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Message: <blockquote data-quote="Mr. OAM" data-source="post: 389623" data-attributes="member: 9816">The cam timing does actually change the dynamic compression.Static compression is what you calculate with your build spec's. My wife's 331 has 9.6:1 static compression.Dynamic compression is what results from cylinder volume and valve timing events. When your valves open and close will have an effect on how much actual volume of air/fuel is captured in the cylinder and the resulting compression based on stroke and combustion chamber volume. To get nitty gritty, time actually factors in as well (RPM).A cam with a shorter valve lift duration creates more dynamic compression. A cam with a longer valve lift duration creates a lower dynamic compression. That is why you run higher compression with long duration cams. This is a general statement because valve timing on the valves is what actually effects dynamic compression, but higher readings are typical with short duration cams and low readings with long duration cams. This is how what cam you are running effects your octane requirement. On my wife's 331 with 9.6:1 compression and aluminum heads she runs 89 octane all the time. The timing is at 10 degrees BTD center. At 14* degrees it doesn't get any knocks or pings but it doesn't idle as nice as it does at 10*. It also idles faster at 14* than she likes. This is with 215* duration on the intake side but when the valves actually open and close in relation to each other (adjoining intake and exhaust events) comes into play. Anywaus this is what works on the street. Taking it to the strip I might bump it up to 14.Steve</blockquote>