165 psi compression...couple questions. - MustangForums.com


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Old 11-16-2006, 05:52 PM   #1  
Twisted
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Default 165 psi compression...couple questions.

Sup fellas. Last night, I bunged a compression tester into one cylinder of my motor, just to see what it would read. It read 165...actually more like 167. I didn't have the correct short reach adapter, so I used one whose threads were exactly as long as the spark plug was from the bottom thread to the tip. My questions are:

1. How much higher might the test read because I used a longer adapter?
2. Depending on the first question, why might my compression be so high (aside from the "carbon theory.")? Thanks again.
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Old 11-16-2006, 05:54 PM   #2  
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Default RE: 165 psi compression...couple questions.

By the way, if anyone says anything even slightly unflattering, I'll cry like a little girl and leave the forum forever!
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Old 11-16-2006, 05:56 PM   #3  
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Default RE: 165 psi compression...couple questions.

The adaptor shouldn't make any difference and there would have to be an aweful lot of carbon to raise the compression. If they are all in line w/ each other, I wouldn't worry.
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Old 11-16-2006, 06:05 PM   #4  
Twisted
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Default RE: 165 psi compression...couple questions.

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The adaptor shouldn't make any difference and there would have to be an aweful lot of carbon to raise the compression. If they are all in line w/ each other, I wouldn't worry.
Thanks man. I've always thought that if the adapter was protruding into the cylinder, displacing more air, then the compression would read a bit higher. Am I silly for thinking that?

By the way, I'm not worried, I've just had this nagging suspicion that my motor may have been breathed-on slightly, due to a couple of clues. Also, I once tore into a bike that had compression like 15-20 psi higher than the factory spec, and found a Wiseco high compression piston kit. Just wondering.
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Old 11-16-2006, 06:32 PM   #5  
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Default RE: 165 psi compression...couple questions.

to get an accurate reading you have to have all the plugs out if you just pulled the one plug and crannked it over its going to change when you do it right..
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Old 11-16-2006, 06:40 PM   #6  
ZachW04Stang
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Default RE: 165 psi compression...couple questions.

From the factory it should be 180-190 so really that isn't that high.
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Old 11-16-2006, 10:32 PM   #7  
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Default RE: 165 psi compression...couple questions.

I'd test all 8 cylinders and see if they were all within 10% of each other.
Thats the important part of a compression test.....
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Old 11-16-2006, 11:05 PM   #8  
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Default RE: 165 psi compression...couple questions.

Thanks fellas. Hey Zach, thanks for the info. I had heard that the factory spec was 150, so that answers my question perfectly.

Mike, so you're saying that the compression reading for the cylinder that I tested will be different once the rest of the plugs are out of the motor? Interesting.

Fellas, I know that testing one cylinder tells me little to nothing; although if it had come up reading 100 psi, then I'd have been worried. I just did it because I had an extra few minutes to mess around in the garage. I wanted to pull a plug to see if it looked different from the last time I looked at it, and figured why not screw in the tester while it was out.

At any rate, thanks for helping answer my question.
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Old 11-17-2006, 01:13 AM   #9  
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Default RE: 165 psi compression...couple questions.

yes sir it will change..youve got to remember that with all 8 plugs out the engine is going to crank much much faster so you will see higher cylinder pressure..also tinman is 100% correct its not so much the number as being sure all 8 are within 10% of each other..if you have one that is way off then you go from there..
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