Compression ratio at sea level vs Denver

I had a conversation today with a member of the Early Mustang Club here in Colorado. He said that he's running 10.5:1 compression and is running 85 octane without a whimper from the motor. He said it was due to the altitude. Basically saying that, yes, the motor may be 10.5:1 @ sea level, but... take it up a mile high dropping the air pressure, and having less O2 in the air also, drops the equivalent compression to 9.5:1. Thus the ability to use regular pump gas. Truth or BS? Judges??

Scott Williams '66 High Country Special

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Scott Williams
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Reply to
Michael Johnson, PE

compression ratio is a calculated figure of total cylinder/chamber volume to the compressed volume. the difference in volume from bottom dead center to top dead center basically. it doesn't change!! compression pressure on the other hand does. that's where the difference is. Chip

Reply to
Chip Stein

That's why I used the terms static and dynamic compression. The calculated ratio using volumes does not change. Most refer to effective CR to descibe how the engine responds under differing atmospheric pressures due to high altitude or use of forced induction. At 4,500+ feet the effect is like lowing the CR 1 point at sea level.

Reply to
Michael Johnson, PE

So my 10:1 302 is effectively only a 9:1 here in Denver? Scott

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Scott Williams

Reply to
Michael Johnson, PE

Cool, thanks alot.

Scott

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Scott Williams

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