compression ratio vs. leakdown test(something is wrong)
compression ratio vs. leakdown test(something is wrong) I had a new shortblock instaleed 1000 miles ago. My compression is very ugly, 135-140. My leakdown test shows 4-8 % leakdown, which is pretty good. The only way that I know for this to happen is a timing/lobe issue with the cam. The cam was degreed in with a cloyes chain setup and came out right on cam card specs. To me that rules that out. I do have oil on all plugs and I'm using oil. I broke in the rings as per instructions. I don't understand how my compression can be so low. I even used two different gauges because i thought something maybe wrong with the gauge. I understand that on a normal engine using 17 to 20 x compression ratio should give you your ratio. New engine closer to 20. My setup is stage II Ls6 heads and the hammer cam.thanks,Dean
Teching In
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From: Austin
Re: compression ratio vs. leakdown test(something is wrong) we know that overall cranking CR drops due to larger cam. Here is a link: http://www.motortecmag.com/archives/.../imageset.html
here is a comparision with leakdown:
http://www.pumaracing.co.uk/comp.htm
You will notice that the only thing that leads to a low Cranking Compression while still having good leakdown is a miss indext cam. That is my general experience also.
Hate to say this but either try one of two things:
new gear and chain set
different cam
none of the above leads to loss of oil, blow by and wet plugs, these all indicate oil ring failure and /or all ring failure.
My best guess is: it could be a misshaped bore, i.e. the sucker is round at the top so leakdown is good but **** poor at the bottom so cranking CR sucks and blowby is bad during operation. Hmm, that would make sense as these early blocks have very thin walls and GM went to a much thicker wall in 99 up (mid year change). GM don't change anything if it works.
One thing we should note, that would backup the above is in order to use low tension rings as well as thin rings, GM relies on a very concentric bore and close tolerances pistons. One would assume that the LS1 has a much closer taper tolerance vs say a LT1 1970 forged piston where out of round and or tapper maybe as great as .002+ and piston to bore up to .007+ Anybody got a spec on LS1 out of round and tapper. I bet you it is in the .000x range of life.
Dean you got the tapper and out of round specs what are they?
here is a comparision with leakdown:
http://www.pumaracing.co.uk/comp.htm
You will notice that the only thing that leads to a low Cranking Compression while still having good leakdown is a miss indext cam. That is my general experience also.
Hate to say this but either try one of two things:
new gear and chain set
different cam
none of the above leads to loss of oil, blow by and wet plugs, these all indicate oil ring failure and /or all ring failure.
My best guess is: it could be a misshaped bore, i.e. the sucker is round at the top so leakdown is good but **** poor at the bottom so cranking CR sucks and blowby is bad during operation. Hmm, that would make sense as these early blocks have very thin walls and GM went to a much thicker wall in 99 up (mid year change). GM don't change anything if it works.
One thing we should note, that would backup the above is in order to use low tension rings as well as thin rings, GM relies on a very concentric bore and close tolerances pistons. One would assume that the LS1 has a much closer taper tolerance vs say a LT1 1970 forged piston where out of round and or tapper maybe as great as .002+ and piston to bore up to .007+ Anybody got a spec on LS1 out of round and tapper. I bet you it is in the .000x range of life.
Dean you got the tapper and out of round specs what are they?
Re: compression ratio vs. leakdown test(something is wrong) Louie the tapper is .0007, i can't find the bore. Louie also my other block with 37k on it came in at 140 compression with a stock timing chain setup and same cam and heads also using alot of oil and thats why i went with a fresh block and different rings and bearings. I don't understand the low compression. To me it seems physically impossible to have low compression on my setup. We know the cam was put it right, you helped me boss.
Re: compression ratio vs. leakdown test(something is wrong) Louie are you saying that the duration of the hammer is dropping my compression 60-8-points? I don't think this cam or any cam would drop it that much
D(irecto)r Pepper
Joined: Nov 2001
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From: Houston Raceway Park...in TEXAS.
