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Discussion Starter · #1 ·
There must be an engine builder/guru here.
what's the difference between dished pistons and dome top pistons in an msx 125 for instance? lets say for instance the msx has the combustion chamber in the head and a dished piston, what would the effect be if a dome top piston was fitted, obviously one that would not strike the valves. I know you would get higher compression but is that the only reason?
I have been looking into piston design and there are 3 basic designs flat, dome and dished, read a lot of conflicting stuff on the effects of the different pistons, can anybody enlighten me.
 

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There's no short, concise, answer...primarily because the effects are varied and piston types are spec'd based on engine (primarily head) design.

In a large coconut shell, it's a toss-up between flat top & dished as to which offers the most efficient, and detonation-resistant, combustion chamber. Both types leave one side of the combustion chamber with a smooth, unobstructed, path for flame travel. Domed/pop-up pistons present the greatest obstacles to flame propagation...as well as even distribution of the incoming air/fuel mix and are thus more prone to detonation...as well as requiring a richer "optimal" air:fuel ratio.

Broadly speaking: flat-tops work best with a wedge-profiled, cardioid, combustion chamber; dished pistons work best with a shallow-roofed combustion chamber; domed/pop-up pistons work best with tall-roofed combustion chambers, such as pent-roof or hemispherical, that have a large volume.

From an efficiency/detonation-resistance viewpoint, the ideal head would have high-swirl/high-velocity ports and a shallow, hemispherical dome that's paired with dished piston that has a good "squish band" at its perimeter and a centrally-located spark plug. The downside is limited compression, it's tough getting much above ~9.5:1 and still fit decent-sized valves.

The flat-top piston & cardioid combustion chamber combo has typically been seen with pushrod engines, which are mostly automotive applications.

A hemispherical chamber allows larger valves, due to their angles. And, this type of engine architecture is well-suited to small-bore & OHC engines...as are used in bikes. Mostly, bike pistons have odd-shaped domes with valve-reliefs...and a squish band around the outer perimeter. It's an ass-backward compromise that works better than seems possible. However, there are also other factors in play...like bore size and rpm range. Bike engines tend to have small (sometimes outright tiny) bores. That has a big effect, since the flame front doesn't have to travel anywhere near as far as with a typical 6000-rpm automotive engine with a 100mm+ bore diameter.

The bottom line is that unless you're doing your own engineering, this topic is mostly good mental exercise and that's about it. If you try using a dished piston in an engine that came fitted with a pop-up type, you'd lose more to stupid-low CR than to the "inefficient" domed piston design needed to get 10:1+ static CR. Consider the compression height, combustion chamber volume/configuration, piston (and cam - which should be selected to match static CR) as a matched assembly...and follow the recommendation(s) of the outfit manufacturing/supplying the parts. There aren't likely to be enough ala carte choices to worry about.
 

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Too much knowledge on a monday!! :nerd: Thanks for the info.
 
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