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Old 01-26-2012, 08:54 AM   #1 (permalink)
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Dish vs flat top pistons

DanH mentioned that #5 is his standing.
I wonder about the idea of #4 too. If more of the combustion chamber was in the piston, what would the effect be? Anybody have some thoughts on that?

Here is what I found on another forum about dish vs flat. Every time my dimly lit bulb gets a little brighter, I get a little more anxious to try it out. I have to hold back to make sure the projects are started in order, and others get finished on time. >>>


If the 'dish' is the same shape as the combustion chamber above it (so quench area isn't reduced) - then it's probably a wash. But if the 'dish' is basically slightly smaller than the full circumference of the piston, then quench area will be reduced. This can result in a less efficient burn - meaning less power, less efficiency, less mileage, etc. Do some searching around the net - lots written on this stuff. If I had my choice - I'd have the chamber in the head and flat top below that maximizes quench area. And then be sure to have things carefully machined and assembled so the quench dimension stays nice and tight -- area between piston top and head (not inside the combustion chamber) in the .035"-.040" range.

Additionally - the head is the best equipped to remove heat from the chamber. When you flatten out the chamber (reduce chamber size) and dish the piston - you remove heat transfer area from the head, and you increase heat transfer area to the piston. Over simplifying - but likely means more thermal work the oil has to do, and less thermal work the coolant has to do. While contemporary oils are designed to both lubricate and remove heat -- you're better off if you minimize the heat transfer function of the oil and let it concentrate on lubrication.

Here's a piece I found while searching around....

"Now if you have a flat top piston with a .040 inch clearance on the squish/quench side, the effects of squish and quench are maximized by how close the piston and head deck close with each other. The effect is the same with a "D" dish piston where the dish is all under the valve pocket. However, a piston with a circular dish cannot close any more than the raised rim of the piston crown. So even if this crown is .040 inch away from the head's squish/quench deck the floor of the dish is a considerable distance away and it cannot and does not expel the mixture toward the plug with sufficient force upon compression to throughly agitate the mixture for the most effective burn. At the post ignition point, it does not provide the high surface area to volume ratio of a flat top so that the late burn's excessively high temps are not damped. Both of these conditions lead to reduced power as on the squish phase the burn is slower and weaker requiring more timing lead which has the effect of first trying to drive the piston backwards and second increases cylinder pressure and temperature too early which combined with inadequate quench leads to detonation and preignition."

#3

Here is a link to some other's opinions. I am not an engine builder or even have that much experience, but lots of info.
Dish piston at 10.50 WILL make more power than a flat top at 10.50???

#4
Several years ago, I talked with a guy who built big blocks. He said the best for normal driving is a flat top. There was talk of having dished pistons because they bring the combustion chamber down into the cyl which if the circumstances are right, it improves the power because the explosion happens more at/in the piston. I personally like flat tops.


#5
The reason a dish works better (especially at high rpms) is because it increases the VE of the engine.
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Old 01-26-2012, 09:18 AM   #2 (permalink)
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Re: Dish vs flat top pistons

Reading in the #3, I went to the link and read that forum. I didn't get al the way through, but, it was very interesting to read the thought processes. The key ingredient thus far, for me, was that for a dished piston;

"You will have a larger volume @ BDC to fill."
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Old 01-26-2012, 09:33 AM   #3 (permalink)
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Re: Dish vs flat top pistons

True, a dished piston will absorb more combustion heat that will work to push on the piston. The upper cylinders are cooled by the water jacket first before the heads (at least in a SBF) so heat transfer isn't a problem. Personally, on a low rpm street engine, I prefer flat top pistons with better squish/quench areas. For higher rpm applications it's not as big a deal, IMO.
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Old 01-26-2012, 10:29 AM   #4 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by bmcd66250 View Post
True, a dished piston will absorb more combustion heat that will work to push on the piston. The upper cylinders are cooled by the water jacket first before the heads (at least in a SBF) so heat transfer isn't a problem. Personally, on a low rpm street engine, I prefer flat top pistons with better squish/quench areas. For higher rpm applications it's not as big a deal, IMO.
From the way I understood it, if you have a flat top vs dished, the dished piston (same CR) gives the effect of more stroke, because the top of the piston (majority of it) is lower than the flat top piston would be at BDC. Also, a small volume of air will also be lower in the cylinder with a dished piston. More air volume at BDC, more air movement. More air, more energy, more power.

