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Old 08-14-2003, 05:25 PM   #1
mike270
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Default Compression at shutdown

When you turn your car off I would assume one of the cylinders would stop near maximum compression. If you leave your car alone for a while wouldn't that pressure cause a lot of wear to the rings or the piston? Or is the seal not tight enough to hold that pressure for more than a split second. Or maybe one of the valves opens to release this pressure as part of shutdown?

I hope this makes sense. Just a weird thought
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Old 08-14-2003, 07:53 PM   #2
IggDawg
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yeah, I don't think the rings make an airtight seal. It might hold the pressure for several seconds, but not much more.

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Old 08-14-2003, 11:47 PM   #3
mike270
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Yeah I kindof figured the seal wasn't air tight for more than it needed to be for ignition. If it were there would be a lot of problems with the rings if it didn't release that pressure somehow.

Also how many kpa or psi roughly is compression on the RS. I don't even know where to begin with a ballpark.
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Old 08-15-2003, 03:18 AM   #4
IggDawg
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depending on which bottom end you have (stage I is '98 and some '99, Stage II is 99.5+ IIRC), the CR is either 9.8:1 or 10:1. for example, a stage 2 bottom end has 10:1 compression ratio. atmospheric pressure is 16 PSi, so it will compress that 10x. ideally, when the combustion chamber is fully compressed, you'd have an absolute pressure of 160 PSi. when doing a compression test, this would only show up as 144 PSi since your gauge would have 16 PSi pushing on it because of the atmosphere. that's called "gauge pressure." whenever you see a boost gauge, vacuum gauge, or any gauge that measures pressure the readings are always in "gauge pressure" unless it's sealed and specifies "absolute pressure."

I think all those numbers are right. someone step in and correct me if I'm wrong.

-IggDawg
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Old 08-15-2003, 01:46 PM   #5
mike270
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That makes sense.

Thanks a lot for the responses. I think its pretty impressive that a car can last for 200,000 miles when the engine is building up 150 psi 2000-3000 times every minute
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