Page 1 of 1
Compression
Posted: Sat Dec 11, 04 3:58 pm
by Anonymous
Hi all,
Just compression tested my motor and thought I'd put the results up for your views.
All readings are in PSI, I cannot guarantee the accuracy of the gauge and the readings are somewhat approximations.
1 - 155
2 - 170
3 - 155
4 - 160
5 - 165
6 - 155
7 - 155
8 - 160
Any flares going up there?
Posted: Sat Dec 11, 04 4:14 pm
by Alex
All readings are in PSI, I cannot guarantee the accuracy of the gauge and the readings are somewhat approximations.
I think the readings are meaningless
you can't guess something like this, get a good guage and read it properly.
If they were accurate I would be pulling the heads and looking at the seats.
Posted: Sat Dec 11, 04 6:10 pm
by Dave-R
And are you holding the carb wide open when you are cranking it?
Posted: Sat Dec 11, 04 7:39 pm
by Anonymous
Actually Dave no I wasn't
The gauge has a guide table on it and suggests that 160 is correct for a 9:1 compression. Is it telling porkys?
To be fair it is just a cheap gauge that you press and hold in the plug hole, I got it from a mate with a bunch of other bits for a fiver.
I am wondering about the condition of the seats since there is an intermittent poping on one of the banks at idle and I have been thrashing it on unleaded fuel.
Posted: Sat Dec 11, 04 9:45 pm
by MrNorm
Def need to hold it at WOT.
If those numbers remain similar, then not great but OK I would have thought. All within 10% of each other, the usual rule of thumb, and not horribly low......but do it at WOT.
As long as the guage is repeatable (most important) and accurate within 10psi I wouldn't worry too much - and I would have thought even a cheap guage should be easily that accurate.
This is for a 'driver', not a race engine. If you do have a weak cylinder(s) try the 'oil in cylinder' and see if the readings go up - if not it's prob a seat.
Or use a leakdown tester - should be relatively easily made with a spark plug and an air fitting
Posted: Sun Dec 12, 04 11:31 am
by Dave-R
Actual cylinder pressure depends not only on static compression ratio but also valve timing and even rpm.
Years ago I had similar results to yours above on an engine. Then I remembered I should have opened the carb so I did it again. The result of that was not only much more pressure but the readings were closer to each other. So try it again. Like Gavin says it is not important that the gauge be very accurate. It is the difference between cylinders that matters. A very small amount of oil will show you if sealing the rings makes a big difference or not.
Posted: Sun Dec 12, 04 2:14 pm
by Guy
mmmmmm mine are lower than that mind you I have only got 8.2-1 compression and a 284 duration cam.
