compression vs compression ratio
Thread Starter
Racer
Joined: Nov 2006
Posts: 264
Likes: 0
From: McLouth Kansas
compression vs compression ratio
Tonight I checked my compression and found all the cyls to be 150 to 160 pounds + or - due to the tester accuracy.
In a ball park sort of a way can anyone give me a idea what my compression ratio is.
Thanks
In a ball park sort of a way can anyone give me a idea what my compression ratio is.
Thanks
Le Mans Master
Joined: Dec 2006
Posts: 5,557
Likes: 9
From: Oquirrh Mountains
St. Jude Donor '09
not really because there's too many other variables involved. The largest being the camshaft in your engine. Valves don't open at tdc and bdc.
Race Director
Joined: Dec 1999
Posts: 11,094
Likes: 990
From: Sierra Foothills CA
There are many factors in determining the compression ratio of your motor. This link has some good info-
http://www.carcraft.com/techarticles...tio/index.html
Your cam can make a difference in how much cylinder pressure you have, so I wouldn't venture a guess on the compression ratio. This link has some info that sheds some light on that-
http://kb-silvolite.com/article.php?action=read&A_id=36
The good news is that your cylinder pressure is consistent in all cylinders. The closest thing I could find that relates compression ratio values to cylinder pressure (during compression testing) values was this statement from an article on the subject-
A useful rule of thumb states that cylinder pressure in an engine in good condition with a mild or "street-type" cam should range (at sea level or a little above) from 17 to 20 times the specified
compression ratio.
That would ballpark your compression ratio at something between 8.0-9.4 to 1. A larger cam would obviously change those numbers.
http://www.carcraft.com/techarticles...tio/index.html
Your cam can make a difference in how much cylinder pressure you have, so I wouldn't venture a guess on the compression ratio. This link has some info that sheds some light on that-
http://kb-silvolite.com/article.php?action=read&A_id=36
The good news is that your cylinder pressure is consistent in all cylinders. The closest thing I could find that relates compression ratio values to cylinder pressure (during compression testing) values was this statement from an article on the subject-
A useful rule of thumb states that cylinder pressure in an engine in good condition with a mild or "street-type" cam should range (at sea level or a little above) from 17 to 20 times the specified
compression ratio.
That would ballpark your compression ratio at something between 8.0-9.4 to 1. A larger cam would obviously change those numbers.
Advanced
Joined: Jan 2007
Posts: 62
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From: LaGrange Georgia
I think some clarification is due for sanity's sake. The earlier posts were referring to dynamic compression ratio, which takes into account cam size and other characteristics, but you can calculate the static compression ratio fairly easily. The static compression ratio is simply the volume of the cylinder when the piston is all the way down divided by the volume when the piston is all the way up. Be sure to include combustion chamber and compressed head gasket thickness on both values. That's enough for a rough estimate.
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
I think some clarification is due for sanity's sake. The earlier posts were referring to dynamic compression ratio, which takes into account cam size and other characteristics, but you can calculate the static compression ratio fairly easily. The static compression ratio is simply the volume of the cylinder when the piston is all the way down divided by the volume when the piston is all the way up. Be sure to include combustion chamber and compressed head gasket thickness on both values. That's enough for a rough estimate.
I did this a very long time ago. Don't recall all details... not sure how I figured out the displacement of the crown of the piston.. may have been given by the manufacturer?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
No. You cannot [accurately] calculate the compression ratio of the engine from compression test data. {for the reasons stated above}
However, the Chevrolet Chassis Service Manual does have a specified range of acceptable compression test readings for the stock Corvette engines, and you can compare your numbers to that, if you wish. [Assuming that you performed the test as that manual requires.] From my 1971 CCSM, the 150-160 psi range is just what is expected for all Corvette engines (hi-perf engines @ 150; others at 160)
However, the Chevrolet Chassis Service Manual does have a specified range of acceptable compression test readings for the stock Corvette engines, and you can compare your numbers to that, if you wish. [Assuming that you performed the test as that manual requires.] From my 1971 CCSM, the 150-160 psi range is just what is expected for all Corvette engines (hi-perf engines @ 150; others at 160)
Race Director
Joined: Dec 1999
Posts: 11,094
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From: Sierra Foothills CA
Only way I know to really tell would be to cc the heads, combustion chamber, unless you already know the volume. You'd also need to know the dimensions of the piston, how the top surface is shaped. You already know the gasket thickness.. I calculated my compression ratio while heads were off. CC'd the chambers, figured out the displacement of the crown of the piston and took gasket thickness into account..
