Best shape for piston dish

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David Redszus
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Re: Best shape for piston dish

Post by David Redszus »

MadBill wrote: Wed Jul 11, 2018 10:19 pm 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
I was quite surprised to see a Porsche engine using a domed piston with a dish bowl in the center.
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Re: Best shape for piston dish

Post by hoffman900 »

From Randy Gillis (formerly of JE Pistons, now at Racetec. Also ‘piston_guy’ on here):
I did the first "tapered" or conical dish pistons for (then) Busch series engine builder Frank Leeson of Bill Davis Racing . He approached me with the idea and we made some test parts for him. Several dimensions were changed and those changes had a very clear affect on performance. One critical aspect was the width of the "perimeter squish band". Frank and I morphed the piston into the "spherical radius" from the conical due to the need for increased negative volume. After a couple of weeks I was contacted by Bob Fisher ( then of Ernie Elliott Inc) who was building engines for Bill. I gave both of them a 1 year exclusive on the design and development. Both did extensive back to back ( spherical to mirror image dish) testing . The spherical required at least two degrees less timing and always made a significant improvement to torque with a smaller improvement in HP. We figured combustion efficiency was responsible for that. There was also a stability condition. the feeling was the load was focused in the center and not offset by the mirror image dish. After the year was up , I offered the design to a west coast Craftsman truck ( then) engine builder. He was extremely skeptical and wanted no part of this "dumb design". I offered to ( and did) make two sets of equal pistons weight , rings, skirt profile, dish volume , etc . except for the dish design. The deal was to test the "conventional" design he was using first and then ( while still on the dyno)) pull it down , change the pistons and use the same used rings , and test it again. The result was a 10 hp 13 ft lb increase in power with 2*s less timing required. We didn't "invent" the concept , it was already out there on Hondas and other imports, we just adapted it to the V8 engine. There would eventually be a few cases where results were neutral as far as power increase but we did the design on all kinds of pistons. None made LESS power. I still use it today

Warp speed should remember when the concept hit and how soon it became available from every piston supplier. He might even have some comments on it.
-Bob
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Re: Best shape for piston dish

Post by andyf »

travis wrote: Wed Jul 11, 2018 12:33 am
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.
If you can't afford custom pistons then you should reword your question. The best shape for a piston dish is a very complex subject. All you want to know is which off the shelf piston you should buy for your engine. No need to get into the theory, there are probably only a few choices.
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Re: Best shape for piston dish

Post by piston guy »

MadBill wrote: Wed Jul 11, 2018 10:19 pm 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
Thanks Bill,
The conical and later spherical dish designs gave significant torque increases and some hp increases with less timing that a mirror image. This was a surprise to those who tried it and disputed by those who didn't. What was funny was how quickly the other manufacturers began offering it. There were some design "rules" we found when using the design that had to be followed. With Cup guys , you never get a "straight" answer and you only know something is good when it's reordered or the design becomes the "new standard". There were many offshoots too. Irregular shapes with radiused perimeters that could only be done on the 5 axis. When the cutback on engines ( no qualifying engines , ONE engine per race) I got out of the Cup side of the business.
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Re: Best shape for piston dish

Post by BradH »

piston guy wrote: Thu Jul 12, 2018 12:09 pm The conical and later spherical dish designs gave significant torque increases and some hp increases with less timing that a mirror image... There were some design "rules" we found when using the design that had to be followed...
Anything in particular that you can recall, or feel OK discussing today? Thanks - Brad
Last edited by BradH on Thu Jul 12, 2018 12:26 pm, edited 1 time in total.
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Re: Best shape for piston dish

Post by BradH »

Warp Speed wrote: Tue Jul 10, 2018 10:26 am 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.
This is why I just asked "piston guy" if he can talk about the design "rules" he mentioned they found applied to the conical/spherical dish. Thanks - Brad
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Re: Best shape for piston dish

Post by piston guy »

David Redszus wrote: Thu Jul 12, 2018 10:58 am
MadBill wrote: Wed Jul 11, 2018 10:19 pm 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
I was quite surprised to see a Porsche engine using a domed piston with a dish bowl in the center.
David,
I made one set using that design for a multi car top fuel team 12 years ago . The crew chief I sent them to left that organization and took them with him. I don't know if they ever got tested. I haven't done any since.
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Re: Best shape for piston dish

Post by piston guy »

BradH wrote: Thu Jul 12, 2018 12:25 pm
Warp Speed wrote: Tue Jul 10, 2018 10:26 am 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.
This is why I just asked "piston guy" if he can talk about the design "rules" he mentioned they found applied to the conical/spherical dish. Thanks - Brad
Brad,
The "squish band" or flat around the perimeter had to be a minimum of .375. We tried and edge to edge radius and it was BAD. The piston had to have a flat on it . Let me also say this is not something that wasn't already being done. We couldn't patent the design because MANY import 4 cylinder pistons already were using the shape! Another thing we found was when the volume needed was greater than 20ccs, we could "step" the edge , effectively lowering the radius cut into the piston. We tried other odd shapes too with an upper and lower quench pad and a "V" shaped trough. They cost more in machine time but were no more powwerful than the spherical.
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Re: Best shape for piston dish

Post by piston guy »

I will never say the spherical dish is 'the best" as there are too many different combustion chamber shapes and volumes. It worked where we tried it and caused guys to rethink what was happening inside the engine. MANY high end builders scoffed at the concept saying it was a fluke until they proved it by using it. One other piston company owner said to me "I think it's BxxxSxxx but when the customer wants it I'm damn well going to make it for him to keep him from going to you!" It was a fun time back then.
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Re: Best shape for piston dish

Post by hoffman900 »

piston guy wrote: Thu Jul 12, 2018 12:50 pm I will never say the spherical dish is 'the best" as there are too many different combustion chamber shapes and volumes. It worked where we tried it and caused guys to rethink what was happening inside the engine. MANY high end builders scoffed at the concept saying it was a fluke until they proved it by using it. One other piston company owner said to me "I think it's BxxxSxxx but when the customer wants it I'm damn well going to make it for him to keep him from going to you!" It was a fun time back then.
Considering the piston top makes the bottom of the combustion chamber, I think it’s greatly overlooked. The import OEMs seem to be running ahead of the domestic OEMs in this regard, but things have seemed to level out.
Ducati has figured out how to make a 4.57” bore run on pump gas with 12.6:1 compression, and spin over 11,000rpm. No doubt chamber / piston top shape plays a role.

Also, thanks for helping me out with that Yamaha piston order a couple weeks ago.
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Re: Best shape for piston dish

Post by piston guy »

My pleasure Bob.
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Re: Best shape for piston dish

Post by BradH »

piston guy wrote: Thu Jul 12, 2018 12:41 pm Brad,
The "squish band" or flat around the perimeter had to be a minimum of .375. We tried and edge to edge radius and it was BAD. The piston had to have a flat on it . Let me also say this is not something that wasn't already being done. We couldn't patent the design because MANY import 4 cylinder pistons already were using the shape! Another thing we found was when the volume needed was greater than 20ccs, we could "step" the edge , effectively lowering the radius cut into the piston. We tried other odd shapes too with an upper and lower quench pad and a "V" shaped trough. They cost more in machine time but were no more powwerful than the spherical.
I appreciate the explanation. - Brad
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