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Old 04-09-2008, 09:05 PM
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compression ratio?

does anyone know if it is possible to find out the compression ratio of an engine that is already finished? I really don't want to have to take it apart, if I don't have to.

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Old 04-09-2008, 09:13 PM
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yeh you can check it with a gauge
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Old 04-09-2008, 09:43 PM
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Hi,
Is the engine installed or is it
on a stand?
Rich
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Old 04-09-2008, 09:44 PM
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thanks! some other threads popped up when I posted this and one had a chart to convert cylinder pressure to compression ratio. I'm going to put it here to help others.
Compression ratio table from cylinder pressure:

8>1 = 102.9 psi
8.5>1= 102.9 - 110.25 psi
9>1= 110.25 - 117.6 psi
9.5>1= 117.6 - 124.95 psi
10>1= 124.95 - 132.65 psi
10.5>1= 132.3 - 139.65 psi
11>1= 139.65 - 147 psi
11.5>1= 147 - 154.35 psi
12>1= 154.35 - 161.7 psi

the engine should be warmed up first. this is suppose to be close.
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Old 04-09-2008, 09:51 PM
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Quote:
Originally Posted by richard stewart 3rd
Hi,
Is the engine installed or is it
on a stand?
Rich
it's in the car and running about three years now. the reason that I want to figure ratio is because I want to run it on E85 fuel and they say that it works better if you have 11>1 or 12>1 compression ratio.
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Old 04-09-2008, 10:01 PM
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Quote:
Originally Posted by 47coupelg
thanks! some other threads popped up when I posted this and one had a chart to convert cylinder pressure to compression ratio. I'm going to put it here to help others.
Compression ratio table from cylinder pressure:

8>1 = 102.9 psi
8.5>1= 102.9 - 110.25 psi
9>1= 110.25 - 117.6 psi
9.5>1= 117.6 - 124.95 psi
10>1= 124.95 - 132.65 psi
10.5>1= 132.3 - 139.65 psi
11>1= 139.65 - 147 psi
11.5>1= 147 - 154.35 psi
12>1= 154.35 - 161.7 psi

the engine should be warmed up first. this is suppose to be close.
This gauge works well for... well I'm not sure... My suggestion would be to find the cranking compression with the throttle open, and the cam timing from the moment your valve leaves the seat and returns and use that to back calculate your static compression... Though static compression is a fairly useless number compared to dynamic compression and even plain ol cranking compression is a better guide. As a rule 150psi cranking compression maximum for cheap pump gas, 180-190 for premium and about 220 for ethanol.
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Old 04-10-2008, 12:41 PM
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Quote:
Originally Posted by 47coupelg
thanks! some other threads popped up when I posted this and one had a chart to convert cylinder pressure to compression ratio. I'm going to put it here to help others.
Compression ratio table from cylinder pressure:

8>1 = 102.9 psi
8.5>1= 102.9 - 110.25 psi
9>1= 110.25 - 117.6 psi
9.5>1= 117.6 - 124.95 psi
10>1= 124.95 - 132.65 psi
10.5>1= 132.3 - 139.65 psi
11>1= 139.65 - 147 psi
11.5>1= 147 - 154.35 psi
12>1= 154.35 - 161.7 psi

the engine should be warmed up first. this is suppose to be close.
I had an ~8.5:1 motor with a smallish cam (267 adv, 210/216 @ .050) that would crank at 150+ psi
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Old 04-10-2008, 12:48 PM
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yeah... I'm a little skeptical about this chart. My 79 Scout with 300k miles cranks 130 psi, and I know for a fact that it is 7.8:1 because I built it myself. The cam timing is more responsible for cranking pressure than compression is.
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Old 04-11-2008, 09:10 AM
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The "compression ratio" air gauge that dirt track tech officials use to find "cheaters" in dirt classes that restrict the compression ratio eg: 9:1 class.
Do not really tell the engines cr. All they need to do is look for a higher than average cranking compression reading and tear that guy's motor down. A very good chance his motor is illegal. They all run near the same stuff and any one who runs a small duration cam that would create a higher reading, without excessive static cr, would not be leading the pack anyways. (cause he won't be making enough HP) They only "tear down" the winners. So the cheaters are relatively easy to catch. There is a CID check air gauge thou that can tell the CID of one cylinder.
"Nutin' stock about a stock car"
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Old 04-11-2008, 09:25 AM
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http://www.katechengines.com/corpora...s/whistler.php

This is the piece of equipment that Wissota uses to verify that the B Mod engines are 9.5:1 it does fancy calculations based upon the amount of air in the cylinder and the pressure that it creates using that air.

It is very accurate and will only cost you $2000 bucks!!! However it seems that the static compression calculators online give a falsely high reading compared to it so if you calculate 9.5:1 it might only read 9.3:1. Which can be frustrating when you spent the money on all the fancy machining to get the motor at 9.5:1 when you could have gone 9.7:1 instead. But if you are even 9.51:1 then you forfeit all your points...so better safe than sorry.
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Old 04-11-2008, 10:07 AM
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Sorry, but that chart doesn't work. Cam timing has a huge effect on cranking pressure. All my engines have about the same cranking pressure and the cr's range from 8.5:1 to 11:1.

do a compression test. ap72 has it right.
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