The "compression ratios" you see quoted are the static compression ratios, and only depend on the physical geometry of the combustion chamber. Specifically, divide the combustion chamber volume with the piston at the very bottom of its stroke by the volume with the piston at the very top of its stroke, and that's your static compression ratio.

But an engine's "octane" requirements are determined by the dynamic cylinder pressure - that is, the actual pressures in the combustion chamber when the engine's running. Dynamic cylinder pressure is affected by the static compression ratio, but it's also affected by things like intake/exhaust valve timing, camshaft lift, whether you've got a turbocharger or supercharger forcing in additional air/fuel, the temperature of the cylinder head & engine block, and whether you have significant restrictions in the air intake system.

 

Don't forget that the pressure you're talking about is also controlled by the spark timing. The earlier the spark, the higher the peak pressure.

 

I have been keeping up with this thread, so I decided to run a test. My car came ith the high octane in it and then I started running 87. I went back to the good stuff for two tanks, driving the exact same way and there was no difference in mpg at all. The car ran the same and got the same mpg either way.

Jim

 

I think a lot of it depends on what type of driving you're doing. When I'm driving on the interstate, octane rating doesn't seem to make a darn bit of difference. In fact, I drive from California to Texas and back several times a year, and I purposely alternate each tank between the highest octane I can find and the lowest I can find, and I run the tank almost dry between fillups. I haven't been able to find any correlation at all between octane and MPG on the highway. In the city, when the loads on the engine are higher, it might make more difference.

 

Good point. I think you could be correct.

Jim

 

What you should do it run several tanks of low octane gas while driving through the same route and average the results. Just testing one tank is too small of a sampling to rule out variations in driving style, traffic, weather, etc.

When I ran my tests, I was careful to try to maximize the mileage through technique and ran several tanks of each type of fuel. I found the difference was about 3 to 5 mpg, not a lot and I have to admit that may be within the error range of the test.