Originally posted by Port:

1. 9.8:1 is the cuttoff for running 91 octane pump gas at sea level.

If you run high octane where you don't have the compression to get it to ignite, you are wasting fuel and potential energy in the wasted fuel every time the cylinder fires. Resulting in lost power and fuel mileage.

2. Wow, seems like the ocatane rating may have something to do with performance and fuel economy according to cylinder pressure. Whadda ya think guys?


1. You have contradicted your first statement yourself at least twice.

Just to pick one manufacturer, Ford, has several engines at 9.8 to 10.3 to 1 that run on 87 octane fuel. I mean that's what is recommended to run. That's not including any of their vehicles tuned for premium that are capable or running lower octane fuel.

Several folks have run 11.25 to 1 hybrid 3L's on 87-89 octane fuel. Even some using ethanol blended fuel with all it's drawbacks.

Your ignorant statement that certain compression ratios automatically require higher octane fuels is missing the rest of the equation entirely. Compression ratio is just one part of the formula/puzzle.
Just the timing curve alone throws all of your statements into the garbage. It's as simple as this; "BUILD A NEW TIMING CURVE" That easily trumps compression ratio.
Your way of thinking is very 50's and 60's. That's a polite way of saying it's horribly out of date and mistaken.

Not only that but the engine in this post is 10 to 1 and most discussion revolves around engines with a range of 9 to 1 - 11.3 to 1 CR. Yes that includes supercharged and turbo charged engines so it encompasses cylinder pressures much higher the typical NA engines.

2. Octane is ONLY a fuel's resistance to preignition. It has little to do with the fuel's stored energy and performance or efficiency ability. Higher octane will "ALLOW" (not generate) use of higher cylinder pressures, leaner fuel mixtures, and/or greater ignition timing so the engine can make more horsepower. It's called tuning. A higher octane rating itself does not make more power or efficiency.
This same TUNING is why running more octane then needed can decrease the efficiency of the engine. The engine is not "TUNED" to run with fuel of a significantly higher flash point and slower burn rate.
Most cars now days that require premium are backwards adaptable to run lower octane fuel. (That's that technology you do not seem to know about based of your posts btw) However adaptive timing trim is caused by the engine experiencing pre-ignition or detonation and adjusting itself. (timing retard)


2000 SVT #674 13.47 @ 102 - All Motor! It was not broke; Yet I fixed it anyway.