Alright, sorry if this topic has been posted before, but I tried searching and didn't get a real straightforward answer. So I'll try one more time.
Here's what I'm trying to figure out for my application. I'm aiming for a 10.3 to 10.5 CR on a 383 with AL heads on 93-94 octane. Basically I have two choices. Run a flat top piston with valve reliefs and a larger chamber (69-70 cc) or D-shaped dish piston with a small chamber ( I think a 62cc will yield around a 10.3 cr, but I haven't done the exact calculations). Again, the quench distance is the same on both of about 0.040". Which is the better choice? Does one have a better combustion effeciency over the other? Differences in power?
I have heard agruements for less surface area on the Flat top piston therefore increasing thermal efficiency. On the other hand, I have heard that the small combustion chamber with a D dish will concentrate the flame front more over the piston than in the head. Also, If you take a 68cc head and have it milled then you get the benefits of unshrouding the vales. I guess you can do the same to a 74cc head and get a similar effect.
Any thoughts or experiences on this is much appreciated.
Thanks.

 

dished will have better squish effect

 

The overall smaller, rounder chamber shape (piston + chamber) will run better. That means a small chamber head, the chamber just large enough for the valves to be unshrouded and so forth; and an "inverse dome" piston, with the dish shape matching that of the head portion of the chamber.

Think of 2 equal volumes in different shapes; like the shape of a golf ball, compared to a sheet of cardboard. Light a fire in the center of the shape. In which one, will the flame travel all the way across first?

For best thermal efficiency, have the head chambers and the piston tops ceramic-coated.

 

ede - would you care to elaborate more on why the squish effect would be better?
My guess would be that the squished mixture would be able to fill the chamber better with the dish shape. In other words, instead of forcing the mixture in one direction with a flat top it can now move more freely (lack of better wording) in the chamber with the dish?

RB- your example is very good with the golf ball/ sheet of cardboard. The volumes are similar, but the flame moves across the golf ball quicker because of the smaller surface area? Is that the right direction?

So what I am getting from this is that the small chamber + D-shaped dish matching the chamber is going to promote better squish/ mixing as well as a faster, more complete burn. Does this translate into more power across rpms or more towards peak power output?

Thanks for bearing with me, still learning.

 

Yes more power everywhere and better detonation resistance.

 

Close but not quite.

The flame front moves faster through the golf ball, from center to outside surface, because it has less distance to travel.

Improving combustion efficiency helps power output, coolant temps, gas mileage, and resistance to detonation, under all running conditions. It just makes the engine work better, period; after all, these are "heat engines", which take chemical energy from the fuel, turn it into heat energy, and then turn that into mechanical energy. Anything you do that increases the efficiency of either of those conversions, is a step in the right direction.

 

Ok, I think I got it now. Thanks RB for the explanation. I guess its hard to imagine how the mixture would burn in the chamber. But it does make make sense now about teh shapes of the volumes through which the flame must travel. Thanks again, I'm more of a visual learner where I can see examples and how things work more than just a textbook telling me it. I guess that is why I found lab more interesting.

 

Like the other guys said:

The D-Dish pistons with the smaller chamber will do ya the best job.

The essence of this principal is simple, talked about already here.

What occurs with the D-Dish is that it allows for the flame front to burn 'down' and in more direction then a flattop. This providing for a more rapid expansion of the flame front, and better burn overall.