Way to Determine My Static Compression??
Way to Determine My Static Compression?? Well to make a long story short there is confusion about what the exact static compression of my forged motor is and I need to know.
How can I go about testing this? Compression tester gives you a PSI right? I heard there is a formula or what not to determine the compression based on the results?
Thanks in advance for any help.
How can I go about testing this? Compression tester gives you a PSI right? I heard there is a formula or what not to determine the compression based on the results?
Thanks in advance for any help.
TECH Resident
Joined: Nov 2001
Posts: 894
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From: Chicago, IL
I think you can add the displacement of the cylinder to the size of the combustion chamber and whatever size valve reliefs you might have. Then divide that total by the size of the combustion chamber and valve reliefs. Seems like they showed this on Shadetree Mechanic once.
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Staging Lane
Joined: Jan 2004
Posts: 74
Likes: 0
From: East Hartford, CT
Originally Posted by SilverGhost
Guess would be better than nothing right now.
I have TEA Stage II 5.3 heads, can't remember what cc that is.
Diamond flat top pistons with -2CC reliefs in them.
I have TEA Stage II 5.3 heads, can't remember what cc that is.
Diamond flat top pistons with -2CC reliefs in them.
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On The Tree
Joined: Dec 2003
Posts: 179
Likes: 0
From: Bloomington, MN
call some race engine machine shops in your area and ask them if they have a whistler[its a tool that is used at race tracks to check compression, it's inserted into the spark plug hole and using sound waves/frequency.it can determine exact compression[static]. i have one but it wont do you any good because i live in mn.
Originally Posted by triumphman
call some race engine machine shops in your area and ask them if they have a whistler[its a tool that is used at race tracks to check compression, it's inserted into the spark plug hole and using sound waves/frequency.it can determine exact compression[static]. i have one but it wont do you any good because i live in mn.
I'll just buy it if its not too expensive, would be a good tool to have around.
yeah but i have valve reliefs in my diamonds.
my mechanic buddy( steve from park ridge John) said he can test the compression and convert it with a chart he has so i'm gonna try that..
my mechanic buddy( steve from park ridge John) said he can test the compression and convert it with a chart he has so i'm gonna try that..
TECH Apprentice
Joined: Jan 2004
Posts: 337
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From: Frisco TX (Dallas Area)
Here's a quick formula for static compression on a stock engine:
Combustion Chamber Volume = heads cc x .061 (for c.i.) + valve relief cc x .061 + gasket compressed thickness x bore x bore x .7854 (PI devided by 4) for c.i..
Total Cylinder volume = Swept Volume (346 c.i./8cyl) + Combustion Chamber Volume
Static Compression Ratio = Total Cylinder Volume/Combustion Chamber Volume.
For a 62cc head and 2 cc valve relief:
Combustion Chamber = (62+2) x .061 + .053 x 3.900 x 3.900 x .7854
= 3.904 (Heads & valve relief) + .633 (gasket volume) = 4.537 c.i.
Total Cylinder Volume = 43.25 + 4.537 = 47.787 c.i.
Static Compression Ratio = 47.787 (total) / 4.537 (combustion chamber)
= 10.53:1
If the piston is slightly above or below the block deck (stroke/2 + rod length + piston compression height not exactly = 9.255), then the combustion chamber volume needs to be adjusted for that, but it won't change the ratio much. The deck height is normally 9.255 on a stock, unrebuilt block. A typical rebuilt block will have a deck of 9.250 (.005 off) and a bore of 3.905 (.005 cylinder hone) so the number will be slightly different, but again, not much. HTH
Can't proof read!
Combustion Chamber Volume = heads cc x .061 (for c.i.) + valve relief cc x .061 + gasket compressed thickness x bore x bore x .7854 (PI devided by 4) for c.i..
Total Cylinder volume = Swept Volume (346 c.i./8cyl) + Combustion Chamber Volume
Static Compression Ratio = Total Cylinder Volume/Combustion Chamber Volume.
For a 62cc head and 2 cc valve relief:
Combustion Chamber = (62+2) x .061 + .053 x 3.900 x 3.900 x .7854
= 3.904 (Heads & valve relief) + .633 (gasket volume) = 4.537 c.i.
Total Cylinder Volume = 43.25 + 4.537 = 47.787 c.i.
Static Compression Ratio = 47.787 (total) / 4.537 (combustion chamber)
= 10.53:1
If the piston is slightly above or below the block deck (stroke/2 + rod length + piston compression height not exactly = 9.255), then the combustion chamber volume needs to be adjusted for that, but it won't change the ratio much. The deck height is normally 9.255 on a stock, unrebuilt block. A typical rebuilt block will have a deck of 9.250 (.005 off) and a bore of 3.905 (.005 cylinder hone) so the number will be slightly different, but again, not much. HTH
Can't proof read!
