Building 350
12-01-2017, 05:17 PM
#22
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This is straight out of a hot rodder form that is an interesting post
Quote:Originally Posted by NWayne
I need a little help understanding.
Let's assume you have two engines with 10.1:1 compression, one with flat tops and the other with dished pistons. Both run a .040 quench. Why is the one with the flat top's better?
The problem is you can't make things equal on the quench, if you could it wouldn't make a difference, but you can't, therefore, it does make a difference.
OK, take a wedge chamber typically it consists of a flat portion which is called a squish or quench deck, it does both, we'll get to that. Opposite that part of the chamber is usually a depression that contains the valves and sparkplug. There have been designs that mill the head flat and put the depression and squish/quench deck in the top of the piston. The 348/409 Chevy being an example as is the 383/410/430/462 Lincoln. These engines surprisingly have breathing problems as do conventional chamber in head designs where the chamber is large, like nearly anybody's SMOG head.
Wedge engines like three things because of their port geometry and valve arrangement. These are:
First, a tight chamber on the exhaust side to reduce flow on the side entering to the bottom of the port. Too much flow here cuts off overall port flow because inertia causes this flow to swing toward the top of the port which then blocks the flow trying to enter the port from the center of the cylinder.
Second, is intake swirl. This is enhanced when the cylinder wall side of the chamber swings in a mild parabolic shape from the side of the valve toward the spark plug. An open chamber allows the flow to slow too quickly and swirl is lost.
Third, is spark plug location. The most effective location is near the center of the cylinder favoring the exhaust valve. This reduces burn time since the burn radiates in all directions from a central point rather than starting in one corner and progressing the entire bore distance. The exhaust valve is the hottest item in the combustion chamber and is a known source of preignition or a cause for detonation. By moving the spark plug close to that point, the burn is initiated there and this provides better control since ignition or explosion caused by the hot valve doesn't happen when it's already covered with burnt products. Of course we're not talking mich time the difference between old and lazy heads and modern fastburn chambers is a hand full of milli-seconds. But what a difference those few milli-seconds makes.
OK, I'm getting to dish versus flat tops versus domes.
Remember I mentioned squish and quench, these are functions of that flat side of the combustion chamber. Both of these functions are aided by as close a closure of the piston's deck to the head's deck as is possible to achieve within mechanical limitations.
First in the compression cycle squish happens as these two sections close on each other. This drives the mixture in this area back into the chamber by the spark plug with great force. The resulting turbulence and high density makes for a fast and complete burn when the plug ignites it. This reduces the amount of timing lead which reduces early forces in the combustion process that want to drive the piston back the way it came, which results in power lost. It also delays peak cylinder pressure and temperature to a moment when the piston is headed down the bore in the proper direction. This effect is to reduce the tendency to detonate.
The second event after ignition is that the close proximity of the piston and head decks perform what's called "quench". This is where the hot end-burn gases that like to detonate are trapped between two relatively cool surfaces with a lot of area compared to the volume. This sucks a lot of heat out and prevents detonation.
A third feature of modern heads is the beak that protrudes from the squish/quench deck between the valves. This helps prevent incoming mixture from being sucked thru the exhaust during the cam's overlap phase. The result is a denser mixture upon compression and more power and efficiency. Of course this dampens out the rough idle of a big cam. But it trades the rumpty rump idle for more power, better fuel economy and lower emissions.
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.
The problem with a domed piston is that it slows the burn speed by both reducing mixture agitation toward the spark plug and causes the burn to have to travel over and around it. The response to this is to add more timing lead and to run excessively rich mixtures to use evaporation of the fuel to cool the burn under the detonation limit. The optimum end game becomes one of finding the best chamber size against the dome size that results in the most power. Efficiency and emissions not being a consideration at these elevated compression ratios over about 10 to 1.
Whew, that's a lot of rocket science. Hopefully your eyes haven't glazed over and you've not passed out.
BogieLast edited by BogiesAnnex1; 05-17-2007 at 07:21 PM.
#24 (permalink)
05-18-2007, 05:47 AM
Quote:Originally Posted by NWayne
I need a little help understanding.
Let's assume you have two engines with 10.1:1 compression, one with flat tops and the other with dished pistons. Both run a .040 quench. Why is the one with the flat top's better?
The problem is you can't make things equal on the quench, if you could it wouldn't make a difference, but you can't, therefore, it does make a difference.
