I’ve been comparing Summits CR calculator to several others including Wallace racing and Diamond pistons website. They are almost .75 in cr higher than other sites. Anyone notice that? Maybe CYA?

It's simple math. Not sure how they manage to come up with different answers. Ratio of volume at BDC vs. volume at TDC.

Do they factor in deck height, dome volume, valve relief notches, gasket width?

The compression calculator I use was about $8 at Staples.

If the Summit calculator disagrees with the figure that Wallace, Diamond and everyone else comes up with, I would just discount the Summit number.

Make sure you have you valve relief/dome/dish pluses and minuses correct.

Seems accurate to me.

Fundamentally, it produces the correct result for what inputs there are...... but there aren’t quite enough.

No input for gasket bore(so I have to assume it’s using the same as the cylinder bore), and it doesn’t take into account the area above the rings.

Modeling a 446 with dome pistons down the hole .010 and a 4.41 bore gasket that’s .045 thick, my staples calculator and the Summit calculator differed by .15:1 ....... the summit one being higher, since there was no input for the bigger gasket bore or the area above the rings.

If I remove the volume for the area above the rings and make the gasket bore the same as the cylinder bore, the result is within .02:1(which is likely from a difference in how far out the decimal point is taken for each entry).

Edit- it does appear there is a slight error with what’s its using for a conversion from CI to CC though.
4.35 x 3.75 x 8 cyls is 445.85ci..... which it has as correct.
But the conversion to CC is 16.387...... which is 7306.14cc.
It’s showing the displacement cc as 7309.02(16.393 conversion).

I use Diamond piston calculator. Its the best most accurate one I have found yet.

Summit does not use the head gasket diameter nor the area about the first ring. The diamond one does use more measurements. I had everything right and used it several times. Not sure why it’s so much higher.

Well, what are all your specs?

Btw...... my staples calculator agrees with the Diamond calculator.

with only a few things to factor I think they have it set up for higher results so it's more of "Max" # , and not a exact # that way the novice builder errors on the high side...let be honest if you build engines for a living your not using that calculator......I use it to bench race and get a vague idea.

I use United Machine calculator and I have used the Wallace calculator on the same motor and numbers used on both and saw about .3 ratio differences between those two

Mine is the button at the upper-right corner of my keyboard. Seems okay so far, is configurable to scientific and does unit conversions.