Last edited by builder; 01-26-2012 at 10:32 AM.
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Old 01-26-2012, 10:31 AM   #5 (permalink)
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Re: Dish vs flat top pistons

I'll just comment only enough to pull the thread toward the common factor in the various discussions. The effort is toward a condensed charge (dictating a spherical shape) with ignition at the center of this sphere. The sphere is (for compression) considerably smaller than the bore and automatically provides the quench and area reduction for detonation suppression. This will enable rapid and complete combustion for greatest power and efficiency. A problem is that our spark plugs may protrude into, but generally cannot be centered in this sphere, and indeed most are relatively centered where there is room between the valves. Therefore we must modify the shape to keep the charge compact, yet initiate the burn from an edge. This leads to the modern chamber and matching piston dish shapes that resemble a short, fat, upside-down ice cream cone, with the spark plug at the top.

By looking at the shape, the radius from the spark is relatively even for complete and rapid burn. In-fact, the shape in 3D is widened to more of a cheese wedge, as we have to fit valves in there also. If the same chamber is used with a flat-top, note the radius from the burn distance from the spark is now uneven and less efficient, as we have more of a cone with no ice cream dip in the piston. So, very oversimplified with little other explanation, we have the trend in design.

Now, why do we have the chambers and pistons we use? They are all compromises away from the 'ideal' due to valve placement, spark placement, valve clearances, height and depth limitations, and other necessary stuff. To make-up for wonky shapes, designers add other features to promote faster burning, such as turbulence and swirl generators, a.k.a. - 'fast-burn' chambers that aren't really fast, but an improvement over versions without those features.

If you apply some of these basic ideas to various chamber and piston shapes, and discussions about them, you can begin to pick apart the theories from the guesses, and identify where weaknesses lie in various features. Hope that helps.

David
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Old 01-26-2012, 11:57 AM   #6 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by PSIG View Post
...........If you apply some of these basic ideas to various chamber and piston shapes, and discussions about them, you can begin to pick apart the theories from the guesses, and identify where weaknesses lie in various features. Hope that helps.

David
Yes. We are making wide general guesses using the knowledge we posses to make justifications to our own ignorance.
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Old 01-26-2012, 03:45 PM   #7 (permalink)
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Re: Dish vs flat top pistons

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Yes. We are making wide general guesses using the knowledge we posses to make justifications to our own ignorance.
David hit on some of the limits . .

#5 is just part of it . lets say there is a #5a fuel burn .
catch 22 . VE is high but can't burn the charge or can burn it but can't get the VE

Modern race are disign to make both VE/Burn work togther

10.5 with common OTS heads on a small cube engine , not that easy and the real question is there a gain by modifying a common head

high flow Pro type heads cost around $10,000 .bare or more plus another $5/6 K for valves and rockers etc.
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Old 01-26-2012, 05:02 PM   #8 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by builder View Post
if you have a flat top vs dished, the dished piston (same CR) gives the effect of more stroke, because the top of the piston (majority of it) is lower than the flat top piston would be at BDC. Also, a small volume of air will also be lower in the cylinder with a dished piston. More air volume at BDC, more air movement. More air, more energy, more power.
The dished portion is lower in the cylinder at BDC but doesn't move as high in the cylinder at TDC as a flat top piston, the stroke is constant. And can a big dish cause problems with exhaust evacuation, vacating the gasses that are down in the dish?

Kind of an afterthought, a really big piston dish would require a shallow chamber or even a flat head deck to maintain compression. With a conventional chamber, shapes and features can be designed to promote tumble, swirl and aid intake mixture pull during overlap. Not sure how that can be accomplished with a really shallow chamber or flat deck.
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Old 01-26-2012, 10:01 PM   #9 (permalink)
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Re: Dish vs flat top pistons

Look at a dish vz other piston designs a different way .

