Flat top or Dish

[Terry Keeley]

A DISHED PISTON IS NOT DOMED. SHOULD I REPEAT THIS AGAIN ? IT CAN BE DOMED AND DISHED BUT THAT IS NOT WHAT THE ORIGINAL POSTER ASKED ABOUT. TOP FUEL DRAGSTERS RUN 100% NITRO AND DISHED PISTONS USUALLY BECAUSE THEY HAVE MADE MORE POWER WITH THEM THAN FLAT TOP. ALL BECAUSE IT BURNS THE FUEL BETTER/FASTER.

TF piston run by 99% of the teams, no dish here:

 

[dr gary turner]

Good question Lohring.
My next question was about FLOW
WOULD A FLAT TOP FLOW BETTER THAN A DISH??

This is both flow from fuel coming in and flow coming from the tinned pipe??

 

[Charles Perdue]

This is according to an engineer that worked in the engine department of Yamaha.

The purpose of the piston dish, when matched to the head bowl (same outer diameters), in a two stroke engine is in use when the fuel/air mixture that is between the piston top and the squish area of the head is sprayed toward the center of the combustion chamber as the piston reaches TDC. The piston dish allows more of the fuel/air mixture to stay away from being in contact with the piston surface so that it will remain vaporized and can burn properly. Any fuel that is in contact with the surface of the combustion chamber ( being piston top, cylinder head and cylinder walls) will try to condense and be too cool to burn properly because the cooler surfaces pulls the heat from the fuel droplets keeping them below ignition temperature.

Charles

 

[SayMikey]

Pretty sure Steve Wood used to mill the piston to get rid of the dish on 21 motors...

 

[John Beardslee]

Pretty sure Steve Wood used to mill the piston to get rid of the dish on 21 motors...

I did some on my lathe.

 

[dr gary turner]

Good comments
So Charles do you like the dish pistons?

 

[Charles Perdue]

There are many arguments both ways.

According to the EXPERTS (I am not one of these), the optimum dish set up is with a taper around the dish on top of the piston that matches the corresponding taper on the head squish band. This is supposed to project the spray at TDC upwards towards the center of the combustion chamber at the glow plug.

Charles

 

[Tom Foley]

The problem with the dished pistons that I have used is that they still lighten the piston under the dome without taking into consideration the loss of thickness of the dome . Stupid . A lean run will hole the piston simply because it is too thin to dissipate the extra heat .

 

[Timbo Whalen]

TF piston run by 99% of the teams, no dish here:

Correctomundo..... valve relief, and that's it..... they adjust CR per track, blower percentage, and weather in a manner similar to

what we do- adding and subtracting head gasket thickness.

 

[steve wood]

never,ever,ever, have i machined top of any piston to remove the dish. not enough metal left to do this. if you do that you would have to re set the liner to fix the timing you just screwed up. if every one would just pay attention to the timing numbers,squish angle, head clearance,volume,nitro %,pipe,glow plug heat,WATER flow,and PROP,PROP,and did i say PROP, oh,and set up, then you will be on the right track

 

[Tom Foley]

It allows a shallower chamber in the head button without sacrificing volume. While allowing the element to be more centered.

But it thins the dome of the piston right in the worst spot . unfortunately .

 

[Danny King]

But it thins the dome of the piston right in the worst spot . unfortunately .

Only because they dished a casting made for flat top finish.

 

[Tom Foley]

Only because they dished a casting made for flat top finish.

Exactly .