How does boost affect compression?

[GTEATER]

A stock bore 231 with 150# of cylinder pressure cold will have what ultimate compression @ 25# of boost?

Is there a formula to figure this?

thanks

 

[1badTTA]

OK,
Assuming it is a GN engine with 8:1cr, you are looking at 14.7psi (at lea level) x 8.0(cr) x 1.1 (for each time the engine turns over on the comp stroke which should be 4 times to get 150#) - 14.7 (correcting for the gauge reading 0 to begin with) = 150#

example (14.7x8x1.1x1.1x1.1x1.1)-14.7=150psi

now add boost of 25 psi and it would be 14.7+25=39.7

example (39.7x8x1.1x1.1x1.1x1.1)-14.7=450.298psi

now assume that you will get more blowby with higher cylinder pressures and will also bleed more off between Comp strokes.

example (39.7x8)-14.7=302.9psi

now I am not sure what the real formula is for a running engine under boost since it depends mainly on the effiency of the engine, but the above numbers should give you a ballpark figure. I know the first formula has always been pretty close when I have done compression checks turning the engine over 4 times.

HTH

BTW 302.9psi is roughly equal to 20:1cr

 

[GTEATER]

Its actually a TTA but I understand the theory.

So at 25# with a fresh engine the cr would be well over 20:1? Had no idea it was that high.