Cranking compression variances.

[Batmanntexas]

o, during my trip to Camaro Fest, I felt the car was down on power a little bit. I drove it back home and let it sit for a few days before I got around to looking at it.

Today, I performed a compression check and got the following readings:

CYL #1 = 190psi CYL #2 = 175psi
CYL #3 = 200psi CYL #4 = 185psi
CYL #5 = 175psi CYL #6 = 175psi
CYL #7 = 170psi CYL #8 = 200psi

Engine specs are:
Stock rotating assembly (4.065" x 3.622")
Custom Cam (227/243 115LSA 110 ICL)
ported & decked stock heads (can't remember how much was cut, maybe .020")
LS9 head gaskets

Using some online calculators (using stock chamber volume, 70cc), the calculator indicated I should have around 190psi gauge pressure, with about 210psi actual cranking pressure.

Other than 'feeling' down on power, the car doesn't exhibit any symptoms, i.e. doesn't smoke, consume oil, run hot, etc.

What do you guys think?

-Scott

[bayarea408]

If it was me id pop the valve covers and closely inspect all the springs. When its cold in the morning open the radiator cap take a look at the coolant see if you see oil.

[silversleeper]

Suspect rings going bad. Do the old school test of squirting in some oil in a low compression cyl. #7 would be good as it's lowest but probably harder to get to than #2 which is similar. Squirt in enough but not so much as to hydro lock it. Like about less than a teaspoon of oil. Crank it over and see if it jumps way up to like 190psi. Then you know it's rings.
You can either tear it all down and replace them, or run it as it is. If I did run it like that I'd make damn sure I had a good valved breather on the crankcase. IDK if the Metco Valved , valvcover breather could flow enough for that many cyl blowby under boost but it'd be the minimum thing I'd do. Or you'll blow out the rear seal and spin your bearings like our Arkansas bro who's doing a rebuild now.
If you do the test please share the results for forum knowledge.
Oh, if you put in lots of oil just the extra CC's of it will make the psi jump because of it filling up the chamber more. So don't put in tooo much oil.

[Batmanntexas]

Originally Posted by silversleeper

Suspect rings going bad. Do the old school test of squirting in some oil in a low compression cyl. #7 would be good as it's lowest but probably harder to get to than #2 which is similar. Squirt in enough but not so much as to hydro lock it. Like about less than a teaspoon of oil. Crank it over and see if it jumps way up to like 190psi. Then you know it's rings.
You can either tear it all down and replace them, or run it as it is. If I did run it like that I'd make damn sure I had a good valved breather on the crankcase. IDK if the Metco Valved , valvcover breather could flow enough for that many cyl blowby under boost but it'd be the minimum thing I'd do. Or you'll blow out the rear seal and spin your bearings like our Arkansas bro who's doing a rebuild now.
If you do the test please share the results for forum knowledge.
Oh, if you put in lots of oil just the extra CC's of it will make the psi jump because of it filling up the chamber more. So don't put in tooo much oil.

Thanks! I will do another compression test sometime this week, and will post the results.

I am running the CFM breather on the passenger valve cover, and no oil leaks to date.

-Scott

[eLeSthree]

Which valve springs do you have?

Since you decked the heads, did you do a ptv test during assembly? Was there a valve job performed when you did the heads?

Have you ever over revved it?

The easiest and first thing I would check out is my valve springs.