Cold vs warm compression test

I no the manual the says to do the compression test with a warm engine. But whats the difference?

The reason I ask is because I did a cold test. I wanted to see what shape the motor was in before I spent to much money on a bad engine.

These are the number I came up with.

1~ 85
2~ 85
3~ 90
4~ 90

This is on a 550 max. The low range of compression is 100 for the 550's. I'm assuming my number will go up with a warm engine? Just curious what the difference was.

Thanks
Jim

 

What happens when something is warm? it expands ,cold contracts ...your rings, valves all effect compression if a valve(s) are too tight this can lower compression because the valve is held off it's sealing seat a little bit. Have you checked valve clearance? When you did compression test did you have throttle wide open (not a big deal if carbs are off) Have you added a little oil to cylinder? One other thought and I'm not trying to insult you in anyway or offend you ,but I have to ask is your gauge a known good gauge ? I had a friend we were working on his mustang he thought is engine was toast compression was low 60 psi was highest , we put my gauge on it 130 psi ...mine was a made in the US of A Craftsman his was a Chinese Harbor freight special just for giggles he tried it on his truck 60 PSI . Any way that is just my thought on this ..Hope you can get better readings Good luck

 

cold vs warm is for comparison of numbers. so is adding oil after first check helps to ID possible problems. info overload has a good writeup on Identifing problems/ possible issues.
did you do a shim/valve check yet?
"Not only that, but remember that there is a moment when both valves are open (called overlap" in camshaft-speak), and if the intake valve is OPENING SOONER, and the exhaust valve is CLOSING LATER (both due to being too tight, i.e. not enough clearance), then this overlap time (measured in degrees of cam rotation) is going to increase = lost compression." (from info over load)
auto parts store may free rent tool but my store does not have the 12 mm adaptor.

550 engines:
Minimum: 100 psi
Standard: 121 psi
Maximum: 135 psi
Max. variance between lowest and highest: 14 psi

more from info
Now Analyze This!:
1) If one or all of your cylinders are too low in pressure, it means that:

a) you did the compression test incorrectly
b) the tester gauge is inaccurate, or was not used properly.
c) your engine has piston ring, piston wall, or valve leakage problems.
d) you valve clearances are too tight, and should be adjusted, or aftermarket camshafts have been fitted that have altered the camshaft timing.

read the info overload for more info ie head gasket warped head ect

 

Try a different gauge and perhaps a third gauge just to be sure it's not the measurement device. If you get similar readings on different gauges, then proceed with the advice of Jetfixer and XJ550H.

 

That was very insulting. What makes you think I got a cheap Chinese harbor freight speacial? Hahahaha! I did and I just went and rented another from the auto store. So we'll see if anything changes after I get off work. Thanks for the info!

Thanks for the reply. I sometimes forget that everything can be pretty much answer in the information overload.

I have not had time to check the valves. Im just about done with the carbs and happen to see the compression tester at hf so I said what the hell.

Valves are gonna check this weekend.

Thanks again for the info.

 

I don't know if I would trust the one from the auto store either. I rented one from O'Reilly Auto Parts and it was junk...not even close to accurate.

 

may show better results than a habor fright tester

 

Yeah, you're probably right.

 

Or just different results

 

Well this gauge does hold any pressure.
But the needle does go up to about 120.

Anyone no of a decent compression gauge?

Ive not had much luck with them lately.

Thanks

 

I bought this one from O'Reilly Auto Parts and really like it. It has a hard long reach adapter that quick connects directly to the gauge so you don't have to worry about screwing a hose into the spark plug holes. Just don't overtighten it.

http://www.oreillyauto.com/site/c/d...&keyword=bosch+compression&pt=N0717&ppt=C0374

 

Thanks.

This is the one I just bought.

http://m.harborfreight.com/quick-connect-compression-tester-95187.html

And it's the sameone I'll be taking back.

 

My craftsman gauge works great....think it was around 30$