Static v. Running Compression
Thread Starter
Drifting
Joined: Oct 2001
Posts: 1,699
Likes: 1
From: Westampton NJ
Static v. Running Compression
Well, I was wondering a bit about my compression readings...then I came across this, and wondered if it was good info:
http://www.imperialclub.com/Repair/Engine/dynamic.htm
I did the "snap" readings and got this:
87 PSI "snap"
14 in vacuum "snap"
29.97 in Hg barometric pressure
29.97 - 14 = 15.97 in Hg
15.97 in Hg = 7.843731024 psi
87 / 7.843731024 = 11.092
Therefore 11.092:1 compression ratio. I may still have the stock pistons in the engine then...hmm...
Suggestions, comments? :smash:
http://www.imperialclub.com/Repair/Engine/dynamic.htm
I did the "snap" readings and got this:
87 PSI "snap"
14 in vacuum "snap"
29.97 in Hg barometric pressure
29.97 - 14 = 15.97 in Hg
15.97 in Hg = 7.843731024 psi
87 / 7.843731024 = 11.092
Therefore 11.092:1 compression ratio. I may still have the stock pistons in the engine then...hmm...
Suggestions, comments? :smash:
Instructor
Joined: Aug 2001
Posts: 201
Likes: 0
From: Daytona Beach
Re: Static v. Running Compression (rainman69)
I don't see the point in this. I never heard of "running the engine" during
a compression test.
To find out the condition of your engine do a normal compression test-
plugs out, ign.& fuel disabled, throttle valve open. Crank it 4 or 5 times
compare readings between cylinders.
also you can do a Leak Down test to check for ring seal.
If you want to what know your static compression is use this formula...
CR = SV+CCV+HGV+DHV+EDV / CCV+HGV+DHV+EDV
Where CR = compression ratio
CCV = combustion chamber volume
HGV = head gasket volume
DHV = deck height volume
EDV = effective dome volume
SV = swept volume
Your Dynamic compression depends on you static compression and takes into account your cam duration and timing.
In other words if you have a long duration cam you need a higher static
compression ratio to maintain a good dynamic (running)compression .
If your static compression is low (say 8 to 1) you wouldn't want a big cam with alot of overlap because your dynamic compression would be to low to make good power.
[Modified by vettedave, 7:12 PM 1/20/2002]
a compression test.
To find out the condition of your engine do a normal compression test-
plugs out, ign.& fuel disabled, throttle valve open. Crank it 4 or 5 times
compare readings between cylinders.
also you can do a Leak Down test to check for ring seal.
If you want to what know your static compression is use this formula...
CR = SV+CCV+HGV+DHV+EDV / CCV+HGV+DHV+EDV
Where CR = compression ratio
CCV = combustion chamber volume
HGV = head gasket volume
DHV = deck height volume
EDV = effective dome volume
SV = swept volume
Your Dynamic compression depends on you static compression and takes into account your cam duration and timing.
In other words if you have a long duration cam you need a higher static
compression ratio to maintain a good dynamic (running)compression .
If your static compression is low (say 8 to 1) you wouldn't want a big cam with alot of overlap because your dynamic compression would be to low to make good power.
[Modified by vettedave, 7:12 PM 1/20/2002]
Tech Contributor
Joined: Aug 2001
Posts: 472
Likes: 39
Re: Static v. Running Compression (rainman69)
Rainman69, cool link. Very interesting. I like what you did there... took the compression differences and then calculated your dynamic compression ratio.
What were the RPMs when you were doing the "snap" measurements? You could take different readings for different RPMs - they should change (get smaller) as the RPMs get higher, right?
It would be interesting to compare the calculated static compression ratio that Vette92 posted vs. the measured static compression ratio. I think I'll do that when I build my engine (about a year from now).
What were the RPMs when you were doing the "snap" measurements? You could take different readings for different RPMs - they should change (get smaller) as the RPMs get higher, right?
It would be interesting to compare the calculated static compression ratio that Vette92 posted vs. the measured static compression ratio. I think I'll do that when I build my engine (about a year from now).
Thread Starter
Drifting
Joined: Oct 2001
Posts: 1,699
Likes: 1
From: Westampton NJ
Re: Static v. Running Compression (aobrien)
Not my idea. A friend found the page, and I just did the test. I'll have to pull the heads at some point to check to find out if it's good info, but I'm guessing it is. :smash:
Drifting
Joined: Dec 1999
Posts: 1,462
Likes: 1
From: Reno nevada
Re: Static v. Running Compression (rainman69)
I wish I could find the article printed a couple of years back. It gave some real insight on static vs dynamic compression ratio. If I remember correctly, static compression ratio is always higher. Your figures would have to way off or that sight is giving incorrect info.
The article talked about motors like the modern Lt1 with 10.5 c/r static. But do to the cam it's dynamic was in the 8.5 range. 92 octane would ping in motors not much over 9 dynamic c/r. They had figures for a 12.5 static hot cammed motor and the dynamic was 9.9 c/r and it required 100 octane.
The only disclaimer the article had and it's only common in highly refined motors is the motor that runs with more than 100% VME. Which means that the cylinder durring a narrow rpm range is actualy having the cylinder over filled. Then of course dynamic would exceed static.
As to your figures do you really get 29.97 inches at 6000 feet or is this a corrected figure?
The article talked about motors like the modern Lt1 with 10.5 c/r static. But do to the cam it's dynamic was in the 8.5 range. 92 octane would ping in motors not much over 9 dynamic c/r. They had figures for a 12.5 static hot cammed motor and the dynamic was 9.9 c/r and it required 100 octane.
The only disclaimer the article had and it's only common in highly refined motors is the motor that runs with more than 100% VME. Which means that the cylinder durring a narrow rpm range is actualy having the cylinder over filled. Then of course dynamic would exceed static.
As to your figures do you really get 29.97 inches at 6000 feet or is this a corrected figure?
Safety Car
Joined: Feb 1999
Posts: 3,504
Likes: 4
From: Mexico Beach FL
Re: Static v. Running Compression (vettedave)
I agree with vettedave, if you really want to know the condition of your engine, do a leak-down test.
If done correctly, a leak-down test can tell you the condition of your rings, valve seats, and head gasket. A leak down test is not a good test for valve stems and seals.
Since a leak-down test is done statically, a wild cam will have no effect on it.
The biggest problem I've had with leak-down testing is the engine should be at operating temperature -- I've never been able to get all eight plugs out of my '92 LT1 and still have a warm engine.
Tom Piper
If done correctly, a leak-down test can tell you the condition of your rings, valve seats, and head gasket. A leak down test is not a good test for valve stems and seals.
Since a leak-down test is done statically, a wild cam will have no effect on it.
The biggest problem I've had with leak-down testing is the engine should be at operating temperature -- I've never been able to get all eight plugs out of my '92 LT1 and still have a warm engine.
Tom Piper
