Hot Rod Forum banner

How to tell what your compression is.....

5K views 21 replies 10 participants last post by  chucksrt 
#1 ·
Hey guys, I know alot of people dish out the classic "I have 9:1, or 10.5:1cr" without really knowing what their CR is. Can anyone tell me how you find out after the engine is built?
 
#8 · (Edited)
adantessr said:
If I remember right, subtract 14.7 from the compression psi and then divide that number by 14.7 and it will be close.
No no no...
that isn't even close!!
My cranking cylinder pressure is 200,
200-14.7= 185.3
185.3÷14.7=12.60

My 383 is 9.79 static CR and 8.7 dynamic CR.

12.60 compared to 9.79 is off by 2.81 points. That's worlds apart! If my engine had 12.60 compression with the cam I have, I would have to run uranium for fuel!!!

The only way to know for sure would be to know the engine specs! If you figure them out and still need help post them here and we will be glad to help!
 
#9 ·
CJ_1080 said:
No no no...
that isn't even close!!
My cranking cylinder pressure is 200,
200-14.7= 185.3
185.3÷14.7=12.60

My 383 is 9.79 static CR and 8.7 dynamic CR.

12.60 compared to 9.79 is off by 2.81 points. That's worlds apart! If my engine had 12.60 compression with the cam I have, I would have to run uranium for fuel!!!

The only way to know for sure would be to know the engine specs! If you figure them out and still need help post them here and we will be glad to help!
I already said I was sorry for the bad info. Why are you beating me up ?
 
#10 ·
Sorry bud, must have been typing while you were posting. Not trying to beat ya up! I just like to give examples instead of just telling someone they are wrong. That's how I've learned, and after all, that's why we all gather here, to share and learn! Sorry if I came off as a jerk! Just heard someone say at work today "I should run my pulling tractor off uranium due to the compression" and thought it sounded so funny I just had to use it somewhere!
 
#11 ·
Bad66 said:
Hey guys, I know alot of people dish out the classic "I have 9:1, or 10.5:1cr" without really knowing what their CR is. Can anyone tell me how you find out after the engine is built?
If you know the actual cylinder displacement of the block, you get the swept volume of the piston.

If you can get the casting number off the head you can get the combustion chamber volume.

These are two big players.

What you can't get with the engine assembled is the volume either positive or negative of the piston crown, that's a large player. But if you can get the piston part number that can be backed into, or if you know them to be stock to a certain model you can get close.

The remaining questions are how thick is the head gasket, again if you can get a part number that data cam be found, or even a type of material which you might be able to see a place where some hangs outside the head dimension. One piece steel tends to be .015 to .020 inch; compositions tend around .040 to .050 inch. The last unknown is whether the block's deck has been milled. OEM for Chevy is close to .020 to .025 inch. Other brand engines tend to stay fairly close to this figure as well.

So with some sleuthing you can get started and some assumptions about what could be the dimensions you can't measure you can at least get into the neighborhood.

I'm not to sure about the math of taking the compression pressure, subtracting a standard atmosphere dividing that result by 14.7 PSI to derive the CR. This is close on my old Harley and old Yamaha it computes about half a ratio low. For my L31 Chevy truck with an OEM cam and rockers it's nut's on the advertised. For the GMC with the built 350 it's a useless number.

Bogie
 
#12 ·
adantessr said:
I already said I was sorry for the bad info. Why are you beating me up ?
Because unfortunately, that's what a lot of people use the web for.

If you want to understand why there's a discrepancy between the two numbers, here goes.

Compression ratios, both static and dynamic are purely mathematical expressions based on geometry, not physics.

Air, or a fuel/air mixture heat up when compressed. Instantly.

If you look at the Ideal Gas Law;

PV = nRT

and move things around a bit, you have

PV/T = nR

"nR" is a constant, based upon what gas we are dealing with.

So as the volume decreases, "P/T" has to increase, so that it all still equals "nR", which hasn't changed.

Since we've established that compressing a gas increases "T", then "P" or pressure has to increase by an even larger factor, in order to keep up with the reduction in volume.

In essence, pressure in a cylinder does not rise at the same rate as volume falls. It rises faster. You will often see cranking pressures be 20 or more times the static compression ratio.

I'm sure my explanation was a clear as mud.
 
#13 ·
nofearengineer said:
Because unfortunately, that's what a lot of people use the web for.

