cylinder pressure

[polywideblock]

 I've got a 340 that is supposed to have less than a 1000k on a rebuild I've done a compression test and got 7 cylinders at 155 psi and one at 157 so its sealing alright .can i work out my compression ratio from my cylinder pressure?

[Cooter]

According to some here, there's no way to get an acurate ratio using the "14.7" method. (Formula using Cyl. pressure)

[XH29N0G]

There are several issues.  First, the valves do not necessarily close at the bottom of the stroke, so some air bleeds off, and second, during the compression stroke, the air inside the cylinder heats up due to adiabatic processes (work done).  Since pressure depends on temperature, the pressure will be slightly off (I do not know how much - and have not done the calculation).  Finally, your engine block may also change the temperature of the gas due to simple heat transfer - also influencing pressure.

All in all, my best guess is that the biggest issue is where on the compression stroke the valve closes due to the cam profile.

You also can change pressure by leaving the butterflies closed (drawing a vacuum in the manifold) but I do not know how much that does.

[Mebsuta]

You can play with this calculator to extrapolate, but you have to know intake closing point of your camshaft, seat to seat, but that can be measured at different points.  I think it's usually at .006 lifter rise.  Comp and some others measure that way.  Isky and Crane do it a little differently.    

http://www.wallaceracing.com/dynamic-cr.php