CR calculation, deck height/effective dome volume?

nm9stheham said
Just working out the volume above deck for the .036" above deck, I get 7.7 cc's above deck with no eyebrows, and since the eyebrow volume is likely in the 5 to 6 cc range, then your 2.2 cc number looks about right. If you want to use this number in a formula, then treat it like a domed piston with deck height = .000" and a dome volume of 2.2 cc's.

However, there are easy tool to compute the SCR without all of this rigamarole. You can just directly enter numbers like the +.036" deck height and the eyebrow volume, etc.
See here for the one I like: Dynamic CR

BTW, .036" deck height is not normal for a 340 piston. If it is a stock-like replacement (which I would expect your Speed Pro pistons to be), then the deck height would be around +.018". Are you sure you measured this right? Or has the block been decked?

Actually, the .017-.018 positive deck height was where the factory wanted it. Most were way lower than that.

If you had heads that had chambers on the low side of volume, rods on the long side of tolerance, Pistons with the pins low on the tolerance, then, if you were lucky and the decks we cut so you had .018 positive, you could get close to the 10.5:1 CR the 340 was advertised at. If you were real lucky, you get everything on the end of tolerance for max CR, you could end up at about 10.75:1...if you were lucky.

The most I'd stick one out is .075 and that would be with .055 gasket. If the gasket is thinner, you can move the piston down accordingly.

Right now I'm .045 out on mine, so his isn't unheard of.