compression ratio
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
compression ratio
I have my 402 big block on an engine stand, with the heads attached.
I need to know the existing compression ratio. I am hoping it is 10 -1.
I have a burrett and I know how to use it. Is there a way to fill the cylinder with fluid (marvel mystery oil) and determine, based on how many cc's the cylinder holds, what my compression ratio is? How many cc's of oil equals a 10-1 compression ratio?
I need to know the existing compression ratio. I am hoping it is 10 -1.
I have a burrett and I know how to use it. Is there a way to fill the cylinder with fluid (marvel mystery oil) and determine, based on how many cc's the cylinder holds, what my compression ratio is? How many cc's of oil equals a 10-1 compression ratio?
Race Director
Joined: Oct 2005
Posts: 12,468
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
I have my 402 big block on an engine stand, with the heads attached.
I need to know the existing compression ratio. I am hoping it is 10 -1.
I have a burrett and I know how to use it. Is there a way to fill the cylinder with fluid (marvel mystery oil) and determine, based on how many cc's the cylinder holds, what my compression ratio is? How many cc's of oil equals a 10-1 compression ratio?
I need to know the existing compression ratio. I am hoping it is 10 -1.
I have a burrett and I know how to use it. Is there a way to fill the cylinder with fluid (marvel mystery oil) and determine, based on how many cc's the cylinder holds, what my compression ratio is? How many cc's of oil equals a 10-1 compression ratio?
Should get you there. Assuming you know the bore is stock.
Doug
Burning Brakes
Joined: Jul 2003
Posts: 786
Likes: 15
From: Emporia, pay no attention to that man behind the curtain, KS
Static compression ratio (as you are trying to determine) is a ratio of the total volume of the cylinder/combustion chamber with the piston at BDC vs. TDC. In theory, you can fill the cylinder/combustion chamber with the piston at TDC, take your reading, then move the piston to BDC and fill again (with both valves closed, of course). Divide the larger number by the samller number to get the CR. The amount of oil needed is determined by such variable as piston configuration, combustion chamber volume, head gasket compressed thickness and such.
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Race Director
Joined: Jan 2003
Posts: 17,652
Likes: 3,662
From: Central Arkansas
Will this really work on an assembled engine? I would think in the time it took to fill the cylinder that some oil (MMO is fairly thin) would have started to leak through the ring end gaps. Maybe not enough to matter?
Race Director
Joined: Apr 2015
Posts: 17,571
Likes: 3,460
From: Fresno California
You need to do the math on the cylinder/piston and then use the oil or fluid to cc the combustion chamber in the heads. You also need to factor in head gasket thickness. Filling the cylinders with oil is not the way to go about it.
Burning Brakes
Joined: Mar 2005
Posts: 1,231
Likes: 34
From: Ontario
Whistler ?
Years ago when I was involved in stock car racing we had a compression ratio rule . The tech inspector had a machine we called a whistler . It was put in the spark plug hole and the engine was cranked over to determine the comp ratio .
I don't know if they are available now .
Here it is ! Bill
http://store.katechengines.com/whist...ster-p174.aspx
I don't know if they are available now .
Here it is ! Bill
http://store.katechengines.com/whist...ster-p174.aspx
Last edited by 63split63; Oct 7, 2015 at 07:22 PM.
Race Director
Joined: Aug 2004
Posts: 15,295
Likes: 2,261
From: NorCal
This is a deep memory from a long time ago, but I seem to remember a simple mechanical device like an air pressure gauge that you could just screw into the spark plug opening, and then crank the engine over. It would record the maximum and minimum pressures (two indicator needles), and from that you could calculate the CR. I'm talking about a looong time ago, like back in the 70s. Does that sound familiar to anyone else?
Team Owner
Joined: Apr 2008
Posts: 31,705
Likes: 2,375
From: Middle TN by way of KY, OH, VA, IL, CA, FL, NY, SC, HI
This is a deep memory from a long time ago, but I seem to remember a simple mechanical device like an air pressure gauge that you could just screw into the spark plug opening, and then crank the engine over. It would record the maximum and minimum pressures (two indicator needles), and from that you could calculate the CR. I'm talking about a looong time ago, like back in the 70s. Does that sound familiar to anyone else?
Probably like the racing test device mentioned above.
However, all this could be done by pulling a head and taking a small number of measurements.
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Racer
Joined: May 2008
Posts: 361
Likes: 22
I use this on my turbo car, it seems to be spot on for tuning purposes. there is also the squish, that is important on the turbo car, but it may be no affect on N/A engine.
http://www.csgnetwork.com/compcalc.html
http://www.csgnetwork.com/compcalc.html
Race Director
Joined: Jan 2003
Posts: 17,652
Likes: 3,662
From: Central Arkansas
The last engine I had built was an iron head Ford engine the buider said he wanted to hold to 9.0 compression. From his build sheets I was able to find enough information to calculate my compression ratio with the exception I think I had to assume a compressed head gasket thickness. I had the head chamber volumes after his head work, the machined bore and stroke dimensions, and the piston dish volume. Amazingly, it calculated out to 8.99 compression ratio.
My point being that if you have engine blueprinting and build sheets from a builder you may be able to caluclate it with a fair degree of accuracy.
My point being that if you have engine blueprinting and build sheets from a builder you may be able to caluclate it with a fair degree of accuracy.
Le Mans Master
Joined: Apr 2001
Posts: 5,426
Likes: 1,244
From: Sarver Pa
2021 C1 of the Year Finalist - Modified
Filling the engines chamber with oil is the most accurate way to do it. BUT , the rings need to be sealed off. I smear grease or vaseline around the top of the piston then put the head back on with a used head gasket. You must also tilt the engine so the plug hole is at the top. Fill the chamber to the bottom of the threads and you've got it ! I used to race NASCAR short track and we had a compression rule. I stayed out of trouble using this method and went fast.
Thread Starter
Racer
Joined: Sep 2014
Posts: 425
Likes: 32
From: richmond virginia
Filling the engines chamber with oil is the most accurate way to do it. BUT , the rings need to be sealed off. I smear grease or vaseline around the top of the piston then put the head back on with a used head gasket. You must also tilt the engine so the plug hole is at the top. Fill the chamber to the bottom of the threads and you've got it ! I used to race NASCAR short track and we had a compression rule. I stayed out of trouble using this method and went fast.
Race Director
Joined: Jan 2003
Posts: 17,652
Likes: 3,662
From: Central Arkansas
Filling the engines chamber with oil is the most accurate way to do it. BUT , the rings need to be sealed off. I smear grease or vaseline around the top of the piston then put the head back on with a used head gasket. You must also tilt the engine so the plug hole is at the top. Fill the chamber to the bottom of the threads and you've got it ! I used to race NASCAR short track and we had a compression rule. I stayed out of trouble using this method and went fast.
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,264
The attached article may be useful, and here's the link to the CR calculator I use: http://www.csgnetwork.com/compcalc.html
Duke
Duke