Re: compression ratio vs. leakdown test(something is wrong) [quote]Originally posted by Louie:
<strong>
none of the above leads to loss of oil, blow by and wet plugs, these all indicate oil ring failure and /or all ring failure.
AND
My best guess is: it could be a misshaped bore, i.e. </strong><hr></blockquote>
The two snips are where I would be looking.
I feel your pain amigo.
<strong>
none of the above leads to loss of oil, blow by and wet plugs, these all indicate oil ring failure and /or all ring failure.
AND
My best guess is: it could be a misshaped bore, i.e. </strong><hr></blockquote>
The two snips are where I would be looking.
I feel your pain amigo.
TECH Addict
Joined: Nov 2001
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From: College Station, Tx
Re: compression ratio vs. leakdown test(something is wrong) Try squirting a little motor oil in the cylinder before you do your compression test - if your numbers shoot up it's definitely your rings.
chris
chris
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On The Tree
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Re: compression ratio vs. leakdown test(something is wrong) As a comparison for you,
My 98 showed 165-175psi cranking compression with stock cam.
Adding a hotcam in at 111 centerline showed 195psi.
After 3 seasons of nitrous and a few months of 10psi boost, leakdown at 9% on most cylinders and 20% on 1 hole with a cracked 2nd ringland in the piston.
Even with a completely seperated chunk of 2nd ringland about 2" long held in place on the piston only by the cylinder bore, the cylinder still showed 170 psi cranking compression.Plus the cylinder has quite a bit of mild scoring from piston debris caught between the rings from the broken land.
So I kind of doubt your rings are the cause of the low compression since the engine leaks down good.
Steve
My 98 showed 165-175psi cranking compression with stock cam.
Adding a hotcam in at 111 centerline showed 195psi.
After 3 seasons of nitrous and a few months of 10psi boost, leakdown at 9% on most cylinders and 20% on 1 hole with a cracked 2nd ringland in the piston.
Even with a completely seperated chunk of 2nd ringland about 2" long held in place on the piston only by the cylinder bore, the cylinder still showed 170 psi cranking compression.Plus the cylinder has quite a bit of mild scoring from piston debris caught between the rings from the broken land.
So I kind of doubt your rings are the cause of the low compression since the engine leaks down good.
Steve
On The Tree
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Re: compression ratio vs. leakdown test(something is wrong) If you want to test out the bad bore theory which I kind of doubt as you measured taper already,
just loosen the rocker arms and use a cheater bar against the floor to hold the crank and you can leak it down through out the stroke.
just loosen the rocker arms and use a cheater bar against the floor to hold the crank and you can leak it down through out the stroke.
Teching In
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From: Austin
Re: compression ratio vs. leakdown test(something is wrong) If it is taper the leak down can still be good at various places. The taper is going to cause the rings to flutter at rpm but it ain't gonna prevent them from sealing stop. Check the taper spec at .0007. You can’t even read this out of round with most micrometers as you cant set them straight (perpendicular) across the bore. Use the old Pythagoreans’s theory you better be able to line up the ends of the mic to less than .0001 for an accurate reading. Sure some super humans will say no sweat, I mic bores all the time to .0000X accuracy…. Not.
duration vs cranking CR, check the first graph in my first link, with 11 CR (your setup with LS6 heads milled) and a 280 gross cam, we are still looking at 190 to 200 PSI cranking.
If the cam is ground really off you can have poor compression as discussed in the second link above. To be down 30 to 60 PSI a hole the cam would be off MORE than a tooth IMO. We degreed the cam in at the default location on the new multi-index gear and it was right on the money (did it twice for grins). But of course we only checked one lobe.
duration vs cranking CR, check the first graph in my first link, with 11 CR (your setup with LS6 heads milled) and a 280 gross cam, we are still looking at 190 to 200 PSI cranking.
If the cam is ground really off you can have poor compression as discussed in the second link above. To be down 30 to 60 PSI a hole the cam would be off MORE than a tooth IMO. We degreed the cam in at the default location on the new multi-index gear and it was right on the money (did it twice for grins). But of course we only checked one lobe.