Last edited by TeeKay; Feb 28, 2004 at 03:19 PM.
TECH Enthusiast
Joined: Sep 2003
Posts: 730
Likes: 66
Pistons come .007 out of the hole and 3.905 is a .007 hone. Piston displacement out of the hole of .007 is the same as a head mill of .012 and it is significant.
The tool for measuring the compression in the car is for dynamic compression and takes into account the lagging intake valve closure and gives the compression after the valve closes and is much lower than static compression.
The tool for measuring the compression in the car is for dynamic compression and takes into account the lagging intake valve closure and gives the compression after the valve closes and is much lower than static compression.
Try my site. (You have to log in to use it).
The only way to determine static compression ratio is to measure it. If you have the numbers go here.
Compression Ratio Calculator
I ran some numbers and here's what I came up with:
You Entered:
Bore 3.9 (inches)
Stroke 3.62 (inches)
Rod Length 6.1 (inches)
Cyl. Head Vol. 62 (cc)
Deck Height 0 (inches)
Head Gasket Bore 4.166 (inches)
Head Gasket Thickness 0.052 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -2 (cc)
Compression Ratio:
10.365 : 1
Bore / Stroke Ratio:
1.077 : 1
Rod / Stroke Ratio:
1.685 : 1
Total volume:
784.451 cc's
Compressed volume:
75.679 cc's
You can change the numbers and figure out what you have.
As for determining the compression ratio from a compression tester, you can't. With valve timing, overlap, and other things it doesn't come out right. I have a calculator on the site for that too but I would NOT use it to determine my compression ratio.
The only way to determine static compression ratio is to measure it. If you have the numbers go here.
Compression Ratio Calculator
I ran some numbers and here's what I came up with:
You Entered:
Bore 3.9 (inches)
Stroke 3.62 (inches)
Rod Length 6.1 (inches)
Cyl. Head Vol. 62 (cc)
Deck Height 0 (inches)
Head Gasket Bore 4.166 (inches)
Head Gasket Thickness 0.052 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -2 (cc)
Compression Ratio:
10.365 : 1
Bore / Stroke Ratio:
1.077 : 1
Rod / Stroke Ratio:
1.685 : 1
Total volume:
784.451 cc's
Compressed volume:
75.679 cc's
You can change the numbers and figure out what you have.
As for determining the compression ratio from a compression tester, you can't. With valve timing, overlap, and other things it doesn't come out right. I have a calculator on the site for that too but I would NOT use it to determine my compression ratio.
On The Tree
Joined: Dec 2003
Posts: 179
Likes: 0
From: Bloomington, MN
Originally Posted by SilverGhost
Awesome. Can you tell me where you got it and how much it was? It will determine exact static compression in ratio terms like 10:1 etc?
I'll just buy it if its not too expensive, would be a good tool to have around.
I'll just buy it if its not too expensive, would be a good tool to have around.
Nice, thanks for all the replies everyone, I appreciate it.
I really hope my **** isn't at 10.3:1, that would mean its WAY WAY less than what I ordered.
I know my gaskets are thinner than the .52 I think I saw in one of the calculations posted here(thanks for doing all that by the way) I think they are .41.
Looks like I need to try and gather some info here..
I really hope my **** isn't at 10.3:1, that would mean its WAY WAY less than what I ordered.
I know my gaskets are thinner than the .52 I think I saw in one of the calculations posted here(thanks for doing all that by the way
) I think they are .41. Looks like I need to try and gather some info here..
Originally Posted by Spinmonster
The deck height assumes that the TDC is at zero but the pistons come out of the hole by .007". Would that be +.007 for the deck height or -.007?
Originally Posted by SilverGhost
I know my gaskets are thinner than the .52 I think I saw in one of the calculations posted here(thanks for doing all that by the way) I think they are .41.
You Entered:
Bore 3.9 (inches)
Stroke 3.62 (inches)
Rod Length 6.125 (inches)
Cyl. Head Vol. 62 (cc)
Deck Height -.007 (inches)
Head Gasket Bore 4.0 (inches)
Head Gasket Thickness 0.040 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -2 (cc)
Compression Ratio:
10.993 : 1
Bore / Stroke Ratio:
1.077 : 1
Rod / Stroke Ratio:
1.692 : 1
Total volume:
779.702 cc's
Compressed volume:
70.93 cc's
Ok, so looks like we are getting closer to an answer.
I need to find out if my block was decked, that would change that deck height obviously and therefore raise the compression..I think the block was in fact decked.
Thanks for you help, I like your site by the way, the calculator is pretty sweet..
I need to find out if my block was decked, that would change that deck height obviously and therefore raise the compression..I think the block was in fact decked.
Thanks for you help, I like your site by the way, the calculator is pretty sweet..