® Where did the compression go?


The following is an exerpt from a popular forum leader's declaration that it's okay for your SOHC Honda to have low cylinder compression because he contends, it's not really low; it's just that Honda's published numbers are not real-world, being derived using a tool no one has access to.

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.

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.

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.

Exerpted from this August 31, 2008 forum post: Compression test numbers, yours vs. book value


In typically inane forum fashion, sohc4.net extemporaneously proposes a fatuous concept, then ignores what the industry has known for almost sixty years. Telling the reader that his engine tests low on compression because Honda's published numbers are derived using a tool no one has access to is ignorant. That is not why the test results are low. They are low because Honda used soft valves for about a twenty-year period. The result is that by 10,000-15,000 miles you can expect an almost 25 percent loss in cylinder compression in virtually any Honda made between about 1965 through about 1985. This includes the singles, twins, and of course the fours. The V4s introduced in 1982 and having improved valves are an exception, as are the Gold Wing introduced in 1975 and having soft valve seats (which slowed valve wear). Every experienced Honda tech knows about this issue, having experienced it innumerable times in the dealership. It's called extreme valve recession, and even a Honda service bulletin, Service Letter #84, a 1971 publication, indirectly explains the reason for it. Somehow this fact most basic to vintage Hondas and so crucial to their ownership has escaped the sohc4 forum "experts". And it's funny: these folks will absolutely lose their minds if one day they are forced to accept it, so much do they hate everything associated with the Honda factory. And don't get me started on that "should be within 10% of each other" business! Ack!


Suggested further reading on this site:
Reasons for low compression
The truth about cylinder compression
Forum backwardness
Lies from the ether
Real issues
Forums
sohc4.net
Don't touch
Powersports orphans
Forums and me
They hate Honda
Valve recession


Last updated October 2022
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