I did this a very long time ago. Don't recall all details... not sure how I figured out the displacement of the crown of the piston.. may have been given by the manufacturer?
I did this a very long time ago. Don't recall all details... not sure how I figured out the displacement of the crown of the piston.. may have been given by the manufacturer?
"I use a syringe primarly used to inject medication into cattle and other farm animals. Holds 100 cc and I have plate cut from plexiglass with a hole in the top. Seal it off to the chamber with white grease and measure away."
81fixerupper,
The top link I posted described all the factors in measuring the static compression ratio.
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
That sounds very familiar, I recall using a piece of plexiglass and some sort of syringe to measure fluid...
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Race Director
Joined: May 2003
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From: California
i am running 10.5:1 with an Edelbrock retro-roller signature cam
my static compression is 170+- 5psi
but as stated, the cam has a direct effect on static CR... this is what I have with this cam...#2207
ENGINE: CHEVY Signature Series 383 (1987 & LATER)
RPM RANGE: 1500-6500
Duration at 0.006" Lift: Intake: 305° Exhaust: 314°
Duration at 0.050" Lift: Intake: 242° Exhaust: 240°
Lift at Cam: Intake: 0.396" Exhaust: 0.396"
Lift at Valve: Intake: 0.594" Exhaust: 0.594"
with 1.52 magnum RR's from compcams
my static compression is 170+- 5psi
but as stated, the cam has a direct effect on static CR... this is what I have with this cam...#2207
ENGINE: CHEVY Signature Series 383 (1987 & LATER)
RPM RANGE: 1500-6500
Duration at 0.006" Lift: Intake: 305° Exhaust: 314°
Duration at 0.050" Lift: Intake: 242° Exhaust: 240°
Lift at Cam: Intake: 0.396" Exhaust: 0.396"
Lift at Valve: Intake: 0.594" Exhaust: 0.594"
with 1.52 magnum RR's from compcams
Thread Starter
Racer
Joined: Nov 2006
Posts: 264
Likes: 0
From: McLouth Kansas
Thanks guys,,,,,,I now know more that when I posted,,,,Later next month I will have the heads off and I can get it figured more accurately.
I ran a compression check and was curious as to about what it could be. I want to change my heads to a set of AFR's. The current heads are Bowtie heads ported and big valves,,,too big for my engine/cam combo.
I ran a compression check and was curious as to about what it could be. I want to change my heads to a set of AFR's. The current heads are Bowtie heads ported and big valves,,,too big for my engine/cam combo.
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
Its always a good idea to do a leak down test as I have seen good compression tests followed by a leak down test that showed up to 40% leak down on an engine that showed 190 to 200 on compression test.
Le Mans Master
Joined: Apr 2001
Posts: 5,941
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Thanks guys,,,,,,I now know more that when I posted,,,,Later next month I will have the heads off and I can get it figured more accurately.
I ran a compression check and was curious as to about what it could be. I want to change my heads to a set of AFR's. The current heads are Bowtie heads ported and big valves,,,too big for my engine/cam combo.
I ran a compression check and was curious as to about what it could be. I want to change my heads to a set of AFR's. The current heads are Bowtie heads ported and big valves,,,too big for my engine/cam combo.
What makes you think they are too big? What are the valve sizes and port sizes in the heads?
