Compression Question

[chriskinzel]

I just changes spark plugs and did a compression test while I was there. My compression ranged from 165 to 175. Is that OK for a non-modified engine. It seems a little low to me.

Thanks
Chris

[mike100]

I'd expect 205-210, but it does make a difference on how many times you crank it. It may also make a difference if the engine hasn't run recently- put a little heat in it.

On most engines, you will see 90 psi on the very first compression stroke/ needle swing. On the LT5, it should be even more. I'd say warm it up a little, pull the plugs, and then crank it 4-5 times watching the needle rise on the gauge. All cylinders should behave the same- especially on the first rise of the needle.

watching and comparing the initial compression rise is like a poor man's leakdown test- easy to see the weak cylinder if there is one.

[LGAFF]

Seems low...could be the tester? The #s are consistent...does it seem down on power? Most run 190+ I believe, I think my 90 rebuilt was 225 or so

[chriskinzel]

I did the test last year when I changed plugs and got the same result. The engine was cold when I have done this. It's had this much power since I bought it 3 years ago. I am not sure, but I think it's a little week. I have only drove my one ZR-1 so I have nothing to compare it to.
Chris

[LGAFF]

Might want to do a leakdown test

[Franke]

You may not get the kind of compression the guys at sea level get if you are at altitude like me. I'm 5500 feet above sea level and the compression is lower. What is your altitude there in SLC?

[Dynomite]

All spark plugs removed, fully charged battery.........with starter hold in start position for at least three Compression strokes on the cylinder being tested.....(keep three compression strokes as the test basis for all 8 cylinders).

The following Compression tests where accomplished with a cold engine and foot off throttle.

Got 210-215 lbs on 90' with 68K miles.
Got 230 lbs on a 90' with 38K miles (just last week).

Of significance is that the compression of each cylinder does not vary more than plus or minus 5 lbs.

You may not get the kind of compression the guys at sea level get if you are at altitude like me. I'm 5500 feet above sea level and the compression is lower. What is your altitude there in SLC?

Compression Test Altitude Compensation Factors Colorado Evolution Club (http://www.coloradoevo.com/forums/showthread.php?t=4683)

Alt Factor
0500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

At 4500 feet a compression test of 215 lbs (sea level) would be .88 x 215 or 189 lbs.
At 5500 feet a compression test of 210 lbs (sea level) would be .85 x 210 or 178 lbs. Which is close to your top compression.

[Bearly Flying]

" I have only drove my one ZR-1 so I have nothing to compare it to."


Well, therein lye's the problem, buy another Z to compare to......


Pretty simple to figure that problem out......lol....

[chriskinzel]

My altitude is about 5000 feet. I keep hoping I find a rich relative that has left me a ton of $$$. so far, not luck.:p I guess I am not that far off the mark. I will go to the auto parts store and rent one of the compression testers and see what I get on a few of the easy to get to cylinders. If its close I guess it is what it is. Time to save lunch money to give to Marc.

Thanks for all the help!
Chris

[Hog]

Ive always held the throttle at WOT., keeps the injectors from firing on most ECM/PCM's.

[chriskinzel]

I have a jumper switch that I use. Never touch the key. No power to fuel pump, ECM or anything else.

Chris

P.S. This is because I have a 'No Start' solenoid that is mounted by the battery. Connection it right there.

[WARP TEN]

Using Dynomite's altitude conversion chart, at 5,000 feet it looks like you are fairly close to normal, about 195 to 205 or so. -- Bob

[scottfab]

I'd expect 205-210, but it does make a difference on how many times you crank it. It may also make a difference if the engine hasn't run recently- put a little heat in it.

On most engines, you will see 90 psi on the very first compression stroke/ needle swing. On the LT5, it should be even more. I'd say warm it up a little, pull the plugs, and then crank it 4-5 times watching the needle rise on the gauge. All cylinders should behave the same- especially on the first rise of the needle.

watching and comparing the initial compression rise is like a poor man's leakdown test- easy to see the weak cylinder if there is one.

Right on all counts:
1 at least ~210psi on all +/- 10
2 ~90psi on first stroke is critical
3 watching and comparing compression rise on each stroke.

I'll add:
warm engine
forget altitude unless you're >5000ft
use 5 stokes
pull fuel pump fuses (both) and relieve fuel rail pressure.
and run each cylinder twice consecutively.