dished pistons make more hp than flat top?

[HPaddict]

can a dished piston make more power than a flat top? both with the same stroke and bore and comp ratio lets say 10.0.1 on both, my way of thinking is maybe the dished piston could be an advantage. seeing as when the pistons are at bdc there is more area to fill as this is the time where the air fuel mixture is really moving, maybe allowing more mixture to be crammed in ?. i realise that at tdc the dished part of the piston is lower than the flat top to begin with but at that stage the air mixture is not really moving as much at that point. but a bdc its a different story. what do you think?

[DavidHarsay]

Here’s what I see:

Some advantages of a flat top piston:
Less surface area
Better squish/qwench possible – very important in some cases

Advantages of a dish:
More efficient and compact combustion chamber possible
Eliminates squish where it’s undesirable, as in some all-effort Nitrous applications

Best of both worlds would be a well designed small dish as seen in the engine masters, where the piston dish is nicely mated to the combustion chamber on the cylinder head, resulting in a very compact and efficient combustion chamber, where there is plenty of squish to cause the desirable charge motion and turbulence to have a faster more complete burn.

[automotivebreath]

I use a flat top because the heads I use have a large combustion chamber
and I need the compression.

This quote you might find interesting:

"Here's the last piston design we cam up with for a NASCAR NW Tour engine.
For those that don't know the rules, these motors are 23* raised runner heads
with unlimited portwork (no welding) ported cast intake manifolds, roller cam,
any bore/stoke/rod combo (358" max),9.5-1 comp. max, and a untouched 390
carb.

These photos are of pistons that made about 8hp more than a reverse dome in
the same exact motor with the same exact specs on the same dyno. Nothing
was changed but the piston design"

[Jim Sams]

I have heard this to be true also.
However I can't see and reason why a dog bowl dish makes more power than a mirror image of the chamber dish given the same compression ratio....
I must not understand flame travel fully...

[rookie]

Originally Posted by Jim Sams

I have heard this to be true also.
However I can't see and reason why a dog bowl dish makes more power than a mirror image of the chamber dish given the same compression ratio....
I must not understand flame travel fully...

On the other hand, Judson hoped to eek out a few extra ponies by running concave dish pistons instead of the more common reverse domes. "A concave dish sacrifices some quench compared to a reverse dome, but the Nextel Cup guys have been using them for years and say they're good for an extra 3-4 hp."



A quote made by Judson Massingill that is in the paper version of the mag. Article but does not appear on line is.
“Some people swear that a concave dish piston is worth some power over a reverse dome, but when five guys that finished ahead of you have reverse domes, it makes you think twice.”

Small Block Ford - Popular Hot Rodding Magazine

[big block fiero]

The theoretical, least amount of chamber surface area would occur if the "clearence volume" chamber shape at top dead center was a round ball. The lower half (piston) would be dished such that a ball would fit in perfect, and the upper half (head chamber) would also fit a ball perfect. If the head and piston are both aluminum and the thermal losses simular (but there not because oil splashes the bottom of the piston) round might be good. when the losses are dissimular the more thermally wastefull surface (piston)needs to have less of the shared total surface area and so becomes nearer to flat.

[rookie]

Originally Posted by big block fiero

The theoretical, least amount of chamber surface area would occur if the "clearence volume" chamber shape at top dead center was a round ball. The lower half (piston) would be dished such that a ball would fit in perfect, and the upper half (head chamber) would also fit a ball perfect. If the head and piston are both aluminum and the thermal losses simular (but there not because oil splashes the bottom of the piston) round might be good. when the losses are dissimular the more thermally wastefull surface (piston)needs to have less of the shared total surface area and so becomes nearer to flat.

Have you read David's article about burn rate Turbulence and Combustion Dynamics and how the fuel burn is to heat the air, it is the expanding air that pushes the piston down the cylinder.
Given the short amount of time it has to burn the charge, full burn is probably helped with an open chamber the flame has nothing to burn around.

[Stan Weiss]

Originally Posted by HPaddict

can a dished piston make more power than a flat top? both with the same stroke and bore and comp ratio lets say 10.0.1 on both, my way of thinking is maybe the dished piston could be an advantage. seeing as when the pistons are at bdc there is more area to fill as this is the time where the air fuel mixture is really moving, maybe allowing more mixture to be crammed in ?. i realise that at tdc the dished part of the piston is lower than the flat top to begin with but at that stage the air mixture is not really moving as much at that point. but a bdc its a different story. what do you think?

A couple of questions
1) If the swept volume of the cylinder is the same and the CR is the same why is there more area to fill when the pistons are at bdc when it has a dish
2) at tdc the dished part of the piston is lower than the flat top to begin with but at that stage the air mixture is not really moving as much at that point. If this is an N/A engine how does the dish effect the squish are? In a N/A engine max. squish velocity will be around 10 BTDC.

[automotivebreath]

Two things to consider:

Swirl will decay faster with the 90 degree corners created with a flat top piston
in the bore, this same swirl should last longer contained in a concave dish,
Dusty helped me to realize this.

With the dish, squish to bore area is reduced and the squish widow area is
increased. This should result in less squish velocity over a larger squish outlet
area. More turbulence to enhance initial flame kernel development combined
with a short burn distance just after TDC. This should result in increased mass
fraction burned near TDC of the power stroke.

[HPaddict]

i guess what i am trying to get across here is think outside the square, forget the squish part and tdc. once the piston is moving down the bore the air is really rushing in the cylinder and once the piston is at bdc the dish part is lower than a flat top so there is actually more room to cram in a bit more air for good measure, then when the piston comes back up it still compresses the mixture in the cylinder the same amount as the flat top because you have used a slightly smaller combustion chamber to up the comp ratio to equal the flat tops comp ratio, which will give the same static compression but because more mixture got trapped earlier on the dynamic comp is actually higher. so getting to the point these are still all the same:
static compression
swept volume
stroke
so nothing has changed to much but:
at bdc there is a bit more area to fill still with a dished piston because the dish sits lower than a flat top for the same stroke.