ztrips

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?

C-4 Now

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

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!!!

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??

and in case no one told you, you've got a #$@%$%@#$%ing HUGE turbo stuck in your grill

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....

MaxLean

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.

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.

CFI-EFI

Buy a piston with a "D cup". Don't laugh, that's what it's called. The dish is "D" shaped so that the flat portion of the combustion chamber has the flat portion of the piston under it. Squish AND dish! I had to use pistons with a 16 cc dish, just to get the compression down to 11.75:1, and they have a great quench area.

RACE ON!!!

Bilyk

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?

olman

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.

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

ztrips

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.

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.

Baldturbofreak

Why thankyou, lol

Baldturbofreak

Why thankyou, lol

tjwong

All the points brought up are valid, but no one mentioned the fact that if you have a larger chamber it unshrouds the valves which will promote better flow which of course promotes more power 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.

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.....

tjwong

Or if Dart would only cast a Little M block for the LTx series engines I would be in Hog Heaven

Baldturbofreak

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.