From Two Tired:
So you did a compression check and the numbers were lower than book value. Does it need an overhaul?
First question you should ask, is “Does my test equipment lie to me?”
Even if you assume the gauge is accurate (Hey, it’s brand new!), the means of connection can alter the readings.
We’ll make two examples A 350 Four and a 750 Four.
The 350 is actually 347cc displacement, making each piston move 86.75cc. It has 9.3 compression ratio, making the combustion chamber volume 9.327cc.
My compression tester, purchased at the local auto parts store (see pic), has a long, fat hose on it. The volume of this hose I measured as 11.5cc. There is also the volume of the gauge itself. But, I chickened out pouring alcohol in that for a volume measurement. I also didn’t account for any rubber hose expansion incurred from higher pressures within the hose, increasing its volume capacity.
Anyway, the volume of test apparatus adds to the chamber volume during compression, because its volume has to be compressed to deflect the gauge needle. So, instead of a 9.237cc combustion chamber, you’d have a 20.737cc total chamber with my compression tester, which effectively lowers the cylinder’s compression ratio from 9:1 to 4.1:1.
What of the 750? 736cc/4 is 184 cc piston displacement per cylinder. Many (not all) 750s are 9:1 compression ratio, leaving a combustion chamber volume of 20.44cc. Adding a test apparatus volume of 11.5cc yields 31.95cc total volume, which changes the actual CR to 5.76:1.
Astute readers will note that the larger the displacement of the engine, the less effect test equipment apparatus volume has on the compression ratio, and the measurement numbers. If you bought your tester at an AUTO parts house, like I did. They expect your 350 to be cubic INCHES not cc. The 350 in the auto world, is 5700cc. Or, 16 times larger in displacement. (Only 7 times of you have 750.) …Now you know.
So, why are Honda published numbers so much higher?
Looking in the Honda 500/550 shop manual, there is picture of someone doing a compression test. The apparatus has a very long, thin, rigid metal tube between the gauge head and the spark plug hole. Clearly this small tube apparatus was used so as NOT to add very much volume to the combustion chamber, which will result in a much closer representation of actual chamber pressures. I don’t have such a gauge. Therefore, I don’t know what the actual volume is added by the Honda test apparatus.
So, unless you have a test gauge like Honda or one that adds little volume to the chambers, your test result numbers will be lower than book values.
The numbers can still be meaningful even if they are lower. All cylinders should still be within 10% of each other. And, not significantly higher after adding a few drops of oil to the cylinder for a wet/dry comparison. Don’t add too much. Oil is not compressible, you just want enough to wet the piston rings to aid their seal onto the cylinder walls. Good rings won’t be affected by adding oil. Leaky rings will be.
Note, adding oil volume increases the compression ratio by removing compressible space. A few drops is plenty for small displacement engines. If your combustion chamber is 9cc, adding a teaspoon of oil, lowers the compressible volume 5cc! So, be careful that your noted rise in measured pressure is not due to too much added oil volume, rather than sealing off leaky rings.
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