Computing "Actual" Compression Ratio's...
Thread Starter
Former Vendor
Joined: Mar 2004
Posts: 1,518
Likes: 7
From: Valencia CA
Computing "Actual" Compression Ratio's...
I get a fair amount of phone calls at AFR with our customers trying to figure out exactly what it takes to achieve such and such compression ratio's etc., etc.....I posted a reply in another thread that I spent a fair amount of time trying to clear up some issues regarding CR and what information is needed to properly calculate it.
I thought it was worthy of a seperate thread to try and reach more people with the information. This was my response "copied and pasted" below....
Phil...
You have to measure how how much your pistons are "out of the hole" to get accurate compression ratio measurements. Every shortblock is different and it is misleading for anyone to claim you will have "X" compression ratio with such and such a gasket and the heads milled a certain amount.
If your pistons are .004-.005 out of the hole, with an .040 gasket and a 62 cc chamber, you will have exactly 11 to 1 CR.
A 62 cc chamber USUALLY requires an .024 mill (.006 flatmill per cc), but there is always slight production tolerances and your stock AFR chamber might have been as small as 65 cc's or as large as 67cc's. Checking all these fact and figures and correcting for them is all part of "blueprinting" and is usually helpfull in achieving the maximum results of ANY engine combination.
If your pistons are sticking further out of the hole than I previously quoted, you would need less milling to achieve 11 to 1....if they are close to zero deck or perhaps down the hole a couple of thousanths, you would need additional milling to achieve 11 to 1. I am of course using 11 to 1 purely as an example....if it's exactly 10.8 CR that you are trying to achieve, all of the information I have touched on still applies.
By the way, Lou's 59 cc chambers yeild 11 to 1 (approximately) due to the fact the cams he typically installs require piston notching which usually are worth -2 to -3 cc's of volume.....his 59 cc combination now acts like a flat top piston with a chamber 2-3 cc's larger (61-62) bringing him right at the example I described above.
Information needed for ACCURATE CR ratio:
Engine displacement? (exact bore and stroke)
How far is piston in or out of the hole? (measure a few and get an average)
Dome volume, dish volume, valve reliefs, or flat top piston???? (flat top would be zero cc's, all others would be some negative or positive figure)
Head gasket thickness? (some manufacturers will give you exact "crushed cc volume)
Combustion chamber volume?? (actually cc'ing a chamber would be preferred if possible)
Also, if you want to very thorough, you also must account for the volume displaced between the top ring land and the top of the piston. It is usually a small amount (less than 1 cc), but if not accounted for your "actual" CR would be slightly less than you think. I have a good computer program at AFR that quickly figures out CR assuming you can input all the data I referenced above. It also uses some given formula to try and account for the ringland cc's automatically.
Hope this info helps out...
Regards,
Tony M.
I thought it was worthy of a seperate thread to try and reach more people with the information. This was my response "copied and pasted" below....
Phil...
You have to measure how how much your pistons are "out of the hole" to get accurate compression ratio measurements. Every shortblock is different and it is misleading for anyone to claim you will have "X" compression ratio with such and such a gasket and the heads milled a certain amount.
If your pistons are .004-.005 out of the hole, with an .040 gasket and a 62 cc chamber, you will have exactly 11 to 1 CR.
A 62 cc chamber USUALLY requires an .024 mill (.006 flatmill per cc), but there is always slight production tolerances and your stock AFR chamber might have been as small as 65 cc's or as large as 67cc's. Checking all these fact and figures and correcting for them is all part of "blueprinting" and is usually helpfull in achieving the maximum results of ANY engine combination.
If your pistons are sticking further out of the hole than I previously quoted, you would need less milling to achieve 11 to 1....if they are close to zero deck or perhaps down the hole a couple of thousanths, you would need additional milling to achieve 11 to 1. I am of course using 11 to 1 purely as an example....if it's exactly 10.8 CR that you are trying to achieve, all of the information I have touched on still applies.
By the way, Lou's 59 cc chambers yeild 11 to 1 (approximately) due to the fact the cams he typically installs require piston notching which usually are worth -2 to -3 cc's of volume.....his 59 cc combination now acts like a flat top piston with a chamber 2-3 cc's larger (61-62) bringing him right at the example I described above.
Information needed for ACCURATE CR ratio:
Engine displacement? (exact bore and stroke)
How far is piston in or out of the hole? (measure a few and get an average)
Dome volume, dish volume, valve reliefs, or flat top piston???? (flat top would be zero cc's, all others would be some negative or positive figure)
Head gasket thickness? (some manufacturers will give you exact "crushed cc volume)
Combustion chamber volume?? (actually cc'ing a chamber would be preferred if possible)
Also, if you want to very thorough, you also must account for the volume displaced between the top ring land and the top of the piston. It is usually a small amount (less than 1 cc), but if not accounted for your "actual" CR would be slightly less than you think. I have a good computer program at AFR that quickly figures out CR assuming you can input all the data I referenced above. It also uses some given formula to try and account for the ringland cc's automatically.
Hope this info helps out...
Regards,
Tony M.
Melting Slicks
Joined: Oct 2002
Posts: 2,200
Likes: 0
Good info Tony.... 
I took my heads off yesterday and I am seeing approximately .007 to .008 piston sticking out of the hole. I was thinking of using the .045 Cometic gasket with 205s milled .024, to 62cc. Supposedly this will put my quench height at .037, right between .035 and .40 (right?). What static compression will this be (approximately)???
I took my heads off yesterday and I am seeing approximately .007 to .008 piston sticking out of the hole. I was thinking of using the .045 Cometic gasket with 205s milled .024, to 62cc. Supposedly this will put my quench height at .037, right between .035 and .40 (right?). What static compression will this be (approximately)???
Last edited by SideStep; Oct 24, 2004 at 10:39 PM.
Le Mans Master
Joined: Nov 2003
Posts: 8,003
Likes: 248
From: Tampa Fl
St. Jude Donor '12-'13-'14
Your more worthy than me doing a cut and paste for you Tony so here you go again
Thank you again
and thank you for being one of the few sponsors who give advise on here. It makes us feel comfortible about ordering your product (lower the price though
) To many sponsors on here ( and I just counted over 80 on the left side) that say nothing and just sell products.
There are about 10 sponsors on here that help us guys
and we know who you are without listing all of them
Carman, Ill call you this week for the cam installation
Thank you again
and thank you for being one of the few sponsors who give advise on here. It makes us feel comfortible about ordering your product (lower the price though
) To many sponsors on here ( and I just counted over 80 on the left side) that say nothing and just sell products.There are about 10 sponsors on here that help us guys
and we know who you are without listing all of them
Carman, Ill call you this week for the cam installation
Burning Brakes
Joined: Nov 1999
Posts: 901
Likes: 83
From: Los Angeles
Thanks for the clarification. I always wondered why LGM's were 59cc when everything else pointed to 62 cc for 11:1. It also serves mentioning that the stock AFR's provide an addl amount of (somebody help me with this) P/V clearance over a stock LS6 head to begin with. What a good post thank Tony.
TTT
TTT