Re: compression ratio vs. leakdown test(something is wrong) well good news and some still not so good. Turns out i had a bad air line for the compression check and used another, actually Louies. My compression is 220-230 on all cylinders except #8 i couldn't get the air line in that sucker. I'm relieved for that but I'm still trying to find out the cause of the oil on the plugs and i forgot to mention that my dipstick is popping out about a 1/4 inch during WOT runs. This is a sign of blow by or bad pcv as someone mention on Big Mikes post a while back. I checked the pcv and it seems ok. whatcha think Louie?
Re: compression ratio vs. leakdown test(something is wrong) [quote]Originally posted by just me:
<strong>If you want to test out the bad bore theory which I kind of doubt as you measured taper already,
just loosen the rocker arms and use a cheater bar against the floor to hold the crank and you can leak it down through out the stroke.</strong><hr></blockquote>
actually i was giving the specs of the tapper for Louie last night. I didn't check it
<strong>If you want to test out the bad bore theory which I kind of doubt as you measured taper already,
just loosen the rocker arms and use a cheater bar against the floor to hold the crank and you can leak it down through out the stroke.</strong><hr></blockquote>
actually i was giving the specs of the tapper for Louie last night. I didn't check it
Re: compression ratio vs. leakdown test(something is wrong) Dean it sounds like you have a lot of crankcase pressure.
I am running a small breather on the driver's valvecover. Other side goes into the normal normal spot in the intake.
I am running a small breather on the driver's valvecover. Other side goes into the normal normal spot in the intake.
Teching In
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From: Austin
Re: compression ratio vs. leakdown test(something is wrong) What weight oil are they running? Yes VENT, VENTing is good. Forget about PCV and the oil catcher dohickeies. If you blowing the dipstick up, you aint got PCV anywhay. Just use CV!!!
Teching In
Joined: Nov 2001
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From: Austin
Re: compression ratio vs. leakdown test(something is wrong) look this is a 1.5 mm ring motor at 30% reduction in tension. GM says:
snip
There is a ton of technology in piston and rings aimed at reducing friction. The rings use the same basic materials as before but the design is different. The LS1 top and second rings have 1.5mm faces vs. the 2.0mm rings used in LT1/4. The tension of all rings have been reduced by about 30%. Reduction in ring face widths and tension would never have proven reliable from a cylinder sealing and oil consumption standpoint, if process control improvements did not result in reduced bore variation and improved consistency in individual bore diameters
snip
Let me cut through the techno bable. If the bore aint true this motor ain’t ever gonna seal. Basically if you ain't with .0007 you are screwed. AND you can't measure .0007.
All that other John J and the .02 LS6 ring pack is just plain out and out BS. The ring pack is to thin, the tension is too little, if you get a motor with perfect bores you are OK, but if you get a motor with an unmeasurable out of round or tapper YOU ARE SCREWED.
my fix is to get a real piston with a real ring. or at the very least run a total seal ring, yes they are out, same guys make the 77 bearings.
snip
There is a ton of technology in piston and rings aimed at reducing friction. The rings use the same basic materials as before but the design is different. The LS1 top and second rings have 1.5mm faces vs. the 2.0mm rings used in LT1/4. The tension of all rings have been reduced by about 30%. Reduction in ring face widths and tension would never have proven reliable from a cylinder sealing and oil consumption standpoint, if process control improvements did not result in reduced bore variation and improved consistency in individual bore diameters
snip
Let me cut through the techno bable. If the bore aint true this motor ain’t ever gonna seal. Basically if you ain't with .0007 you are screwed. AND you can't measure .0007.
All that other John J and the .02 LS6 ring pack is just plain out and out BS. The ring pack is to thin, the tension is too little, if you get a motor with perfect bores you are OK, but if you get a motor with an unmeasurable out of round or tapper YOU ARE SCREWED.
my fix is to get a real piston with a real ring. or at the very least run a total seal ring, yes they are out, same guys make the 77 bearings.