High compression topic, dish vs flat top vs dome.

Ky brings up some points in my thread on the other site, I didn't start this topic to be a dick (for a change), but rather for general information. I'm not talking about 10-1 pump gas crap, but rather 14-17 to one "race" compression and how its achieved...........and how is one type of piston shape better than the other? And at what point is too much for a specific application.

No blowers, no nitrous, no turbo's, I'm talking n/a and even lean more toward an alcohol point of view.

IMHO, the flatter the piston, the better the flame travel. Sometimes a dome is necessary to get the compresson up (like a bunch on a chevy ).
Compression is free horsepower, but everything has it limits.

"an alcohol point of view"
LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
That's all you guys have

I would try to achieve it with a flat top first only because the dome will affect flame propagation...

a dome works good......just not to the point it gets "ugly".

all will work , domes with the right shape give up little if not nothing . depends what head , imo I rather have a dome with A460 heads , then cutting the head for a 65/70 cc chamber . boils down to - do what you have to , just don;t give something else up by doing it

Great subject Jon ... did some reading

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.

Here is a nice dome

Nice research Nick

"the problem with dome pistond" will say it again -- thats way BB Chevys can't make power and are the slowest thing on the track . they got them big ass dome pi`stons

The 466 school bus motor has big well designed domes, ran the same with a 6 al and blaster coil as it did with a 7 al 2 plus so I think that would rule out the flame propagation thing. It ran a 5.51 in my car and that's nothing to sneeze at. for a little engine

bruno wrote:Great subject Jon ... did some reading

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.

I'd think you should get a copy of the 2010 BBF Winter Tech Seminar and review Darin Morgan's talk before you copy/paste .

Come on Lem, let us have some fun with our old flat top love affair. I like them in the brands of Diamond and Gibson.

Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

HotDamn,That means I've got the Hot setup

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

Sometimes you got to do what the customer wants brother....................

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

I didn't know you could get 14 to 17 CR with a small dome in a 468 cube A460 head engine , thats without excessive cutting the head .

bbf-falcon wrote:

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

HotDamn,That means I've got the Hot setup

me too! small dome and milled the heads excessively!

DanH wrote:

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

I didn't know you could get 14 to 17 CR with a small dome in a 468 cube A460 head engine , thats without excessive cutting the head .

The original post was by richter69....he referenced Mr. KY. Both have 562" engines.......that is the context of my statement.

richter69 wrote:Ky brings up some points in my thread on the other site, I didn't start this topic to be a dick (for a change), but rather for general information. I'm not talking about 10-1 pump gas crap, but rather 14-17 to one "race" compression and how its achieved...........and how is one type of piston shape better than the other? And at what point is too much for a specific application.

No blowers, no nitrous, no turbo's, I'm talking n/a and even lean more toward an alcohol point of view.

Lem , if you can show where anyone else was mention . I see it as a general question . there is many specfic applications . . Randy did give one - bus engine with a dome , which is under 500 inch

Lem Evans wrote:

DanH wrote:

Lem Evans wrote:Three options.......small dome, flat top or dish. None of which includes milling a head excessively.

I didn't know you could get 14 to 17 CR with a small dome in a 468 cube A460 head engine , thats without excessive cutting the head .

The original post was by richter69....he referenced Mr. KY. Both have 562" engines.......that is the context of my statement.

Jon also works on a small block

I know all about the school bus domed pistons........they are a custom Wisco that I ordered and installed in Bill Peak's A429 headed engine about 14 years ago. Wisco did a great job on them......a nice shape and a modest hieght.

I use domes in both my smallblock engines..............but to get to the target compression of 14-1.

I have nothing against domes, or using them to get to a certain number.

I don't know enough about the difference of the 2 BUT, I would rather get to that high compression by using a Dome than by cutting the shit out of my head because if you ever wanted to go a different route,and wanted to sell the heads,you might have a hard time selling them because of that. jmo