compression ratio???

[scottp11]

I currently go to school for automotive tech, and we learned about doing compression testing. We were taught that if you take the PSI minus 5, then minus 14.7, then divide by 14.7, you come up to your compression ratio, to one. And the same goes for the opposite if you have your compression ratio you can find what your PSI should be.

Well... I did my PSI on my JDM B16A and came out with 210 PSI for each cylinder.
If my math is correct, that means my CR is 12.9:1. I thought a JDM B16 CR was 10.4:1 and that would be about 175.5 PSI.

Was I tought wrong? Help me out if you know... Thanks.

 

[phunky.buddha]

That's a very inexact way to guess your compression ratio... it depends on your cam profile and a bunch of other stuff too. If you have an intake cam that leaves the valves open after the piston starts moving up, you'll lose compression- you can also gain more depending on how your engine is set up too.

 

[HondaCivicSiR93]

Seems like a guess-timate type figure for a "perfectly" compressed car with absloutley no faults in the engine.

 

[scottp11]

Whats the formula to find the REAL compression ratio? Thanx for your help!

 

[phunky.buddha]

(volume of cylinder and combustion chamber @ BDC) / (volume of cylinder and combustion chamber @ TDC)

There you go.

 

[MikeBergy]

Originally posted by Calesta@May 26 2004, 02:46 PM
(volume of cylinder and combustion chamber @ TDC) / (volume of cylinder and combustion chamber @ BDC)

There you go.

Mike, you need to invert that... other wise your cr comes out to less than one.

 

[phunky.buddha]

Whoops- same thing... it's still the compression ratio- it's just not the same convention that everyone's used to. I guess I'm used to dealing with compression as fractions of a whole, since you're making things smaller after all.

compression ratio = (volume of cylinder and combustion chamber @ BDC) / (volume of cylinder and combustion chamber @ TDC)