(dumb question alert!!)

I've wondered, when getting heads that are say, 15 degrees, or 18 degrees, or those dominion 32valve ones, why its necessary to get "18 degree pistons" or in the case of the dominions, "custom made pistons".

I can understand having to get pistons with the correct thickness to get optimal compression. I thought pistons tops were flat (some of them have little markings, look almost like valve markings or something). All it really does (with respect to air) is create vacuum to suck in air, compress the combustion mixture and push out the exhaust, makes me think it shoudl be able to do taht regardless of the angle that the air takes to come in with respect to the pistons.

 

It is not absolutely necessary, but encouraged. The issue is piston to valve clearance. With heads that have valves opening at an angle, in relation to the bore, different than oem, it is necessary to check piston to valve clearance. The clearance designed into an oem spec piston crown, is intended to accompany a 23* head. Having pistons with clearance designed in, based on the 18* (or less) platform, makes it easier to achieve the necessary clearance. I'd check it, regardless of intended application. In my book, those that believe they can just 'throw' in a set of aftermarket/custom pistons without physically checking any/all relevant clearance issues, should choose to do other things with their time. I for one, like the idea, of knowing exactly what I'm dealing with, regardless of the components I choose/use. It's part of meticulous engine building/blueprinting.

BTW, unless you worded your statement incorrectly, pistons having crowns of any given thickness, has little/no bearing on the compression ratio generated. Thickness having no bearing on a given compression hgt. of the piston.

 

Thanks. Measurements and their implications must be worked out before taking on such a project.

I figured a piston with a thicker crown would result in less total volume of the cylinder + combustion chamber at both TDC and the bottom of the stroke. Stroke volume would remain the same, resulting in increased compression. I must admit my practical knowledge is quite limited, this is based on my understanding of the mechanics involved.