Compression test results...low numbers?
Thread Starter
Intermediate
Joined: Nov 2010
Posts: 40
Likes: 0
From: Eau Claire WI
Compression test results...low numbers?
I just did a compression test on my stock short blocked ls1 with ported & milled 243 heads, blower cam of unknown specs. Results I got:
---Rear---
162---167
165---165
157---165
160---165
---Front--
Is it just me, or are these numbers really low for the small cc'd heads/higher compression? I would have thought the numbers to be in the 200+ range on a healthy motor. The car/shortblock has 112k on it now, 25k of which was with the blower & mods. When the blower was first put on by the previous owner it made 545whp @ 6psi, yet when running against a buddy's 450whp STi it is very close...
---Rear---
162---167
165---165
157---165
160---165
---Front--
Is it just me, or are these numbers really low for the small cc'd heads/higher compression? I would have thought the numbers to be in the 200+ range on a healthy motor. The car/shortblock has 112k on it now, 25k of which was with the blower & mods. When the blower was first put on by the previous owner it made 545whp @ 6psi, yet when running against a buddy's 450whp STi it is very close...
Racer
Joined: Jul 2011
Posts: 491
Likes: 38
Interested in the answer, but I always thought the rule of thumb was if you've got less than a 10% variance across all you're ok.
How many times did you crank the engine over? Did you pull all the plugs at the same time or leave the ones in on the cyls not being tested?
How many times did you crank the engine over? Did you pull all the plugs at the same time or leave the ones in on the cyls not being tested?
Racer
Joined: May 2011
Posts: 329
Likes: 1
From: Bowie Texas
Camshaft duration and lobe centerline, battery charge, throttle position ect will also affect cranking compression. Without any history to see if it has gone down since the engine was built about all you really have to go by is the difference between cylinders and yours doesn't look unusual to me. Like has been said a leak down test tells a lot more about the health of a engine.
Intermediate
Joined: Jan 2009
Posts: 37
Likes: 0
I also, have ported 243 heads and my compression was 192 to 202 but, I have a mild cam. A large cam with a lot of intake duration will scrub off compression. The compression will not build until the intake valve is fully seated. Your numbers are fine IMO
Thread Starter
Intermediate
Joined: Nov 2010
Posts: 40
Likes: 0
From: Eau Claire WI
I cranked the motor over for a total of 7 puffs on each cylinder. I didn't have the throttle open I totally forgot, perhaps I will re-test after work. I do have a leak-down tester and was pondering doing the test, but I don't think my 2 gallon harbor freight air compressor is up to the task.
The cam...I know it is a boost cam, but I forgot the exact specs. I want to say it is a .581/.581, 230/230, 112 LSA? Does that sound right for a comp boost cam? I am not very knowledgeable when it comes to camshafts...
I was thinking even with the cam it should be around 200 psi. The motor runs very good, it is self tuned on e85. It does go through a decent amount of oil though, it doesn't leave any smoke trails, leads me to believe blow-by. Thoughts? I'll post back up when I re-do the test.
Edit: I also forgot to mention that when I did the test I had a battery charger connected supplying 10amps. The battery was initially dead but had been charging for hours on a lower setting.
The cam...I know it is a boost cam, but I forgot the exact specs. I want to say it is a .581/.581, 230/230, 112 LSA? Does that sound right for a comp boost cam? I am not very knowledgeable when it comes to camshafts...
I was thinking even with the cam it should be around 200 psi. The motor runs very good, it is self tuned on e85. It does go through a decent amount of oil though, it doesn't leave any smoke trails, leads me to believe blow-by. Thoughts? I'll post back up when I re-do the test.
Edit: I also forgot to mention that when I did the test I had a battery charger connected supplying 10amps. The battery was initially dead but had been charging for hours on a lower setting.
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Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Throttle open will add a bunch to the psi... Probably 20 psi atleast... If its drive by wire I am not sure how you will get the throttle open unless you remove the throttle body all together.
But yes a leakdown test will tell you how well its sealed up and where it is leaking compression if it is..... Leak down tests can be tricky though to be accurate and the leak down tester results can vary big time. Just mainly look for consistancy amongst the cyls...dont pay a lot of attention to the %s.
But yes a leakdown test will tell you how well its sealed up and where it is leaking compression if it is..... Leak down tests can be tricky though to be accurate and the leak down tester results can vary big time. Just mainly look for consistancy amongst the cyls...dont pay a lot of attention to the %s.
Burning Brakes
Joined: Mar 2011
Posts: 783
Likes: 2
From: Houston Tx
Those are typical numbers for a cammed car. I usually just stick a screwdriver in the throttle black to prop it open. But, a warm motor with the throttle open MIGHT crank 5-7psi more, and that is about it. Your numbers are fine.
Burning Brakes
Joined: May 2011
Posts: 1,216
Likes: 56
From: Houston Texas
The cam and resulting dynamic compression will have the biggest impact here as others have noted. They cylinders are all close enough to each other to not worry about it and sound ballpark for a performance cam with decent overlap. Understand that there are two different compression ratio values, a static and dynamic. The static would be a direct calculation of total cylinder volume over compressed cylinder volume, discounting the valve positions. The dynamic takes in to account the valve positions, and is a more true indication of working compression.
If you are truely concerned with the condition of the powercylinder, a leak down test will tell you much more. Also, you will be able to tell if the source of any possible rapid leak passage being either by the piston/intake valve/exhaust valve.
Again, since the cylinders are all fairly consistent I don't think you have anything to worry about here in my opinion.
If you are truely concerned with the condition of the powercylinder, a leak down test will tell you much more. Also, you will be able to tell if the source of any possible rapid leak passage being either by the piston/intake valve/exhaust valve.
Again, since the cylinders are all fairly consistent I don't think you have anything to worry about here in my opinion.
