Low Compression numbers?

[jefffoxsr]

All my cylinders are
# 1 = 93 psi
#2 = 100 psi
#3 = 100 psi
#4 = 95 psi
#5 = 97 psi
#6 = 102 psi

Right after the rebuild they were all about 110 psi.
I have about 1,500 miles on the rebuild I did 9 years ago.

Last leakdown test check on the gn was on 10-11-06 which is only 600 miles ago. The readings were 7,10,15,10,15,11 percent for the cylinders. This is acceptable.


Normally I would think those compression numbers are very low, but they aren't much lower than right after the rebuild.

I tested with engine cold at WOT. I know guys are reporting 135 psi+ on stock engines, but on modified engines do people get lower numbers?

Thanks.

 

[bosted86]

engine should be at operating temp when you test the compression. yes, those numbers are low even after the rebuild 110 seems low. i have 92000 miles on my car and they are right around 140.

 

[GNVYUS 1]

What cam's in it?

Not looking good IMO if it were done at WOT and the leak down IMO is not acceptable. I have 3% still on my 19k mile Talon I beat the piss out of everyday.

 

[jefffoxsr]

I thought I read somewhere that up to 15% leakage is ok.

I'll have to do some more research on what the acceptable leakdown numbers are.

The cam is flat tappet 225/225.

I don't recall, but I think that's pretty big. I think the lift was .510 or something like that.

Thanks.

 

[Chuck Leeper]

Cam degreed in where it should be??? 110, when new, is a bit low. But that may be a combo of cam, and unseated rings.....
Leakdown air is going where? Into the pan, the intake, the exh???

 

[SloGN]

i think that the cam is retarded some. What type of timing chain does it have? i know the edelbrock double roller set-up are way off.


Also on the very first compression hit how much does it make?

 

[jefffoxsr]

On the compression test at the first hit the needle went all the way up to the final reading. Additional cranking time yielded no additional psi. Is that good or bad?

I don't remember what timing chain I put in when I rebuit the engine 9 years ago.

I do know the the cam was degreed at the time.

I didn't think to listen to where the air was going when I did the leakdown test.

Chuck and SloGN, I could really use your experienced help on the following thread. http://www.turbobuick.com/forums/general-turbo-buick-tech/268978-car-running-weird-need-suggestions.html

 

[WS6]

FWIW, I too got low numbers one time when I made up an extra long hose to see it from the drivers seat. I realized that may be a problem and it was. So maybe try another gauge too and keep the hose length the length it was built with.

 

[SloGN]

On the compression test at the first hit the needle went all the way up to the final reading. Additional cranking time yielded no additional psi. Is that good or bad?

I don't remember what timing chain I put in when I rebuit the engine 9 years ago.

I do know the the cam was degreed at the time.

I didn't think to listen to where the air was going when I did the leakdown test.

Chuck and SloGN, I could really use your experienced help on the following thread. http://www.turbobuick.com/forums/general-turbo-buick-tech/268978-car-running-weird-need-suggestions.html

The fact that it made the max number on the first hit and gained nothing afterthat. I think that the cam is not degreed in correctly or the valves might be held open a tad bit. what kind of heads is on the car and what type of valve train. Do you have solid lifters,hydraulic.

also when the motor was rebuilt was the deck milled? was the heads milled any. If the block and the heads were milled and you are using stock rocker arms and pushrods. the valves could be held open and this could result in low compression #'s as well.

Go back and do the leak down again and listen where the air is going.

 

[jefffoxsr]

I have GN1 aluminum heads. I'm using the stock valve train. The block was sent to PTE and it may have been milled. I'm not sure if the heads were milled or not.
The lifters are standard, not solid, but not stock. They were new at the rebuild time.
The pushrods are not stock, but I don't remember what the measurement is for how long they are.

I don't remember all the details since the rebuild was 9 years ago, sorry.

I really appreciate the help and ideas.

 

[bison]

I tested with engine cold at WOT. I know guys are reporting 135 psi+ on stock engines, but on modified engines do people get lower numbers?

Thanks.

It depends on cam overlap, cranking speed, and compression ratio.

 

[SloGN]

I have GN1 aluminum heads. I'm using the stock valve train. The block was sent to PTE and it may have been milled. I'm not sure if the heads were milled or not.
The lifters are standard, not solid, but not stock. They were new at the rebuild time.
The pushrods are not stock, but I don't remember what the measurement is for how long they are.

I don't remember all the details since the rebuild was 9 years ago, sorry.

I really appreciate the help and ideas.

one thing to try and do is loosen the rocker shaft and then try the compression check again to see if the numbers go up. if they do the valve are too tight.

 

[XLR8]

Stupid questions, but are you sure the gauge you're using is correct? Also are you cracking open the throttle body when you're cranking the engine?

 

[jefffoxsr]

I have two different guages and tried both. I tested at WOT.

Thanks for the tips. I'll try the loosening the rocker idea too.

 

[S10xGN]

It depends on cam overlap, cranking speed, and compression ratio.

Yup! :wink: Low compression with a big stick will show poor cranking compression.

 

[KevinB]

It depends on cam overlap, cranking speed, and compression ratio.

I agree. I did a compression check on engine and they were all in the 100 range. Then went on and beat it at the track ending up with my best time.

I wouldn't worry about it. All the cylinders are close. That bigger cam you have is what creating those lower numbers. There is no way you can compare your numbers with a stocker with a stock cam.

 

[bison]

I agree. I did a compression check on engine and they were all in the 100 range. Then went on and beat it at the track ending up with my best time.

I wouldn't worry about it. All the cylinders are close. That bigger cam you have is what creating those lower numbers. There is no way you can compare your numbers with a stocker with a stock cam.

I know of a high very hp stage 2 that had 70 psi cranking . It was way over cammed. It killed the 330'. Switching to a more sensible grind brought the 330' right where they should have been. If you have a mild cam like a 212 with under 110 degrees overlap you should have at least 120 cranking on an 8.5:1 engine. If you are lower than this it is advisable to advance the cam or use another grind. Overlap, ci, and compression always have to be considered. I see a lot of mis-matched combos on here. If the cam is too big it will really kill the combo.

 

[jefffoxsr]

Thanks guys. The cam issue makes sense to me. I know my rings are not worn after only 1,500 miles.
I think we put this one to rest.
Thanks again.
Now if I can make the car run properly as referenced my other thread that would be great.
http://www.turbobuick.com/forums/general-turbo-buick-tech/268978-car-running-weird-need-suggestions.html

 

[KevinB]

I know of a high very hp stage 2 that had 70 psi cranking . It was way over cammed. It killed the 330'. Switching to a more sensible grind brought the 330' right where they should have been. If you have a mild cam like a 212 with under 110 degrees overlap you should have at least 120 cranking on an 8.5:1 engine. If you are lower than this it is advisable to advance the cam or use another grind. Overlap, ci, and compression always have to be considered. I see a lot of mis-matched combos on here. If the cam is too big it will really kill the combo.

I did mine cold. I was just looking for a dead cylinder. The car popped real bad twice in a row because of a bad cam sensor. The ring was moving. That was some scary stuff there lol.
The gaskets lived, but the block wasn't as fortunate.

 

[bison]

I did mine cold. I was just looking for a dead cylinder. The car popped real bad twice in a row because of a bad cam sensor. The ring was moving. That was some scary stuff there lol.
The gaskets lived, but the block wasn't as fortunate.

That sucks.