I see Posts referring to flat top pistons with valve reliefs as dished pistons, and vice versa. There is a difference, right?

 

yes flat top and dish refer the the top of the piston, totally seperate issue from valve reliefs

 

For some of us, yes.

Flat tops are ..... flat. Their top surface is level with the "compression height" all the way across. They may have 2, 4, or zero valve reliefs.

Dished pistons are .... dished. Their top is level with the "compression height" around the edge, and then they have a scooped-out area in the center. This area may cover all of the piston top except for a narrow strip around the outside; or it may be a "reflection" of the shape of the chamber. It may have 2, 4, or zero valve reliefs. In any case, it is not "flat".

Domed pistons are .... domed. They have a raised area, the "dome", that fills part of the chamber, and almost always have 2 valve reliefs. Not 4, not zero.

I can accept no responsibility for what other people post, and erroneously call the one thing or the other. Regardless of what the unlearned call something, it remains, what it is. Those who wish to get accurate discussion, should be careful to describe their parts accurately.

 

"Dome" or "dish" volume is really the important part (along with quinch height). My pistons have 12cc volume, which includes the valve reliefs and a slight dish.

If the volume is zero, it's an absolutely flat top piston - don't see them very often in SBCs. If the volume is negative, it's got a dome top.

Automotive terminology is rarely purely defined. Know your audience.

 

on my compression calculator if I use a positive number for dome cc then my compression goes up, if I use a negative number it goes down.

 

Thats the way it should work.

 

The CR calculators don't all handle that the same way.... if they ask for "piston volume" or some such, then positive will indicate a dome, and CR will increase; if they're thinking in terms of "chamber volume", then positive numbers indicate a dish, and CR wil go down. You can write one that will handle it either way.

You gotta pay attention to your speicifc calculator, to avoid getting backwards results.

 

A dome is a negative dish, and vice versa.

 

Obviously, no convention there, either.

Know your application...