Compression tester results
Thread Starter
Team Owner
Joined: Jan 2006
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From: Crossville TN
Compression tester results
Can anyone give me what a standard compression tester should read for various CR values? 8.5:1, 9.5:1, 10.5:1, 11:1. [Assuming a standard performance camshaft.]
Team Owner
Joined: Jul 2001
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From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
That's just a guesstimate as it depends on several factors..
Thread Starter
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Problem is that I'm reading about 155 psi...on what is supposed to be an 8.5:1 engine. Does the compression check process cause the compressed gas to heat[thus getting an increase over "calculated" psi]? Or did the PO change heads on me (that would be a nice surprise)? By the way, all cylinders were within 5 psi of each other and all held pressure well.
Race Director
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From: Dayton, Ohio
Le Mans Master
Joined: May 2006
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From: League City Tx
Both of those estimates seem low to me. I had a 406" pontiac that I know we calculated ato 10.1-10.25-1 and it would push 175psi on average with the engine warm, throttle closed, with the throttle open it would push 190psi. We did this with two different gauges many times over the years.
Even my low compression (around 8.5-8.75-1) turbo hayabusa puts 132-135psi across the board with the throttle open and that is a small 83mm cylinder.
My jetski is rated at 6.5-1 compression and it checks out at 125-130 psi.
I have not checked my 8.5-1 L48 yet because I am afraid to see what it may or may not be....LOL
Even my low compression (around 8.5-8.75-1) turbo hayabusa puts 132-135psi across the board with the throttle open and that is a small 83mm cylinder.
My jetski is rated at 6.5-1 compression and it checks out at 125-130 psi.
I have not checked my 8.5-1 L48 yet because I am afraid to see what it may or may not be....LOL
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From: Melbourne, Fla. 6 months- New Middletown, Ohio 6 months
1971 350, 8.5:1 compression, 65,000 miles, warm engine. Average across all 8 with throttle wide open is 150 PSI. Dense air in Florida.
Last edited by Red 71; May 30, 2007 at 09:54 AM.
Burning Brakes
Joined: Dec 2003
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From: Adelaide South Australia
static compression ratio is based on total volume of cylinder at bottom dead centre / total volume at top dead centre.
With 14.7 psi ambient pressure at sea level, you would expect values of 125 @ 8.5:1 , 140 @ 9.5:1 etc.....
Actual values are higher as many posters have noted. Quite a few variables at play here, including valve timing and fact that compressed air heats up, causing expansion and increased pressure.
Figures of ~150 psi sound about right for stock heads and cam 8.5:1 CR
With 14.7 psi ambient pressure at sea level, you would expect values of 125 @ 8.5:1 , 140 @ 9.5:1 etc.....
Actual values are higher as many posters have noted. Quite a few variables at play here, including valve timing and fact that compressed air heats up, causing expansion and increased pressure.
Figures of ~150 psi sound about right for stock heads and cam 8.5:1 CR
Drifting
Joined: Feb 2005
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From: Richardson Tx.
Will this give you an idea of the compression ratio that you have (if you don't know it)?
Team Owner
Joined: Jul 2001
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From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
Bore (Inches)
Stroke (Inches)
Connecting Rod Length (Inches)
Cylinder Head Volume (cc)
Deck Height (Inches)
Head Gasket Bore (Inches)
Head Gasket Thickness (Inches)
Piston to Cylinder Wall Clearance (Inches)
Top Ring Land Height (Inches)
Piston Dome Volume (cc)
Negative number is dished piston. Positive number is a domed piston.