Bigger combustion chamber or piston dish??
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Pro
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Bigger combustion chamber or piston dish??
OK. I'm trying to build slightly less than 10:1 CR. I'm using a 3.75" stroke crank and 6" rods.
Options:
Use smaller combustion chamber heads (54cc) and larger -CC dish
or
Use larger combustion chamber heads (62cc) and smaller -CC dish
This will be a blower motor. What combo (given same CR) gives better performance?
Options:
Use smaller combustion chamber heads (54cc) and larger -CC dish
or
Use larger combustion chamber heads (62cc) and smaller -CC dish
This will be a blower motor. What combo (given same CR) gives better performance?
Drifting
Joined: Nov 2003
Posts: 1,637
Likes: 2
From: Honeoye Ny
a bigger dish will help "point" the combustion pressure down. I might have somthing up your alley. www.baldturbofreak.com Go under the project cars and under btf turbovette there are pics of my pistons. htey are custom wiseco's for a 6" rod 383 they have a -37cc dish and will provide 8.86-1 with a 54cc head. designed for lotsa boost
Last edited by Baldturbofreak; Jan 24, 2005 at 07:32 PM.
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
I don't think it makes much difference. I have always thought that the greater surface area of a dished piston might exert more pressure on the con rod, than a flat top, because of the psi in the cylinder acting against the more "si" of the piston. However, I have never heard this expressed anywhere or by anyone. I have pretty much written it off as one of my harebrained ideas.
Those thoughts aside, I think I would go with the most available, economical, parts. I would, however not lose sight of a tight quench area.
RACE ON!!!
Those thoughts aside, I think I would go with the most available, economical, parts. I would, however not lose sight of a tight quench area.
RACE ON!!!
Thread Starter
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Originally Posted by Baldturbofreak
a bigger dish will help "point" the combustion pressure down. I might have somthing up your alley. www.baldturbofreak.com Go under the project cars and under btf turbovette there are pics of my pistons. htey are custom wiseco's for a 6" rod 383 they have a -37cc dish and will provide 8.86-1 with a 54cc head. designed for lotsa boost
Also, noticed the fuel rails on your intake. Where did those come from??
and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill
Thread Starter
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Originally Posted by CFI-EFI
I don't think it makes much difference. I have always thought that the greater surface area of a dished piston might exert more pressure on the con rod, than a flat top, because of the psi in the cylinder acting against the more "si" of the piston. However, I have never heard this expressed anywhere or by anyone. I have pretty much written it off as one of my harebrained ideas.
Those thoughts aside, I think I would go with the most available, economical, parts. I would, however not lose sight of a tight quench area.
RACE ON!!!
Those thoughts aside, I think I would go with the most available, economical, parts. I would, however not lose sight of a tight quench area.
RACE ON!!!
Pro
Joined: Jan 2005
Posts: 686
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From: San Jose Ca
From a combustion effeciency standpoint, a flat top piston is supposed to be superior when used with a quench type combustion chamber. And as mentioned above, the piston may also be stronger too. At least that's what I've understood.
Safety Car
Joined: Dec 2002
Posts: 3,655
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From: Charleston SC
Unless you can get a great deal that makes a difference I would go with the larger cc heads. Here is my logic, (Kind of an oxymoron LOL), that if something happens the 54cc chamber there is very little left of the heads to work in the event something happens. I have 3K tied up in my heads and pray nothing ever happens to them If I where go back I would have went with flat tops and a bigger cc head.
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Race Director
Joined: Sep 2000
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From: The Top of Utah
Originally Posted by ztrips
Since most dished pistons (at least large dish) the material is taken away from all over the piston top, would your statement about tight quench indicate a "deeper" combustion chamber (or deeper dish with less area measured at the deck) would be better? I just don't see how a tight quench is possible with a dish....
RACE ON!!!
Drifting
Joined: Dec 2001
Posts: 1,419
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From: Mississauga Ont
here's a question
are the 'cuts' in the dished pistons at all likely to promote detonation at all?
also, wouldn't it be possible to bleed some of the compression with a late closing intake valve like on the LTx or LSx motors?
are the 'cuts' in the dished pistons at all likely to promote detonation at all?
also, wouldn't it be possible to bleed some of the compression with a late closing intake valve like on the LTx or LSx motors?
Racer
Joined: Mar 2004
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From: Oakley Ca
Originally Posted by FD2BLK
Unless you can get a great deal that makes a difference I would go with the larger cc heads. Here is my logic, (Kind of an oxymoron LOL), that if something happens the 54cc chamber there is very little left of the heads to work in the event something happens. I have 3K tied up in my heads and pray nothing ever happens to them If I where go back I would have went with flat tops and a bigger cc head.
I kick myself for going with the small chamber heads cause it limits the number of the shelf pistons available if you are looking for lower compression combinations. You can still get a good squish area with the larger chambers with out pushing the compression up too high.
Racer
Joined: Mar 2004
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From: Oakley Ca
Originally Posted by ztrips
Since most dished pistons (at least large dish) the material is taken away from all over the piston top, would your statement about tight quench indicate a "deeper" combustion chamber (or deeper dish with less area measured at the deck) would be better? I just don't see how a tight quench is possible with a dish....
