Whats the lowest octane you run?

[benz-tech]

I was wondering what octane rating you guys are running out there. I am especially curious to see if anyone with a chipped or freshly rebuilt engine chime in. My tired M42 with 230k miles will take regular this time of year without a hiccup. Even with an ebay 91oct chip.
I ask because I'm curious about this engine's ability to resist detonation since I currently am building an 11.4:1 (approx) M44 and plan to run it on pump gas. If my 10:1 M42 is healthy and I can run it on regular, that bodes well for my planned build. But, I'm assuming that at 230k its not an entire healthy beast.
Anyone else's thoughts would be appreciated.

[monty23psk]

Before the car was mine, it was my sister's for about 4 years. Here and here husband only put 87 octane. It ran fine. I started with 93 and haven't looked back even after two top rebuilds.

[deansweet]

91+ here. The 318is I replaced the profile gasket in the pistons and head looked perfect so that's what we use.
Besides, that's what BMW says it should be running so one less thing to worry about.

[DesktopDave]

I tend to run 93 octane here, but only because the car gets better mileage with it. My car is stock, ~290K miles, runs perfectly well with 89. Oddly, I noted that 89 got better mileage than the 91 ever did. I suspect that 89 with a good shot of alcohol gets bumped up to 91.

I'm not sure where you're from, but snow belt states also use winter blends for easier cold-starting. Those blends use more aromatics (mainly butane & MTBE). While they tend to chop 10% off fuel mileage (and may cause cancer) I suspect they may also prevent detonation.

The E36 motors have a big advantage over the early M42...knock sensors, so you'd be ahead of the game there. You could always blend some octane boosters or race fuel too. My local Shell carries 110 race fuel...a bargain at $7.99 a gallon!

[benz-tech]

Dave, thanks I'm thinking that the knock sensors that ill be running will be a good safety net. I will obviously be running the highest octane I can get. It's interesting that you noticed betterfueleconomy with 89. I'm thinking yours and my high-mile M42's aren't compressing as they did a few million piston strokes ago. I think 89 or even 87 might be fine for a tired engine. The "higher the octane the better the fuel economy" statement only applies to engines with knock sensors that would advance or retard ign timing accordingly and thus actually affect fuel usage. I've read many times to run the lowest octane your engine is happy with. Higher burn temps theoretically mean better efficiency I suppose.

[colin86325]

Note that if you have an engine with blow-by, the oil vapors will lower your effective octane rating and can cause pinging...