How do you convert compression to psi? like if I had 9.5:1 compression ratio what would it be in psi? I was going to go to onlineconversion.com but I dont know the units compression is mesured in, if there is one.
any ideas?
thanks
Sorry for the double post, can a mod just delete this one. If they dont get around to it just dont post on this one and let it fall to the bottom.
there is no direct comparison between static compression ratio and doing a cranking compression test. static compression is the ratio of the volume of the cylender at BDC (bottom dead center) to the volume at TDC (top dead center). a compression test on a gauge wil give you psi, and that is a test of your dynamic compression ratio.
Because of the definition of compression ratio, the compression ratio can be 100:1 and yet, with the piston rings missing, cylinder pressure at TDC will be near zero.
Cranking psi is affected by several things:
Piston stroke
valve timing
valve overlap
ring condition
valve seat condition
compression gauge responsiveness
Compression ratio is just that, a ratio. It's a fraction, like 375cc/ 41.7cc , which is the same as 9:1. The pressure developed can be measured in psi, in kPa (kiloPascals) or in any other unit of pressure.
What exactly are you trying to do?
-->Slow
I'm just curious what my compression ratio is in my car. (not the sunbird) the rings are gone and the lowest cylinder was 100psi and the compression ratio is 9.4:1 I was just curious how much lower it is than its susposed to be. I'm putting new rings in it this week but I jwas just curious how much compression it was loosing so I have some idea of how bad it is. I know its pretty bad though cause I'm getting so much pressure in the bottom end the oil pan gasket leaks pretty bad when its running, lol
your compression
ratio is the same as it was when the engine was built. Well, if the chambers are full of carbon or the tops of the pistons are burned away then it's changed.
But the point is that the compression ratio doesn't change just because the rings wear out.
The cylinder pressure does change though, and there is a way to get a rough idea how much pressure you might see during cranking if the rings are perfect. You need a compression gauge and a leakdown tester. The leakdown tester is used to fill the cylinder with air and to measure the amount that leaks past the rings and valves as a percentage. A good engine has less than 2% leakdown. A well used street engine can have something around 7-10% (iirc) and a tired engine will frequently top 10%. You can use the number obtained in the leakdown test along with a compression test to figure out the "like new" cranking pressure. Your compression test shows 100psi, and if you found 10% leakdown, then the total compression pressure would be around 110 psi with everything sealing correctly.
Easy, huh?
-->Slow
Your service manual should give you a cranking PSI range for the engine. As long as all of them are around the same, you should be fine, but no, you really don't know until you actually test it.
For example, I run 8.0:1 static compression, but my cranking PSI on all cylinders is between 184.5 and 185.5 psi.
Curtis
91' Turbo Z24
http://www.turboz24.com
i've got an 8.4:1 compression ratio on my v6, and get about 160 with a compression tester -- all plugs out and throttle WOT
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