Flat tops with small chambers?

Domed Pistons with normal chambers?

Say for a custom nitrous fed beast pushing 300-400shots? I know either way the ringlands need to be spec'd and ring packs, but looking at good clean flame propagation vs. heat soaking issues vs longevity etc. Not to expect the engine to be a Daily Driver either, so a freshing up seasonally would be in line.

Charlie

 

All the best performing engines seem to favor the combustion process happening as close to the center of the cylinder as possible. Because of this, the trend is small combustion chambers and flat or dished pistons. Domed pistons tend to squish the combustion away from the center and away from the spark plug...not good for nitrous or for N/A performance.

 

I have heard the same but then thought I read where designs have changed and have the dome with the flame centering thus keeping temps distributed evenly.

 

Small Chambers and Flat Tops are the best way IMHO. A small dome just to even out what you lose with the valve reliefs would be fine. It's kind of hard to do it with any of the stock LSX castings unless you weld and port and weld and port which can be $$$. The furthest I've heard a LSX factory casting being angle milled is to 51 CC's.

What Head Casting are you using? What amount of SCR do you want?

 

Small chambers and a flat top or effective dish will be the best. Smaller chambers also flow a bit better, at least in the case of the heads I'm getting, but I guess that really just depends on the design of the head you use.

 

i used small domes (6.6) and some milling (.020) for a 64.7cc chamber.

http://www.ls1camaro.net/freehosting/piston-1.jpg
http://www.ls1camaro.net/freehosting/piston2.jpg

12.03 SCR

 

I feel the flat top and a smaller chamber is better.
JRP's setup will work and the pistion isn't the huge of a dish.
However a flat top and 55 or less cc head will get my vote.
Check out ET performances new heads. They are made to order.
You can get a 50cc head very easily with them for no extra.
Might be a great idea for what your doing.
Attached is a pic of a domed piston similar to what JRP is running. +6cc's.

 

Oh yes, I am and have been looking at ETP's new 11* heads Well, guess I'll go with smaller chambers now.

 

Dish with valve reliefs and 59cc chambers seems to have done well for me.

 

Originally I voted for other which would be a dish with small small combustion chambers to keep the majority of the combustion happening in the center of the cylinder to avoid detonation. However, I think the thickness of the piston becomes a very big issue with this kind of hit of nitrous. That being said I guess I would still favor a smallish chamber with a small dish in the piston. Helps keep detonation away and maintains the proper thickness of the piston.

When it comes to n/a performance one need only look at what almost everyone in Engine Masters does, small small combustion chamber with large dish in the piston. Could be an issue for nitrous though because of thickness in the top of the piston.

 

Well, a few differences now I see. I looked at Diamonds and JE tech info on nitrous pistons, seem to be majority of flat tops, of course these are only ratings up to 300shots., but not sure how compression would effect that, as in:

10:1 SCR with 300shot compared to 12.8:1SCR with 300shot. IIRC my pistons were quoted as being 12:1 capable and 300shots? Might have to get the PN off them and find out from Diamond.
Before ordering the heads, I have to know so I can get <55cc chambers or stick with 55> chambers. Looking now at 52cc for 12.75:1 with current .040" Cometics, -.007deck, 2cc vr pistons. Kind of high for some ppl but I think with the tuning and remaining parts I'll manage nicely. Might go lower, and build a more bullet-proof bottom for all out nitrous use. So many choices.

 

A few things I see.
1. you are doing a new bottom end. Your piston height will more then likely be different then the avarage -.007 we use when figuring out compression.
Make sure you find out the new piston deck height as well when figuring compression.

2. Why go 12.8 vs 12:1 compression?
IMO the difference will be almost 0 unless you run a higher octane fuel to help with the added compression. That is just my oponion. Also with compression that high a new started is in you near future as your stocker is going to hate that much compression. I am just tossing out a oponion here.
I myself would order either a flat top or something like a 6cc dish and run a smaller chamber head to make the compression.
Both way will work and work well. I think the bigger issue is making sure you get the best rings for that much spray and some nice coating on the pistons.

 

I verified the deck height, and its there -.007. I thought about the starter vs. compression but forgot to think it through thanks for the reminder. If going higher, I fully intend on running higher octane, and getting the "most" out of the package on race nights We'll see.

 

Looks like the majority here is small chambers. The problem with that is the more that you mill, the thinner the head surface, more possibility of a crack. If you mill too much, the heads sit lower and the intake bolts and ports don't line up. Maybe that is not a problem on LS motors?? I like big domes, and deep pockets for valves. If you have ever had a valve hit a piston due to a weak spring, you know what a disaster that is. Big domes, heads not milled, everything bolts up fine. What I am reading here is that there are performance gains with the small chamber..

 

racecar, with the newer aftermarket casting heads the deck is thicker to allow for custom milling and no loss of integrity.
I have heard arguements from both sides, dome is better and chamber is. I am looking for a street/strip, reliable higher compression, low quench height for a cleaner burn w/ less detonation etc... With that in mind, and I already have a forged bottom, I am looking for ETP 11* to spec chambers for desired SCR. Now all I have to do is make up my mind on how I want to go.

Charlie

 

Equal heat distribution and transfer is a very important part of piston design. A flat top, a flat top with four valve reliefs (427 LS7 version), pop ups, or any dish or reverse dome types are good as long as the dish or pop up is centrally located for a more even heat transfer across top and through piston.

 

If you dont want to mill and you want to use the stock pistons go with a thinner gasket. Commetic makes all kinds. My stock heads are 64cc with 10.5-1 and my afrs are 66cc with the .040 cometic it comes out to 10.8-1 with no milling and stock internals.

 

I have early SDPC ported heads. The porting was done by Lingenfelter. ...anyway, the LS6 castings are milled .055!!!!!! I guess the static compression is over 11 to 1.

No real issues with alignment of the head bolts, manifold, etc.

 

Tiny combustion chamber with reversed dome allowing better atomization of air and fuel.

Chris