which is better assuming both have the kidney shaped fastburn chamber for a forced induction application

1.) Small chamber and D-dished pistons

2.) Large chambers and flat top pistons

 

Flat top would be best. Any edges on pistons get hot and cause pre-ignition.

 

Lots of variables here...assuming head flow doesn't get impeded by smaller chamber, it's generally accepted that a small chamber with properly matched dished pistons will burn better and make more power.

The large flattop just gives more places for fuel not to burn....better to force it into center and burn it there.

JIM

 

When the mixture is ignited by the spark the burn is spreading out from the spark as a ball of flame, which gives a curved flame front. In an ideal world a dished piston would be best as the flame front, being curved, will give a more even pressure across the top of a dished piston than a flat one. This isn't an ideal world & a good theory doesn't always work out good in practice (like a theoretically brilliant chat-up line results in a slap around the face when used ). According to an old rodder I knew who was involved in drag racing in the 60's (with SBC's) a dished piston is best, but sometimes you just can't get the CR you need with one, so he'd use flat top or even domed if he had to.
To raise CR on an engine to about 13:1 I once used pistons that had riduiculous shaped crowns (imagine a wedge of cheese with the point cut off, turn it on it's bottom edge & that's what the piston crowns looked like). No doubt plenty of unburnt fuel was thrown out in the exhaust, but it definitely went better than it did when running 10.5:1. It'd probably be a bit better with that CR if the piston was dished, but the work needed on the head to allow that would have been excessively expensive & would have destroyed the flow, losing far more power than would have been gained by having a dish.
To prevent pinging it's always a good idea to round off any sharp edges. This will help prevent carbon from clinging to the edges where it can glow red & ignite the fuel. I've done this on an engine runing 9.8:1 CR & don't get pinging problems (unless I put poor quality fuel in it). A mate runs the same engine, same ign timing, etc, etc with 9.5:1 CR and gets huge problems with pinging. The only difference (apart from mine having the head skimed) is that he fitted the pistons without smoothing off the sharp edges around the valve reliefs in the pistons.

 

The current thinking at GM has the pistons dished and a small combustion chamber.

 

For forced induction I'm assuming you mean a super charger, I would think the dished would work better.

For nitrous, I've read the a piston with a crown will let you get by with running slightly more timing than with a flat top.

 

#1! small chambers and reverse "D" type dish! the combustion in a large area "open" chamber cylinder head is less efficient and more prone to pre-ignition form poor fuel distribution where the small chamber has more turbulence and better mixing characteristics. Somehow, Ford Motor Company has not "discovered" this fact with the new "modular" motor and that is why they are such "dogs". NASCAR motors have VERY Small chambers and are running over 13:1 with a "flat-top" style piston same for most current racing engines.

 

Look at the Dart Big Chief and Bodix Big Dukes..they have tiny chambers so you can use less piston.

On the old pistons with the huge domes, the trick of the day was to cut "flame slots" in the dome in front of the spark plug so that the flame front could "jump over or through" the dome and get a shot at lighting some of the mixture on the other side. They literally saw 40-50 hp on some engines with huge domes.

Nothing like good 'ole sloppy inefficiency getting re-done to make it somehow run huh??


JIM

 

I am working on a turbocharged engine on an enlisted salary.

The difference is that I can get forged dished pistons with teh D shape cup for like $600 bucks if I want to use heads with a 64cc chamber making a 8.25:1. If I wanted to use 76cc Chambers I can use probe forged flat tops for a 8.5:1 ratio

8.5:1 is about as high as I want to go. 8.25 would be ideal but the pistons are twice as much.