Most OEM Gasoline engines use a dished piston. I'm assuming that there is an advantage in reduced ignition advance timing requirements and/or reduced emmissions. I've asked a number of custom piston manufactures but no one seems to know. Any thoughts or information?

it probably has more to do with less machining time  than anything related to engine performance . maybe the machining for valve cutouts is not necessary with dished pistons.
malbeare.

Most dished pistons are a bit heavier, some a lot.  Many of the applications utilize a 'flat' cylinder head design thereby eliminating a lot of machine work, e.g., the Kent Ford 116E type (Cortina, Fiesta, etc).  Some use a dished piston to lower the CR when the overall engine displacement is increased beyond the ability to increase the combustion chamber sufficiently to compensate, e.g., BMC/Leyland "A" series (Mini, Sprite, etc.ball valve)
I'm not sure that the practice is anything more than a cost saving device for the mfgrs as in a race/performance design, the practice is non existent...save for a couple extreme and, rare cases.

Concentrating the air/fuel mix towards the center of the chamber is generally regarded as a good thing to be doing. Dished piston (and a shallow/compact chamber in the head) does that. Crowned piston does the opposite.

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