The dish is not all the way to the outside edge of the piston, but in about a half or five eights inch. The height in the bore is the same as a flattop. Even with a flat top the squish is at the outer edge. Maybe a little more with a flattop, but not that much. At least that is my experience. I've been wrong before
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Side question... How low is too low for compression ratio on a blower motor? I dont' anticipate running more than 12lbs of boost, right now.. you never know what the future holds... I'd rather be on the low side than have to worry about detonation.
Drifting
Joined: Nov 2003
Posts: 1,637
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From: Honeoye Ny
Originally Posted by olman
The dish is not all the way to the outside edge of the piston, but in about a half or five eights inch. The height in the bore is the same as a flattop. Even with a flat top the squish is at the outer edge. Maybe a little more with a flattop, but not that much. At least that is my experience. I've been wrong before
ya beat me to it. In a naturally aspirated motor he is correct a flat top has better flame propagation and will more quickly burn the charge, but in forced induction the burn time last for so much longer due to the charge density. thats why boosted motors produce soo much torque, not because of the incresed cylinder pressure (not as much as you would think) but the increased average cylinder pressure, especially when the crank is at 90 deg (most leverage). I gave up a little off boost efficiency (not like it will knock @8.86-1) for more capabilty on top. My heads have a .750 deck (afr)so I have plenty of material left should things go awry. My pistons have no less than .500 thickness anywhere on the crown so I have plenty of strength. Brian Nutter set these up with 1000 horse+ in mind, there built to take my (utterly severe 
) punishment.
Drifting
Joined: Nov 2003
Posts: 1,637
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From: Honeoye Ny
Originally Posted by ztrips
That's a dish... I was looking at some "off the shelf" wiseco's with a -31cc dish. How do you like them?
Also, noticed the fuel rails on your intake. Where did those come from??
there holley Tpi rail that were modified a little, the injectors are magnetti-marlli pico 069's.(the shorties) that way I can use 2 sets
and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill
Also, noticed the fuel rails on your intake. Where did those come from??
there holley Tpi rail that were modified a little, the injectors are magnetti-marlli pico 069's.(the shorties) that way I can use 2 sets
and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill
Drifting
Joined: Nov 2003
Posts: 1,637
Likes: 2
From: Honeoye Ny
Originally Posted by ztrips
That's a dish... I was looking at some "off the shelf" wiseco's with a -31cc dish. How do you like them?
Also, noticed the fuel rails on your intake. Where did those come from??
they're holley Tpi rail that were modified a little, the injectors are magnetti-marlli pico 069's.(the shorties) that way I can use 2 sets
and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill
Also, noticed the fuel rails on your intake. Where did those come from??
they're holley Tpi rail that were modified a little, the injectors are magnetti-marlli pico 069's.(the shorties) that way I can use 2 sets
and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Originally Posted by Baldturbofreak
ya beat me to it. In a naturally aspirated motor he is correct a flat top has better flame propagation and will more quickly burn the charge, but in forced induction the burn time last for so much longer due to the charge density. thats why boosted motors produce soo much torque, not because of the incresed cylinder pressure (not as much as you would think) but the increased average cylinder pressure, especially when the crank is at 90 deg (most leverage). I gave up a little off boost efficiency (not like it will knock @8.86-1) for more capabilty on top. My heads have a .750 deck (afr)so I have plenty of material left should things go awry. My pistons have no less than .500 thickness anywhere on the crown so I have plenty of strength. Brian Nutter set these up with 1000 horse+ in mind, there built to take my (utterly severe ) punishment.
If I am working on a blown application I would normally use a 70 to 76cc chamber which would allow the use of a 21cc or less dished piston in a 383 which means theres more material on the crown of the piston to take more abuse if needed. A 70cc chamber with a 21cc dished piston will yield about 8.7:1 in a 383 at a .005 deck height and a .040 thick head gasket.
Drifting
Joined: Nov 2003
Posts: 1,637
Likes: 2
From: Honeoye Ny
agreed a bigger chamber does indeed help with flow. but with our 4.0xx bores not much short of rolling the valve angles helps with that oh if world products would only cast us a nice 4.200 bore HD block.....
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Originally Posted by Baldturbofreak
agreed a bigger chamber does indeed help with flow. but with our 4.0xx bores not much short of rolling the valve angles helps with that oh if world products would only cast us a nice 4.200 bore HD block.....Or if Dart would only cast a Little M block for the LTx series engines I would be in Hog Heaven
Drifting
Joined: Nov 2003
Posts: 1,637
Likes: 2
From: Honeoye Ny
the pattern change would be so minimal, too. but hey if they figure they can't make money on it I guess we get left in the cold, so sad.
we should get together with the f-and b body guys an form a coalition. we'll organize a day where everyone calls dart or world and asks if they have a ltxx series block
maybe then they would listen.
we should get together with the f-and b body guys an form a coalition. we'll organize a day where everyone calls dart or world and asks if they have a ltxx series block
maybe then they would listen.
Last edited by Baldturbofreak; Jan 25, 2005 at 10:47 AM.