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compression test
Posted: Wed Sep 22, 2004 8:11 am
by broook
Why do you have to hold the throttle wide open when doing a compression test?
I know you have to do it but I was just asked "why" and I couldn`t answer. Possibly to have the cylinders full of fuel?????
Volume
Posted: Wed Sep 22, 2004 10:07 am
by Tom Neimeyer
Bob,
I think it is done so the maximum amount of air can flow around the butterfly and fill the cylinder. If you restrict the air going to the cylinder, the compression (ratio) will be less because it does not have as much to compress.
It would be interesting to see it done both ways, throttle open and throttle closed.
Tom
Posted: Mon Sep 27, 2004 1:06 pm
by Passx
Brook, Tom, I've done it both ways and never seen a difference, but I'll also tell you I always check with the throttle cranked open. Also don't be to surprised if the cranking compression isn't the recommended 171psi. This seems to be a number that Honda pulled out of the air on all their bikes but none seem to be a real possibility, probably due to cam duration or lobe centers but they all seem to be in the 135-150psi range. It's more important that all the cylinders are within 15% (not sure of this number, look in the manual for sure) of each other .
Posted: Thu Sep 30, 2004 10:11 pm
by Mike Nixon
Yes, 170 seems to be a popular number with Honda, and few of the bikes actually reach it, not just the CBX. However, the CBX is one of a handful of Hondas that also does not seal its valves terribly well, and of course stretches its cam chains rather quickly. Both contribute to low compression readings. John Seltzer did a great piece on this phenomenon in the Tech Tip Chroniccles -- it's well worth the read.