If you want to understand why there's a discrepancy between the two numbers, here goes.

Compression ratios, both static and dynamic are purely mathematical expressions based on geometry, not physics.

Air, or a fuel/air mixture heat up when compressed. Instantly.

If you look at the Ideal Gas Law;

PV = nRT

and move things around a bit, you have

PV/T = nR

"nR" is a constant, based upon what gas we are dealing with.

So as the volume decreases, "P/T" has to increase, so that it all still equals "nR", which hasn't changed.

Since we've established that compressing a gas increases "T", then "P" or pressure has to increase by an even larger factor, in order to keep up with the reduction in volume.

In essence, pressure in a cylinder does not rise at the same rate as volume falls. It rises faster. You will often see cranking pressures be 20 or more times the static compression ratio.

I'm sure my explanation was a clear as mud.
Well, I am sure that I would have understood, if I had gone on to GMI (General Motors Institute) and studied to be a mechanical engineer, as I had originally intended, instead of going to work at the local Ford dealership in 1970 . As it is, it was as clear as mud. Thanks for trying anyway.
 
#14 ·
nofearengineer said:
Because unfortunately, that's what a lot of people use the web for.

If you want to understand why there's a discrepancy between the two numbers, here goes.

Compression ratios, both static and dynamic are purely mathematical expressions based on geometry, not physics.

Air, or a fuel/air mixture heat up when compressed. Instantly.

If you look at the Ideal Gas Law;

PV = nRT

and move things around a bit, you have

PV/T = nR

"nR" is a constant, based upon what gas we are dealing with.

So as the volume decreases, "P/T" has to increase, so that it all still equals "nR", which hasn't changed.

Since we've established that compressing a gas increases "T", then "P" or pressure has to increase by an even larger factor, in order to keep up with the reduction in volume.

In essence, pressure in a cylinder does not rise at the same rate as volume falls. It rises faster. You will often see cranking pressures be 20 or more times the static compression ratio.

I'm sure my explanation was a clear as mud.
Unfortunatly, adiabatic conditions only exist on the pages of text books. Otherwise your right one has to deal with T and P or P and T being related.

I'm out of here for Easter
Bogie
 
#20 ·
ScojoDak said:
Cylinder temp has absolutely nothing to do with measuring compression ratio.
Actually it would. It's just with a real engine ranther than one defined on sheet of paper, the sinking effects of the surrounding structure on compression heat would render the data useless. But on sheet of paper computing a cylinder's adiabatic compression effects, the temperature of compression (not combustion) would be very real, in a figurative way, and relateable to the compression ratio.

Bogie
 
#21 ·
nofearengineer said:
You apparently failed to read any of this thread before posting.
You know, I think you're right. My apologies :thumbup:



oldbogie said:
Actually it would. It's just with a real engine ranther than one defined on sheet of paper, the sinking effects of the surrounding structure on compression heat would render the data useless. But on sheet of paper computing a cylinder's adiabatic compression effects, the temperature of compression (not combustion) would be very real, in a figurative way, and relateable to the compression ratio.
You're close. Perhap, this will help.

 
#22 ·
Thanks for the info!!

oldbogie said:
If you know the actual cylinder displacement of the block, you get the swept volume of the piston.

If you can get the casting number off the head you can get the combustion chamber volume.

These are two big players.

What you can't get with the engine assembled is the volume either positive or negative of the piston crown, that's a large player. But if you can get the piston part number that can be backed into, or if you know them to be stock to a certain model you can get close.

The remaining questions are how thick is the head gasket, again if you can get a part number that data cam be found, or even a type of material which you might be able to see a place where some hangs outside the head dimension. One piece steel tends to be .015 to .020 inch; compositions tend around .040 to .050 inch. The last unknown is whether the block's deck has been milled. OEM for Chevy is close to .020 to .025 inch. Other brand engines tend to stay fairly close to this figure as well.

So with some sleuthing you can get started and some assumptions about what could be the dimensions you can't measure you can at least get into the neighborhood.

I'm not to sure about the math of taking the compression pressure, subtracting a standard atmosphere dividing that result by 14.7 PSI to derive the CR. This is close on my old Harley and old Yamaha it computes about half a ratio low. For my L31 Chevy truck with an OEM cam and rockers it's nut's on the advertised. For the GMC with the built 350 it's a useless number.

Bogie
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top