What kind of compression should I get from a 1974 twin carb 850 Commando? The valves do not leak and the cylinder is re-bored and new pistons. When the camshaft was "timed correctly" by the book it only had 80-90 psi which I thought was low for a rebuilt engine. I then timed the cam how I thought it needed to be and the compression was 180 psi which I thought was a little high. So what should the compression actually be?
What compression do I need?
- Thread starter zcarman
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It's more important that the cylinders are even say 120/120 would be normal. A fresh build broke in I would think would be around 150/150.
Yes i think 180 per side is high. High comrpession head and pistons may produce this. You may want to get a fresh head gasket for the shelf. Remember we are not speaking of compression ratio here, but cylinder pressure.
Low or high, you want them even.
Yes i think 180 per side is high. High comrpession head and pistons may produce this. You may want to get a fresh head gasket for the shelf. Remember we are not speaking of compression ratio here, but cylinder pressure.
Low or high, you want them even.
I got an indicated 155/170 on mine (73 850, hot engine, WOT) BEFORE replacing rings. Haven't retested since but the engine is notably torquier/crisper and the exhausts have a "POP" to them (you can feel it with your hand held a foot or two behind the exhausts) that they didn't previously.
Dunno if my gauge is 100% accurate, but there's a data point. As pvisseriii points out, what's important is less the absolute values than that they be comparable - within 10% is a common benchmark but closer the better.
Best of luck.
Dunno if my gauge is 100% accurate, but there's a data point. As pvisseriii points out, what's important is less the absolute values than that they be comparable - within 10% is a common benchmark but closer the better.
Best of luck.
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Don't know what your method is, but ...
Open the trottle and kick over several times till the needle stops moving.
One or two kicks usually isn't enough to get an acurate reading.
Open the trottle and kick over several times till the needle stops moving.
One or two kicks usually isn't enough to get an acurate reading.
I was getting circa 90's on a 750 rebuild.
Then it was pointed out to me that the throttle needs to be held wide open to do this check, which I did not previously know or do.
Opened the throttle and rechecked, these pistons ( see below ), combat head and standard cam ( I believe, could not find any markings on the cam ), a lovely consistent 152 PSI per side.
Then it was pointed out to me that the throttle needs to be held wide open to do this check, which I did not previously know or do.
Opened the throttle and rechecked, these pistons ( see below ), combat head and standard cam ( I believe, could not find any markings on the cam ), a lovely consistent 152 PSI per side.
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My combat hits 180ish.
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Would it not be better to do the cam time correctly, rather than setting to what you think it should be?
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Unless you have a calibrated compression tester don't bother with the numbers (if they are reasonable) and go for even between the two cylinders.
You don't say what change you made to the cam timing but I'd guess that you advanced the cam. Do you know how much you advanced it? You will get a higher reading on the meter because the valve closes closer to BDC that way. Standard timing has the valve close with the piston quite a ways on the way up, so effective CR is much less than what it would be if the valve closes a BDC. It's a proven fact though that when it comes to moving air in a running engine, it's the way to go.
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check this site for compression ratio versus cranking compression...pretty interesting.
http://www.not2fast.com/turbo/compressi ... sure.shtml
http://www.not2fast.com/turbo/compressi ... sure.shtml
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