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#15: 04-15-2011, 11:16 AM
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Originally Posted by BAC5.2 View Post
Not really variable.

You have a static compression ratio, 8.2:1. Air is compressed by the turbocharger, and then stuffed into the engine where it is compressed again. This is often called the "dynamic compression ratio", because it changes dynamically. The static compression ratio never changes. Whatever makes it past the valves and into the cylinder is compressed 8.2:1.

8.2:1 is pretty low, but the 8.6:1 in your old Probe is not high. S2000's run up around 11:1. The E30 BMW's ran 10.ish:1.

Octane does little more than require greater activitaion energy to begin a combustion reaction in fuel. This greater energy requirement makes it more difficult to ignite, which means you get greater resistance to auto-ignition. Auto-ignition in the modern world is known as detonation or det. In the old days, you'd call it "pinging" or "pinking" or something silly like that. Det occurs for a number of reasons, but the end result is an ignition of the Air/Fuel mixture at a time not desired, typically before the ignition event was supposed to occur. The damages from det can be catestrophic.

So, thta's kind of it in a nutshell. The description of everything you asked would take quite some time to describe, but there are a number of posts on this board that can help you get a better idea of the answers to your questions.

i was trying to describe the 'dynamic compression ratio'

your description makes more sense though