OK, take a wedge chamber typically it consists of a flat portion which is called a squish or quench deck, it does both, we'll get to that. Opposite that part of the chamber is usually a depression that contains the valves and sparkplug. There have been designs that mill the head flat and put the depression and squish/quench deck in the top of the piston. The 348/409 Chevy being an example as is the 383/410/430/462 Lincoln. These engines surprisingly have breathing problems as do conventional chamber in head designs where the chamber is large, like nearly anybody's SMOG head.
Wedge engines like three things because of their port geometry and valve arrangement. These are:
First, a tight chamber on the exhaust side to reduce flow on the side entering to the bottom of the port. Too much flow here cuts off overall port flow because inertia causes this flow to swing toward the top of the port which then blocks the flow trying to enter the port from the center of the cylinder.
Second, is intake swirl. This is enhanced when the cylinder wall side of the chamber swings in a mild parabolic shape from the side of the valve toward the spark plug. An open chamber allows the flow to slow too quickly and swirl is lost.
Third, is spark plug location. The most effective location is near the center of the cylinder favoring the exhaust valve. This reduces burn time since the burn radiates in all directions from a central point rather than starting in one corner and progressing the entire bore distance. The exhaust valve is the hottest item in the combustion chamber and is a known source of preignition or a cause for detonation. By moving the spark plug close to that point, the burn is initiated there and this provides better control since ignition or explosion caused by the hot valve doesn't happen when it's already covered with burnt products. Of course we're not talking mich time the difference between old and lazy heads and modern fastburn chambers is a hand full of milli-seconds. But what a difference those few milli-seconds makes.
OK, I'm getting to dish versus flat tops versus domes.
Remember I mentioned squish and quench, these are functions of that flat side of the combustion chamber. Both of these functions are aided by as close a closure of the piston's deck to the head's deck as is possible to achieve within mechanical limitations.
First in the compression cycle squish happens as these two sections close on each other. This drives the mixture in this area back into the chamber by the spark plug with great force. The resulting turbulence and high density makes for a fast and complete burn when the plug ignites it. This reduces the amount of timing lead which reduces early forces in the combustion process that want to drive the piston back the way it came, which results in power lost. It also delays peak cylinder pressure and temperature to a moment when the piston is headed down the bore in the proper direction. This effect is to reduce the tendency to detonate.
The second event after ignition is that the close proximity of the piston and head decks perform what's called "quench". This is where the hot end-burn gases that like to detonate are trapped between two relatively cool surfaces with a lot of area compared to the volume. This sucks a lot of heat out and prevents detonation.
A third feature of modern heads is the beak that protrudes from the squish/quench deck between the valves. This helps prevent incoming mixture from being sucked thru the exhaust during the cam's overlap phase. The result is a denser mixture upon compression and more power and efficiency. Of course this dampens out the rough idle of a big cam. But it trades the rumpty rump idle for more power, better fuel economy and lower emissions.
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.
The problem with a domed piston is that it slows the burn speed by both reducing mixture agitation toward the spark plug and causes the burn to have to travel over and around it. The response to this is to add more timing lead and to run excessively rich mixtures to use evaporation of the fuel to cool the burn under the detonation limit. The optimum end game becomes one of finding the best chamber size against the dome size that results in the most power. Efficiency and emissions not being a consideration at these elevated compression ratios over about 10 to 1.
Whew, that's a lot of rocket science. Hopefully your eyes haven't glazed over and you've not passed out.
BogieLast edited by BogiesAnnex1; 05-17-2007 at 07:21 PM.
#24 (permalink) 05-18-2007, 05:47 AM
12-01-2017, 05:37 PM
#24
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iTrader: (7)
here is the link to post 22. Ck out the response he gives to grasshopper. I don't think I'd try and go up against ( bogiesannex1 ). Like anything else though all is debatable. I'm just posting because I think it's interesting and good information to share with others.
dished-vs-flat-top-pistons-116001-2.html
dished-vs-flat-top-pistons-116001-2.html
Last edited by getrdunn; 12-01-2017 at 05:51 PM.
12-01-2017, 06:08 PM
#26
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iTrader: (7)
i was having some iPad issues and the link wouldnt copy and paste for some reason I couldnt post it but if you haven't seen it look up (combustion engine explosion). There's a YouTube video on a Briggs in Stratton engine with a thick clear acrylic head. Kinda cool!
Last edited by getrdunn; 12-01-2017 at 06:10 PM.
12-01-2017, 06:35 PM
#28
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iTrader: (1)
This is a mild 5.7L. No need to over think it.
