9:1 Compression test

[livedsm4g63]

Well I just got done building my motor with a manley/mahle combo and have around 176 miles on the new motor. My compression test came back to be the following:

Cylinder 1: 150
Cylinder 2: 150
Cylinder 3: 155
Cylinder 4: 150

Ive done some research and I see that 9:1 usually is around 190ish per cylinder. I check the studs and they were a little loose at around 80lbs so i torqued them back down to 95lbs. Ive also noticed some oil marks on the backside of the engine that look like oil is running down the back of the block. Those lines looked sort of dry though. My question is how many miles should I wait to tear it open if the number don't go up. The car runs and drives good too, so I'm sorta speech less.

Any good advice would be sweet. Thanks!

 

[mrfister11]

compression does not equal compression ratio.......... do you have big cams? are they timed other than 0*? did you do a valve job? are the first and second ring gaps on opposite ends? there is NO way you can say EX. 190 PSI is 9:1 200 psi is 9.5:1... ect. ect.. compression is how good the cylinder/combustion chamber is sealed.. the timing and duration effects this too

 

[BurrowZ]

Was the motor fully warm and throttle fully open when you tested?

Only reason I'm asking is because I've done hundreds of compression tests and to this day I still sometimes forget to hold the throttle open. It does make a difference.

 

[mrfister11]

and if you take out all the other plugs it makes the biggest difference.. that will also let you know if you have a leak between 2 cylinders.. had quite a few with headgasket out.. compression low with spark plugs in.. 0 with them out haha

 

[livedsm4g63]

compression does not equal compression ratio.......... do you have big cams? are they timed other than 0*? did you do a valve job? are the first and second ring gaps on opposite ends? there is NO way you can say EX. 190 PSI is 9:1 200 psi is 9.5:1... ect. ect.. compression is how good the cylinder/combustion chamber is sealed.. the timing and duration effects this too

Never said that compression equals compression ratio.
I have fp2's with a newer valve job less then 10k. Cams were straight drop in too. All of the rings went in opposite of each other.

and if you take out all the other plugs it makes the biggest difference

They were all out.

Was the motor fully warm and throttle fully open when you tested?

Only reason I'm asking is because I've done hundreds of compression tests and to this day I still sometimes forget to hold the throttle open. It does make a difference.

Car was also warm with wot pressed.

 

[mrfister11]

Never said that compression equals compression ratio.

yes you did

"Ive done some research and I see that 9:1 usually is around 190ish per cylinder"

that is incorrect in every way..


ANY engine even with 7:1 should have 190~psi on a fresh rebuild..


anyways... im betting its just the valves arent sealing perfectly..since all the cylinders have roughly the same pressure

 

[livedsm4g63]

yes you did

"Ive done some research and I see that 9:1 usually is around 190ish per cylinder"

that is incorrect in every way..

Dont start a false argument please, and thank you.
By that statement I said that most people running 9:1 compression ratio come up with a compression test result of around 190 per cylinder.
Check yourself before you wreck yourself

 

[mrfister11]

sorry its just really annoying when people compare compresion ratio with cylinder pressures... its like asking someone how wide their tire is and they say its a 50 series..............

 

[livedsm4g63]

ANY engine even with 7:1 should have 190~psi on a fresh rebuild..

So your trying to argue that no matter how much space is in the combustion chamber it will always be 190 on a new build?

Thank you for your advice,but the head wasn't touched when the motor came out and was put back in. There was no need to since it was just recently rebuilt. It didn't need a surface either because the surface passed inspection.

 

[mrfister11]

no i was saying that a low c/r engine can have the same psi.. just did a compression check on our 14:1 565 bbc... 160-180's.... needed a valve job.. engine had 10 passes on it.. seriously just saying that comparison between the two is almost irrelevant..

 

[livedsm4g63]

Ok, I plan on driving it more to see if they go up, fingers crossed they do.

 

[mrfister11]

do you have adjustable cam gears?

 

[livedsm4g63]

Negative I do not. There is no real need to time the cams. I'm sure it would help performance a little but it isn't needed in the big picture.

 

[mrfister11]

yeah i was just going to suggest to advance the intake cam a few degrees and see if your compresssion increases.. just getting the intake valve closed a little sooner BTDC.. kind of a little experiment ha. have you ran the engine hard any


also a very high elevation on a hot day could possibly make a big difference in your numbers

 

[livedsm4g63]

Yeah I'm running around 8lbs and I've done some pulls and hard decel as well.

 

[livedsm4g63]

Im also low on sea level and it was done at dusk

 

[mrfister11]

k i usually just let an engine warm up to check for any major problems then the engine gets the piss ran out of it.. ive never had an engine with that low of compression on a FULL rebuild.. including a fresh valve job.. so im leaning towards the valves not sealing perfect.. never know how much carbon could get built up in 10k miles. just saying also to back me up, just researched this for the first time and found this.. Compression ratio vs pressure



also i wasnt trying to argue or be rude.. atleast im still trying to help ha

 

[livedsm4g63]

Haha no problem. I may have to do a leak down test for the heck of it, just to see where it's leaking if the numbers don't go up.

 

[mrfister11]

there ya go! exactly what i would do. only way to do it. most people dont have a way to do that

 

[BogusSVO]

Before you go tearing into it, check the timing belt, and check your marks. A tooth off would drop compression some.

 

[livedsm4g63]

Good call. I will check that first.

 

[BogusSVO]

Best of luck and I hope that is the issue for you!

Keep wrenching!

 

[mrfister11]

dam it i said that first... not straight forward but i was thinking it when i posted the first post haha

 

[turboglenn]

don't use that metal extender with the gauge, just thread the braided hose part in and double check... with that metal piece i get 150'ish but with the hose alone i get about 180-185

 

[LordDigi]

compression does not equal compression ratio.......... do you have big cams? are they timed other than 0*? did you do a valve job? are the first and second ring gaps on opposite ends? there is NO way you can say EX. 190 PSI is 9:1 200 psi is 9.5:1... ect. ect.. compression is how good the cylinder/combustion chamber is sealed.. the timing and duration effects this too

You are right, although, one major flaw in your thought process ... It matters when speaking of different engines. Everyone here is talking about the 4g63. One 4g with 9:1 and another 4g with 9:1 should have the same compression given the age. It may very slightly based upon the milling of the head and head gasket used, but should be very close to each other.

I run 8.8:1 but have a mill limit head, and I saw 195 across the board when the rings were seated.

... and have around 176 miles on the new motor ...

I would wait to do another compression test, it sounds like your rings are not fully seated yet.