Small Combustion Chamber Dished Piston N/A Motor Discussion
03-28-2005, 12:58 PM
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Small Combustion Chamber Dished Piston N/A Motor Discussion Guys, I've been seeing a wealth of popularity regarding small combustion chambered motors with dished pistons winning the engine master challenges in PHR. There must be something to this.
I have heard that the smaller combustion chamber allows for less chance of detonation becuase there is less time for flame travel. These guys are running upwards of 12.6 to 13.5 to 1 in pumpgas. We all know the reason for a quench pad is to distribute the a/f mixture evenly through the cylinder to make the mixture completely homogeneous or as close as possible. Now comes in the dished piston. Because detonation happens typically at the perimeter of the cylinder bore, unless induced by a very hot exhaust valve or spark plug, does the dished piston act like a 360 degree quench pad, hence keeping all or most of the mixture in the center of the cylinder and distributing it very evenly limiting its chance for detonation. Is this the reason for the popularity of this style combonation?
Please, lets discuss.
I have heard that the smaller combustion chamber allows for less chance of detonation becuase there is less time for flame travel. These guys are running upwards of 12.6 to 13.5 to 1 in pumpgas. We all know the reason for a quench pad is to distribute the a/f mixture evenly through the cylinder to make the mixture completely homogeneous or as close as possible. Now comes in the dished piston. Because detonation happens typically at the perimeter of the cylinder bore, unless induced by a very hot exhaust valve or spark plug, does the dished piston act like a 360 degree quench pad, hence keeping all or most of the mixture in the center of the cylinder and distributing it very evenly limiting its chance for detonation. Is this the reason for the popularity of this style combonation?
Please, lets discuss.
03-28-2005, 01:35 PM
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Im also interested in this. Typically dished pistons are not completely dished, but dished near the valve reliefs. The added dish allows you to make the chamber flatter on the outside of the cylinder adding to the quench effect. This added quench allows you to run more compression safely due to flame travel like you stated.
The goal with quench is to get as much flat area on the piston and combustion chamber while maintaining proper total volume. The best way to do this would be with a dished piston to make a chamber with the greatest quench area (with regards to surface area) and the thinnest possible before you get into danger zones.
I am still learning about all this, so any professional opinions would be good to know.
The goal with quench is to get as much flat area on the piston and combustion chamber while maintaining proper total volume. The best way to do this would be with a dished piston to make a chamber with the greatest quench area (with regards to surface area) and the thinnest possible before you get into danger zones.
I am still learning about all this, so any professional opinions would be good to know.
03-28-2005, 01:49 PM
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IMO, the best set up is the smallest conbustion chamber you can run without shrouding the valves and then compensate for a given compression ratio with the piston crown design. It keeps flame travel down to a minimum and quench to a maximum 
03-28-2005, 01:51 PM
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I have seen both types. Most have been dished in the manner you state (mirror image of the cubustion chamber), some were more circular. This is a very interesting phenomenom and one I am very interested in hearing about from a professional as well. There has got to be something to this. I may try something along these lines with my 408 which is being built or will begin being built shortly.
03-28-2005, 06:21 PM
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I had always heard dished pistons were bad for det resistance. How do you get that high of CR with a dish anyways?
Edit: Oh, did you mean only dished near the valve reliefs? From my reading, this is how I see it: To get a high CR with a dish that covers the whole CC, everything is going to be really flat. Naturally this decreases flame speed, which is very bad. So you add flat sections on the edges and dish the middle. Since most combustion takes place near TDC, you've got a little circular pocket in the center. If you dish the piston in that area too, you can make it more circular, which of course increases flame speed.
Endyn makes pistons that look like this,
http://www.theoldone.com/components/pistons/
Mainly for hondas though. Some people say Endyn sells snake oil, but I have seen posts by people who have 13+ CR hondas on pump gas. I'm not sure what his 2-valve stuff looks like though.
Edit: Oh, did you mean only dished near the valve reliefs? From my reading, this is how I see it: To get a high CR with a dish that covers the whole CC, everything is going to be really flat. Naturally this decreases flame speed, which is very bad. So you add flat sections on the edges and dish the middle. Since most combustion takes place near TDC, you've got a little circular pocket in the center. If you dish the piston in that area too, you can make it more circular, which of course increases flame speed.
Endyn makes pistons that look like this,
http://www.theoldone.com/components/pistons/
Mainly for hondas though. Some people say Endyn sells snake oil, but I have seen posts by people who have 13+ CR hondas on pump gas. I'm not sure what his 2-valve stuff looks like though.
03-29-2005, 06:56 AM
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I don't see how a flat piston would reduce flame propogation at all. Dome pistons slow the process becuase they protrude into the flame path. Alot of people seem to be going this route to get the high compression and pumpgas compatability, small combustin chamber and dished pistons that is. It does make sense as the quench is much more efficient with a dish and small combustion chamber.
Also to get high compression with a dished piston just simply reduce the combustion chamber volume.
Also to get high compression with a dished piston just simply reduce the combustion chamber volume.
03-29-2005, 08:34 AM
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Originally Posted by DAPSUPRSLO
I don't see how a flat piston would reduce flame propogation at all. Dome pistons slow the process becuase they protrude into the flame path. Alot of people seem to be going this route to get the high compression and pumpgas compatability, small combustin chamber and dished pistons that is. It does make sense as the quench is much more efficient with a dish and small combustion chamber.
Also to get high compression with a dished piston just simply reduce the combustion chamber volume.
Also to get high compression with a dished piston just simply reduce the combustion chamber volume.
Way back when (which really isn't that long ago) Mike Moran was building the ILLEGAL 4 Turbo car, Popular HotRodding ran an article on the setup. Mind you this was for a forced induction drag only car, but some of the principals still apply. Some of which are a little hazey. (HAHA I found the article!! http://www.popularhotrodding.com/eng..._bb/index.html ) If you think about the simple phsyics behind momentum, forces on objects and supporting structures, you begin to undstand what makes a dish piston more adaptable to performance oriented engines. If you have a piston that focuses the energy change in the combustion process to a focal point downward (the center of the piston obviously) as well as place the most force on the center of the connecting rod as possible (where you don't have to rely on the tinsel strength of the wrist pin as well as transfer the energy laterally from the outside of the piston to the con rod) you make more efficent use of the pressure. Considering this, you're also increasing piston speed away from the flame, which means you would increase detonation resistance. Also, the dish is father away from the flame to begin with so, there again, increased det. resistance. Dome pistons become inefficent, maybe not because of detonation resistance, but because of the mechanics structure and the change of energy that they have to deal with. I'm sure, check that, I know, that my analasis of this isn't 100% right (arm chair engine builder/racer/engineer but)...just thinking about the simple physics behinds this and you can begin to understand why this makes for a good engine combination. Besides...why would the likes of the top NASCAR teams and programs as well as the factory backer's like GM design the SB2 and SB2.2 head that gives them the legal limit of 12:1 comp ratio with a flat top piston. BTW if I remember correctly, thos heads have about a 45CC and 72CC chamber respectively (SB2 prior to the 12:1 limit).
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03-29-2005, 09:01 AM
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Like I said...I'm sure that's not 100% correct, the pros can point out my flawed analysis but simple physics and engineering (which those classes from college pay off somtimes.)
