Compression difference on different side?

My Limit

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May 19, 2005
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37
I just did a compression test on my 1998 Volvo 5.7GS with 120 original hours. The numbers on the starboard side were all about 194. The numbers on the port side were all about 190. <br /><br />Why the consistant difference on opposing heads?<br /><br />My friend speculated that one head was machined slightly different than the other where it connects to the heat exchanger. He said a micro difference in measurements would make the numbers slightly off. <br /><br />I know these numbers are great, but I'm curious about the slight difference from one side to the next.<br /><br />Anybody ever notice that?<br /><br />Thanks,<br /><br />ML
 

f_inscreenname

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Aug 23, 2001
Messages
2,591
Re: Compression difference on different side?

A thicker gasket, a tiny bit milled off the block deck or head. There is to just many variables why it is what it is. Just be happy with what you have.
 

My Limit

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Re: Compression difference on different side?

I am very happy about the motor. I am just a very detail oriented person who likes to learn as much as I can about my motor. <br /><br />I'm guessing my buddy was right about the micro difference on the machining of the heads.<br /><br />Thanks,<br /><br />ML
 

ron7000

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Re: Compression difference on different side?

you have a compression gauge with resolution to 1 psi?
 

whywhyzed

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Re: Compression difference on different side?

that's what I was thinking... Mine goes in increments of 10.
 

My Limit

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Re: Compression difference on different side?

No, It's a standard Snap On compression test kit. It is in 10's. If the needle is exactly between 10 I would estimate it's indicating 5. So, I can't be precise on the numbers. I'm just doing the best I can and being consistant with my interpretation of the numbers for each cylinder. If it reads just a a millimeter below 190 I call it 189. If it's a millimeter above I call it 191.
 

ron7000

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Re: Compression difference on different side?

not to be a **** but you mentioned above<br />"very detail oriented person who likes to learn as much as I can about my motor"<br />which is great but taking it to this exteme is more detrimental than good. If the gauge doesn't have close to that resolution you can't infer that much more from it. Maybe if it's in increments of 10 you can pull a 5 (half) out of it, but that's it. On top of that, the gauge only has so much accuracy so a difference of probably 10-20 psi on these should be considered irrelevant anyway, even if it's a snap on gauge. Speculating the heads are machined different is nothing more than speculation. Bottom line is a compression test shows your motor to be 100%, provided the gauge is accurate. You did have it calibrated before you used it? you are a very detailed oriented person, aren't you? just busting your chops :D
 

My Limit

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Joined
May 19, 2005
Messages
37
Re: Compression difference on different side?

Ron7000,<br /><br />I'm not a master mechanic by any means like yourself. I only work on motors for fun because I like learning about them from guys who are willing to teach and because my job is stressful.<br /><br />As far as the number thing. You admit I can figure out when the gauge is halfway between 10 and 20. So you admit I can read a 185 reading. Why then can't I use the same logic and spacial ability to measure the distance between the 10 psi and figure out "about" where 1-9 would be. I think you just said a difference of 10-20 PSI should "be irrelevant". <br /><br />Have you ever owned a clock or watch without numbers? Could you figure out what time it was? <br /><br />If it's irrelevant I can say 190, 191, 192 and it's all the same. Why worry about that little of a difference right? <br /><br />The reason I noticed the difference was there was an obvious and consistant difference between sides of the motor. I thought it was interesting and was hoping and expert could shed some light on the subject. <br /><br />I also learned a lot on the compression test process from Don and others on this site who were willing to lend advice. I plan on doing the test again properly to see what my results are. I did not know I needed to calibrate my compression gauge. I will investigate that further. <br /><br />I don't mind a little chop busting as long as it can go both ways. : ) I'm glad we can all have fun while learning and teaching.<br /><br />ML
 
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