I had this question asked and its a good one!

[grumpyvette]

I had this question asked and its a good one

 

Using the CR calculator on the first link above,

 

http://www.newcovenant.com/speedcrafter/calculators/compressionratio.htm

 

if I input a piston top volume that is positive for a dished piston, then change the piston top volume to zero for a flat-top, the CR goes down. If I add a negative number (for a dome), the CR drops again. Isn't this backwards? Is the calculator messed up, or is there something I'm missing.

 

I inputted the specs into the calculator taken from here. http://www.beckracing.com/page48.htm

 

 

 

yes the instructions on the site seem backwards

but the calculator itself is correct

adding dome volume does increase cpr,removing valve notch volume from a flat top does REDUCE cpr

 

this is listed at +20 cc but it MEANS +20cc (to the total combustion chamber net volume)

this is listed at +5.5cc but its really +5.5cc(to the total combustion chamber net volume)

 

 

 

remember what they are saying is that that plus 20cc ADDS 20cc TO THE COMBUSTION CHAMBER VOLUME, which reduces the TRUE compression, thus when useing the calculator you would ADD a NEGATIVE VOLUME TO THE TOTAL FOR A DISHED PISTON AND SUBTRACT A DOME VOLUME

heres a ZERO VOLUME (FLAT TOP)

this piston has a slight dome and has a NET 1cc REDUCTION IN CHAMBER VOLUME

as the dome has 1cc MORE volume than the combined valve notches

 

it will be easier to understand if you use the listed cc volumes quoted for the pistons and add or subtract them from the combustion chamber volume data and always leave the piston input as a true flat top while figuring the cpr with the calculator