Hot compression test

[dave76]

Went riding yesterday and did a compression test the right way after it was hot with the throttle wide open. Before I did it cold about 40°F with the throttle close and only achieved about 155 PSI. Well yeasterday it was about 80°F and had the motor hot after riding and got a compression of 230 PSI, did it three times just to confirm I was getting a consistant reading and was spot on every time.

Hear is what the plug looks like after some nice tuning and a full day of riding.

 

[dave76]

If the factory manuel is right, then 120 PSI is pretty weak.

 

[jasonp]

if you got 155psi cold I can only amagin what mine would be hot when cold I had 175psi. Maybe this is why it doesn't like to start and runs better with less timing.

 

[dave76]

if you got 155psi cold I can only amagin what mine would be hot when cold I had 175psi. Maybe this is why it doesn't like to start and runs better with less timing.

Yeah was a bit surprized myself, this explains why it likes the 110 in the summer. Dosn't ping but runs better with race gas, as it cools better.

So yeah, check it out man you and I got the same cam so were bleeding off the about same amount of cylinder pressure. I might of had higher cold numbers but didn't know you had to hold the throttle open until you told me about it.

Anyway if you look at that plug, it's running awsome.

 

[courchaine]

god damn ****....I'm getting like Jasonp 175 PSI cold....but never add a problem starting it or anything

 

[jasonp]

if you got 155psi cold I can only amagin what mine would be hot when cold I had 175psi. Maybe this is why it doesn't like to start and runs better with less timing.

Yeah was a bit surprized myself, this explains why it likes the 110 in the summer. Dosn't ping but runs better with race gas, as it cools better.

So yeah, check it out man you and I got the same cam so were bleeding off the about same amount of cylinder pressure. I might of had higher cold numbers but didn't know you had to hold the throttle open until you told me about it.

Anyway if you look at that plug, it's running awsome.

Yeah holding the throttle open with a big cam effects the compression greatly. I think I had maybe 145 or something not holding the throttle wide open and it took a while to build compression. I just wanted to try it and see. I maybe had less or more I can't remember... I know though when I held the throttle wide open it jumped big time. I'll have to do a hot compression check sometime just to see.
I know for a fact I will be running less timing and for starting I'm going to take like 4 degrees out of it. Seems to run better with less timing.

 

[jasonp]

great looking plug. Jetting looks to be right on or very very close to the correct mixture. Mine seems not to color the plug at all when runing perfect.

 

[griff]

What is it you guys are trying to accomplish exactly?

Theres some 'debate' on cold and hot comp' tests. A Leak-down tends to be better for diagnostic purposes (usually).

 

[dave76]

Explain a little more griff?? I was under the assumption that as an engine heats the rings will seal better as the piston will expand, especially the new lite weight forgings like JE uses.

I've never done a leak down test, know they work but don't understand how??? So I'll assume a simple compression tester will not work??

Were just trying to accomplish cranking compression.

 

[jasonp]

What is it you guys are trying to accomplish exactly?

Theres some 'debate' on cold and hot comp' tests. A Leak-down tends to be better for diagnostic purposes (usually).

Compression pressure can be used to find compression ratio.

 

[griff]

Compression pressure can be used to find compression ratio.

Heh, yes it can

Heres a little experiment for you guys to try, now that some of you know how to do it properly

Take a cold reading before any riding.

Go ride the piss out of it, let it cool down for about 10 minutes, then do another comp' test, and post the respective numbers.

 

[griff]

Explain a little more griff?? I was under the assumption that as an engine heats the rings will seal better as the piston will expand, especially the new lite weight forgings like JE uses.

I've never done a leak down test, know they work but don't understand how??? So I'll assume a simple compression tester will not work??

A leak down test is really diagnostic only. You obviously need a leak down tester and a good air compressor. A leak down test will tell you a few things... Valve seat/seal integrity, ring seal integrity, even gasket integrity, etc. It must be done COLD.

With the cam(s) pulled (so that all valves are closed), leave the crank-case breather and/or oil cap open.

You pressurize the cylinder, and the gauge tells you at what % the air is escaping the cylinder (either past the rings or the valves). Depending on the engine, 5% to 15% is typical, and A-OK.

You'll be able to hear the air escaping, if youre gushing air (say more than 20% on the gauge), you can track it down to a specific cylinder (not a problem on our engine, lol), or specific valve(s). A stethoscope is pretty handy for this on multi-cylinder engines. Its real handy for telling if its a valve seat/seal problem, or the rings.

A compression test cant really tell you any of that specifically.

Hope that makes sense...

 

[dave76]

After reading a bit on cranking compression, with the megacycle 252X1 cam having 64° of overlap with a pretty late closing intake valve and my cracking compression reading 230 PSI the static compression is like 12.1 to 12.5:1.