fine for avoiding knocking under race conditions is simplistic. As the fuel-air mixture rushes in and gets compressed inside the compression chamber, there’s regions of the compressed zone of fuel-air that have compression ratios as high as 100:1. We make the mistake of imagining that the compressed zone is a homogeneous mixture of fuel-air, but it’s not. It’s compressed more around the outer edge of the piston and less around the interior. Think of the shape of the piston face and the head’s chamber and it should be clear why. At lower rpms the pressure of the fuel-air has more time to equalize inside the small region allowed it under full squish. But we don’t spend much time at low rpm so our squish zone has local areas of higher compression. And this can lead to knock which tears up pistons.

Note that if you have an overbore engine, the problem is worse. Not an assload worse, the bump in compression isn’t a lot, but the likelihood that you’re getting some knocking certainly is higher.

We drive our cars a helova lot harder then BMW intended. If you have a cherry engine, we should apparently use 89 octane at a minimum.


Beside the point that 87 is faster: I don’t agree about the fear of loosing an engine or partial detonation in a N/A engine. Especially it happens when you are at low RPMS and high loads, exactly what we don’t do 90% of the time.

There is just not enough energy in naturally aspirated engine running with proper electronics to warrant paying attention to fuel pre-ignition. Our engine is so bad at making sure the fuel mixture is correct that after a 100 miles, I already had a carbon layer on my cylinder as if it was driven for 100x that., which would already act as an insulator anyway in terms of thermal conductivity.

To add the engine I rebuilt recently had a more damage to the head then pistons. And even with the holes left by the pinging of fuel mixture done over 100 000 miles, the engine still makes great power.

Can you get away with 89? yes. Is it worth it? Not for the price increase. If this car was turbo, where the volumetric efficiency(Volumetric efficiency - Wikipedia) was higher then 1, then YES, buy the best premium fuel you can get away with, but I don’t think our engines volumetric efficiency is even close to 50%.