compression ratio ???????

[Highbanked Hauler]

 I am not an engine person so I ask.   Is there a way to find the comp. ratio without pulling an engine apart ??

[justcruisin]

If you are unsure of the specs the only way is to pull a head, however, you can get a rough guesstimate  by doing a compression check, and comparing it with idle vacuum. For example if you have a cranking compression of around 180 - 200psi and around 10 - 12" of vacuum @ idle you should be somewhere around 10 - 10.5:1. Not an exact thing as there are a lot of variables. My previous engine which was near stock had 160psi and idled with 16" of vacuum, it was around 8.8:1.

[firefighter3931]

The only accurate way to determine static compression is to remove the cylinder heads and measure piston to deck height and chamber volume. Then you do the math taking into consideration head gasket volume and valve relief area.  


Ron

[Highbanked Hauler]

The only accurate way to determine static compression is to remove the cylinder heads and measure piston to deck height and chamber volume. Then you do the match taking into consideration head gasket volume and valve relief area. 


    Thats what I  thought. I was just looking for an easy way out.

[HPP]

Do you know the cam specs? Knowing cranking cylinder pressure and the cam specs, you could back into possible ratios using a calculator.

Ultimately, the dynamic cylinder pressure is what matters most for octane tolerance.

[Highbanked Hauler]

Do you know the cam specs? Knowing cranking cylinder pressure and the cam specs, you could back into possible ratios using a calculator.

Ultimately, the dynamic cylinder pressure is what matters most for octane tolerance.

  So you can get an idea by how high the compression gauge goes  The 440 in my  69 hits 160 lb. and it was built as a 9-1 comp. motor.   The 440 I bought for my 68 is supposed to be a 12-1 ratio  and it  will idle on premium  but I haven't made it work yet. I am figuring if it is  12-1 comp.it will need  E85 ..