As I look and study about the Big Bore Kits, I seen one that had Domed Pistons and most with flat pistons. Is the domed pistons for higher compression?

 

Yes. They

 

Nobody knows?

 

Domed pistons are normally used to raise compression & flatter pistons are used for lower compressions & to keep pinging down. Certain combustion chamber designs require matching shaped domed pistons to reach a desired compression ratio = A Hemi combustion chamber needs, because of it volume, a matching domed piston to work correctly. The matching of pistons to chambers are seen more often in auto motors vs our HD designs.

This is a general statement but close enough ....
A stock 103 flat top piston equals about 9.4:1 compression
A raised dome 103 piston can take the compression from 9.7 to 10.5:1 & higher.

 

Thanks for the info Jim...

 

Just to add to the info here - domed pistons are less common than they used to be on the older hemi head engines - especially as newer cylinder heads have flatter valve angles. One problem with a domed piston is that it creates a crescent shaped combustion chamber which makes for a long flame travel so giving inferior combustion characteristics - that's why higher performance shovel heads would often have twin-plug conversions - to attack to combustion chamber from both sides.