Posted By: PorkyPig
Which is better, flat or dished piston at same comp.? - 11/23/13 07:42 AM
If the quench is the same and the compression ratio is the same, would a flat top piston with a larger chamber or a dished piston with a smaller chamber be expected to work better?
Posted By: Swedcharger67
Re: Which is better, flat or dished piston at same comp.? - 11/23/13 12:35 PM
My bet is a piston with the dish...
Posted By: jcc
Re: Which is better, flat or dished piston at same comp.? - 11/23/13 05:53 PM
However a proper chamber shape could also recreate the dish effect you suggest is beneficial, thought I read somewhere less piston surface area, ie flat is more efficient, with all other parameters being optimized.
/2
Posted By: Twostick
Re: Which is better, flat or dished piston at same comp.? - 11/23/13 10:58 PM
I believe the flame travel is faster across a flat surface so the flat piston "should" be better.
Kevin
Posted By: 383man
Re: Which is better, flat or dished piston at same comp.? - 11/23/13 11:25 PM
The main reason the flat piston works so good is because you can get quench and a nice flat flame front that can travel very good. I mean which would you prefer to have say 11.0 comp with an open chamber head and a pop up piston or the same 11.0 comp with a closed chamber head with quench and a flat piston ?? Of course the flat piston with quench will be the better as it wont have anything in its way of the flame front that a pop up piston would. The main reason for the D-dish piston is to have quench and a pump friendly comp of about 10.5 and lower. So I would think the flat piston with quench would be best. The D-dish will work fine since part of the piston head is still flat for quench but as I said if you dont need a dished piston to drop comp then most go with the flat piston which I believe will have the best flame travel. I was going to go with flat pistons and quench but when I decided to use 75cc heads instead of the 84cc heads I had no choice but to use a D-dish piston and still have 10.6 pump friendly comp and still have quench. All said I believe the flat piston will work best.
Ron
Posted By: Swedcharger67
Re: Which is better, flat or dished piston at same comp.? - 11/24/13 12:14 PM
My gut feeling says that a flat head, with the combustion chamber in the piston, would be the best. I've seen this design in highly efficient diesel engines. Drawback could be a somewhat heavier piston, on the other hand modern pistons are so much lighter than the old...
Posted By: gregsdart
Re: Which is better, flat or dished piston at same comp.? - 11/24/13 08:24 PM
The real answer to this question may also be "it depends" Some times stuff works like it is supposed to, some times it doesn't. There are many other factors besides flame travel that will effect the end result. Stuff like the effect of tumble and swirl, how it changes as the piston rises, engine rpm used, plug location, and on and on. But figuring that we were talking about my favorite motor, a BB, then I am betting on a dish based on the fact that the majority of the fuel/air mix winds up closer to the plug, (shorter flame travel to the edge of the charge) and the quench area actually increases if you cut the heads a lot like mine are. The reason you see engines at less than 4.00 bore now days is that it is better for emissions, hp and efficiency. Some of that comes from reduced surface to volume ratio.
One clue to my first statement is that I read somewhere that 60 percent of the computer design time spent on the new Corvette was spent on combustion engineering in the new motor. That alone says there is much to learn yet, and all the "rules" are subject to new info.
Posted By: Hemi Allstate
Re: Which is better, flat or dished piston at same comp.? - 11/25/13 12:24 AM
One clue to my first statement is that I read somewhere that 60 percent of the computer design time spent on the new Corvette was spent on combustion engineering in the new motor. That alone says there is much to learn yet, and all the "rules" are subject to new info.
Does the new Corvette engine have "Direct injection"? That is into the chamber rather than the port?
Posted By: Leon441
Re: Which is better, flat or dished piston at same comp.? - 11/25/13 01:02 AM
I can only speak from experience.
The question is kinda wrong to start with. The same quench. I assume to OP means piston to head clearance. Then if you change nothing else the ratio would have to change when going from flat top to dish pistons. Opening the combustion chamber or running more valve angle to deepening the combustion chamber.
I like shallow valve angles although some say a sharper angle slows the flame in say a power adder. So if I run my small chamber heads. A shallow angle then we are talking less than 40 cc. With any stroke we have a compression problem. Running a full dish fixes this problem. Can't see how that would disturb a flame front as the valve reliefs are much less evident in a dished piston. Some heads like P7 have a lot of lumps placed in key areas to make compression with short strokes. When you install these heads on larger strokes you start thinking of getting rid of these lumps.
I have always thought if you are starting with a clean sheet of paper you should make the combustion chamber in the head 2/3 the size of the needed combustion space at TDC. Then make the piston with the other 1/3 mirrored to the head. This way at just before TDC you have better flame front characteristics requiring less ignition advance before TDC. I have always felt it was an advantage to have an engine requiring less timing.
Leon
Posted By: 383man
Re: Which is better, flat or dished piston at same comp.? - 11/25/13 07:59 PM
Basically what I was trying to say is that just like when I built my 493 I was shooting for zero deck and quench with about 10.5 comp for pump gas. I did not shoot for a setup to use the D-dished pistons as I wanted a flattop piston with quench and the comp ratio I was shooting for. But at the last minute I decided to use the 75cc EZ heads instead of the 84cc Eddy heads and the only way to now get comp in the 10.5 area with quench was to use the D-dish piston to have the comp I wanted and still have quench and nothing in the way of the flame travel. So to me I shoot for everything to work with the flat piston and only consider the D-dish piston if I have to much comp and a D-dish piston will get me what I want. I am not aware of any advantage of using the D-dish over the flat top other then letting the builder hit his comp mark and still have a good flame travel. Ron