Street Racer (his real name btw) has a engine that needs 92 grade fuel .

his engine comp ratio doesnt matter . Street a ceap dude and wants to burn 87 pump fuel.

Street has a 400 hp engine with the C/R it has now
he also been reading the dish claim to make more power, but doesnt want more just the same HP

what street wants to know is , what size dish does he need to stay at the 400 HP but lower the CR for the 87 cheaper fuel or a mix of 87 and 92 . Street is a cheap dude

Dish pistons make more power . should be able to be done . No other changed parts but the piston

Last edited by DanH; 01-26-2012 at 10:05 PM.
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Old 01-27-2012, 08:58 AM   #10 (permalink)
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Re: Dish vs flat top pistons

Perhaps Street's squish/quench zones aren't adequate to run the lower octane fuel. Or maybe he needs to cut Singh grooves in the head (which I would love to experiment with some day).
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Old 01-27-2012, 09:14 AM   #11 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by DanH View Post
his engine comp ratio doesnt matter . Street a ceap dude and wants to burn 87 pump fuel.

what size dish does he need to stay at the 400 HP but lower the CR for the 87 cheaper fuel or a mix of 87 and 92 .

Dish pistons make more power . should be able to be done . No other changed parts but the piston
There are many 10.5:1 engines out there running 87 octane fuel.... They just have to work their timing fully into the equation and aren't able to use the engine to it's capacity...

But you said CR don't matter, Then you said Needs to stay at 400 HP but have lower CR to run 87 octane or a mixture of the two.

Simply knowing the advantage in percentage of power that the DT piston has over his current FT pistons would allow an extrapolation of CC's need for any mathematical calculations of the percentages to be applied and taken note of when making the DT properly Dished.

And all of that highly depends on the current combustion chamber design.

Is any of this going to help myself arrive at a juxtaposition of enlightenment on the topic of DT versus FT pistons?

Last edited by FEandGoingBroke; 01-27-2012 at 09:18 AM.
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Old 01-27-2012, 09:49 AM   #12 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by bmcd66250 View Post
Perhaps Street's squish/quench zones aren't adequate to run the lower octane fuel. Or maybe he needs to cut Singh grooves in the head (which I would love to experiment with some day).
Street got a high dollar engine , zero deck , plus anything else anyone can come up with . Street got the best of the best .

Street doesnt like paying for gas. BTW his valve covers are 24k sheet gold . ONLY one thing needs to be done , install dish piston , the best of the best piston , cost is no object .

Now , back to saving Street some gas money
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Old 01-27-2012, 10:04 AM   #13 (permalink)
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Re: Dish vs flat top pistons

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Originally Posted by FEandGoingBroke View Post
There are many 10.5:1 engines out there running 87 octane fuel.... They just have to work their timing fully into the equation and aren't able to use the engine to it's capacity...

But you said CR don't matter, Then you said Needs to stay at 400 HP but have lower CR to run 87 octane or a mixture of the two.

Simply knowing the advantage in percentage of power that the DT piston has over his current FT pistons would allow an extrapolation of CC's need for any mathematical calculations of the percentages to be applied and taken note of when making the DT properly Dished.

And all of that highly depends on the current combustion chamber design.

Is any of this going to help myself arrive at a juxtaposition of enlightenment on the topic of DT versus FT pistons?
Yes , you will be enlighten .

How much of a dish . !/100 cc or 20 cc's . any drop in the compression ratio that shows a power loss . the dish piston power gain is not a fact

1/100 of a cc . lol
with only one thing changed , credit can only be to that one item
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Old 01-27-2012, 10:20 AM   #14 (permalink)
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Re: Dish vs flat top pistons

Ahhhhh, I do get it! Thanks Dan!
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Old 01-27-2012, 11:20 AM   #15 (permalink)
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Re: Dish vs flat top pistons

The last 331 I built I used -13cc dished pistons that is 104 extra cc or 6 cubic inches total. I sized the heads to get 10 to 1 cr.
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