Best shape for piston dish

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travis
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Best shape for piston dish

Post by travis »

For a modern style performance cylinder head combustion chamber, is there any difference in performance/fuel economy/detonation resistance/etc between a round dish with VR’s vs a D shaped dish or in the case of many fords, an oblong dish? Especially when approaching the upper limits of compression with pump gas?
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Re: Best shape for piston dish

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For what engine?
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Re: Best shape for piston dish

Post by travis »

351w

I also forgot to mention the “figure 8” dish.

I’ve got a few choices to achieve the compression I’m looking, but the dish shapes vary widely. Heads will be either AFR’s, Darts, Brodix, etc...no OEM or low $$$ import this time
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Re: Best shape for piston dish

Post by Walter R. Malik »

The "D" shape seems to be about the best compromise for cc area increase and quench area; for most wedge type combustion chambers.
If the engine is NOT octane limited then I would opt for a parabolic round dish.
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Re: Best shape for piston dish

Post by hoffman900 »

This is a pretty high level question - many will have opinions, but you would need a really strict dyno regiment and/or pressure testing equipment. I would hesitate to go by the old “well it needed less timing” statement.

Randy Gillis (‘piston_guy’) on here has shared on certain NASCAR engines 15? years ago that some liked a conical dish while others didn’t respond.
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Re: Best shape for piston dish

Post by Warp Speed »

We did a conical back in the 18*/9.1 days due to the dish having to be such a large volume. Present day seems to favor a mirror of the combustion chamber in most cases.
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Post by dwilliams »

For high compression and maximum turbulence, I would make a tracing of the combustion chamber and mail it to the piston maker.

For lower CR turbo motors, the same. However, if you keep lowering CR, there are limits to how deep a dish can be cut into a standard forging blank, even if you don't run into the rod. Last one of those I built I just used a short pin height and dropped the whole piston down into the bore; the little ring around the dish wasn't doing anything useful anyway.
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Re: Best shape for piston dish

Post by modok »

If modern means modern, then it's four valves, with 2 or 4 small quench pads at the edges.
A gentle concave dish, matches the quench pads, lowest point in the middle probably. But modern also means short pin height, so the rod small end may be directly under the crown, making a flat relief more likely if working with existing design, or even a bump in the middle to clear the rod like the great unholy subaru

If you were referring to the modular fords..... yeah they have some strange chambers and pistons. It made me wonder, when those came out. But, i don't think it they knew anything we don't.
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Re: Best shape for piston dish

Post by travis »

modok wrote: Tue Jul 10, 2018 11:50 pm If modern means modern, then it's four valves, with 2 or 4 small quench pads at the edges.
A gentle concave dish, matches the quench pads, lowest point in the middle probably. But modern also means short pin height, so the rod small end may be directly under the crown, making a flat relief more likely if working with existing design, or even a bump in the middle to clear the rod like the great unholy subaru

If you were referring to the modular fords..... yeah they have some strange chambers and pistons. It made me wonder, when those came out. But, i don't think it they knew anything we don't.
Ok...maybe not THAT modern :lol: How about “fast burn kidney shaped 2 valve chamber” instead of the earlier wedge design?

Trick Flow markets pistons that are a better dish to chamber match. Besides the needed valve relief locations, maybe there is something there?

Custom pistons are kind of beyond the realm (and budget) of what I am building...I just want the best combo of (mostly) off the shelf parts.
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Re: Best shape for piston dish

Post by PackardV8 »

I asked this question some time back and was told I'd pretty much wasted my money having a custom dish matching the combustion chamber; dynos had proved a semi-hemi flat dish made more power.
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Re: Best shape for piston dish

Post by lefty o »

PackardV8 wrote: Wed Jul 11, 2018 12:59 am I asked this question some time back and was told I'd pretty much wasted my money having a custom dish matching the combustion chamber; dynos had proved a semi-hemi flat dish made more power.
but did they show you proof?
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Re: Best shape for piston dish

Post by ptuomov »

Before gasoline direct injection, the car companies seemed to have converged to a round shallow dish when compression ratio allows it.
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Re: Best shape for piston dish

Post by David Redszus »

ptuomov wrote: Wed Jul 11, 2018 11:19 am Before gasoline direct injection, the car companies seemed to have converged to a round shallow dish when compression ratio allows it.
The piston dome (or dish) is the lower half of the combustion chamber. As such, its shape is highly dependent on the shape of the combustion chamber, squish pattern and scavenging route. A good piston company will require a mold of the chamber or the actual head. Unless of course, it is simply using a design previously developed.

Combustion and power is highly dependent on chamber shape, but is often overlooked as being too complex to develop.
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Re: Best shape for piston dish

Post by ptuomov »

David Redszus wrote: Wed Jul 11, 2018 11:27 am
ptuomov wrote: Wed Jul 11, 2018 11:19 am Before gasoline direct injection, the car companies seemed to have converged to a round shallow dish when compression ratio allows it.
The piston dome (or dish) is the lower half of the combustion chamber. As such, its shape is highly dependent on the shape of the combustion chamber, squish pattern and scavenging route. A good piston company will require a mold of the chamber or the actual head. Unless of course, it is simply using a design previously developed.

Combustion and power is highly dependent on chamber shape, but is often overlooked as being too complex to develop.
The car factories used to do more elaborate dish shapes in the late 80's and early 90's. Then they for some reason largely converged to a round shallow dish where the compression ratio would allow it. I think this is pretty clear, but I don't know why that happened.
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Re: Best shape for piston dish

Post by MadBill »

Best practices can change over time, but I clearly recall reading at least 15 years back a very authoritative article on this topic. The testing was done on some fairly high power, low compression NASCAR series engine. (I'm guessing ~600 HP, 9:1 CR)
The baseline was with a chamber-mirror-image dish and they were very surprised to find that a full round shallow dish made significantly more power. :-k
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