Engine compression rating = cilinder lb compression

• posted on April 8, 2006, 12:55 pm
Ive googled and can`t find my manuals for this. What is a new motors cilinder lb. compression equaling a 9-41 or 10-1 compression rating, for a standard new regular octane engine. I thought 9-41 was aproximately 145-155lb. Do new small 4 cilinder motors run near 200lb?

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• posted on April 8, 2006, 1:33 pm
most new gas engines are 125 to 150 pounds of compression.
http://www.minibite.com/america/malone.htm

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• posted on April 8, 2006, 2:25 pm
Ds, alot of motors are 9.41 rating and regular gas, I believe. Computers retard timing when needed to prevent knock, modern engine management is allowing higher compression ratings for regular gas.
What lb. pressure would 9.41 equal, im sure its a simple formula.

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• posted on April 8, 2006, 1:34 pm
compression ratio on new engines is 8 to one or less to burn regular octane gas,..
http://www.minibite.com/america/malone.htm

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• posted on April 8, 2006, 5:49 pm

Not entirely true. A lot of engines are over 9.5:1 these days.
The reason for this is aluminum heads - you can run several points more compression and not worry about spark knock.
-CF

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• posted on April 8, 2006, 5:51 pm

Actually, I know guys that run 11:1 compression with aluminum heads - and run 93 octane no problem.
-CF

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• posted on April 8, 2006, 7:24 pm
OK but what compression rating equals what lb. a factory new 9.5-1 equals how many pounds pressure with a compression tester. It has to be a simple formula.

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• posted on April 8, 2006, 8:31 pm
In article <12433-44380DE0-234@storefull-

PV=nRT ?

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• posted on April 9, 2006, 4:52 am
MikeP wrote:

Assume nRT remains constant (actually they don't because compression increases T which increases the pressure). So P1V1 = P2V2 or solved for V2(P1/P2)x V1 . Actually you just assume the normal pressure is 15psi at sea level so just multiply the 15 psi by the volume change ( in this case 9.5). That would be absolute so on subtract one for the actual pressure gage.

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• posted on April 8, 2006, 10:11 pm
9.5 atmospheres to 1. 1 atmosphere = ~14 psi. 9.5 X 14 = 133 psi.
Having said that, all cylinders should be within 10 psi of each other.
If the highest pressure is 120 psi all other cylinders should be no less than ~110 psi.
If all are within range, compression is usually AOK.
-- PDQ
--
| OK but what compression rating equals what lb. a factory new 9.5-1 | equals how many pounds pressure with a compression tester. It has to be | a simple formula. |

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• posted on April 8, 2006, 11:14 pm
PDQ, that makes sence, thanks.

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• posted on April 9, 2006, 12:16 am
Just a guess but doesn't 9.5 to 1 mean you are compressing 1 lb to be 9.5? So if air pressure at sea level is 14.6 than compressed 9.5 it would be about 139.

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• posted on April 9, 2006, 1:00 am

The compression ratio is by volume. If there are no leaks and the valves are closed when the compression starts , then it will be 14.7 (air pressure) times the ratio.

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• posted on April 8, 2006, 5:15 pm
hi, i rebuild several engines a year,and have been sure to keep them 8 to 1 or less . i knowsome engines from the factory are more,and show 165 pounds compression.but the way i build them,they run cooler and spark knock less.. best regards, lucas
http://www.minibite.com/america/malone.htm