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Old 05-18-2003, 09:00 PM
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Post highest compression on pump gas??

I would like some feedback on how high of compression you can go and still run pump gas. I was running 10.2 on my motor. It had 900 miles on it and I decided to change a few things. When I tore it down it showed no signs of detonation. My elevation is 5000 feet..... Thanks for the info.......

Let me add that I am happy with the 10.2 compression. I was just wondering if I could up the compression with out running into any detonation problems on pump gas.......

[ May 18, 2003: Message edited by: showtime ]</p>

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Old 05-18-2003, 10:25 PM
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it isnt what static compression ratio you are running, its the dynamic compression. is you were to run 13:1 compression and one of those old mileage master cams designed to work with hich CR you could get away with 91 octane, but if you swapped in a cam that allowed better filling of the cylinder, your dynamic compression ratio (which can be estimatad as WOT cranking cylinder pressure) would skyrocked and you would have dedtonation at part throttle on 95 octane, so as with so many other questions in this forum, the best answer i can give you is: It Depends...
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Old 05-18-2003, 10:30 PM
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Deuce if I give you my engine specifics would that help??
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Old 05-19-2003, 10:00 AM
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Not sure,but I think dynamic compression is best around 8.5:1 or so.
Like he said though depends.The bigger the cam the more bleed off of cylinder pressure.So it depends on what size cam as compared to what compretion ratio.Also timing is a big part of things too.For my own peice of mind I usualy consider 9.5:1 max for an average mild street motor.
Also if your pistons are rated at 10.2 it doesn't nessasarily mean you have 10.2:1 it depends on your head gasket thickness and deck hieght as compared to what the factory used for figureing there rating.You may actualy be lower like 9.5 or 9.6:1
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Old 05-19-2003, 11:12 AM
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cylinder pressure is what determines what kind of gas you can run, which if you think about it makes the most sence. the air fuel mixture has to be able to withstand a given PRESSURE before it detonates. This is the exact same as dynamic compression so everyone else has been right on. Alot depends on your altitude, timing, as well as your cylinder pressure.

Unfortunately there is no formula that i have come across that will be able to tell you what will work and what won't.
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Old 05-20-2003, 09:30 AM
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yeah you would have to be alot smarter than me to make your car run any higher compression. the guys in the phr engine masters challange had alot of engines with 13:1 and higher that all ran on pump gas. you're on the right track thinking about static and dynamic compression tho (i think)
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Old 05-20-2003, 09:05 PM
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Interesting info I found:

http://www.zianet.com/SCCoSW/Articles/Elevation.HTML

Ken
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