Steve Bryant

iTrader: (1)

Do you think it is better to have a large combustion chamber and a small piston dish or vice versa while maintaining the same compression ratio?

Let's say that with a .040" gasket you could have a 62 CC combustion chamber with a 20 CC piston dish or a 72 CC combustion chamber with a 10 CC piston dish for a given compression ratio, what do you think are the relative advantages of each? For instance, the bigger piston dish would reduce reciprocating mass and the valves might be less shrouded with the bigger combustion chamber.

Your thoughts please.

Thanks,

Steve

SStrokerAce

iTrader: (2)

Dish and small chamber... well as small as you can get it without hurting the flow. The closer the chamber is to a sphere the better you are.

Bret

DavidNJ

The reason for this recommendation is...???

Wouldn't it also be dependent on the shape of the dish and the piston to head clearance?

Beast96Z

iTrader: (21)

I was told the same when putting together my parts. The smallest safest combustion chamber will work best. Then fix your CR with the dish.

TheBlurLS1

iTrader: (8)

Any notes about quench in this?

I've been thinking about this issue as well. When a company sells a "-12cc dish piston", is it dished under the valves, or under the flat spot of the head as well? It seems they would like to keep it as flat as possible under the flat part of the head surface (for quench), and have a huge volume under the valves.

This volume under the valves would help with compression and PTV clearance.




BTW - I'm an internal engine n3wb, but at least I have the smarts enough to admit it.

SStrokerAce

iTrader: (2)

You've read Larry Widmer's articles on it....

v8pwr

iTrader: (19)

I've never read Larry Widmer's articles but I think it's better to have a small chamber for a few reasons . I would prefer to have more surface area on the piston versus the head . I know it's a small difference but it's still psi , more area = more force . For a given cylinder head casting , the smaller chambered head would have more material to resist deformation and control heat . Most pistons come from the same forgings to reduce costs and the greater dish volume obviously reduces piston weight which would be great till durability begins to be sacrificed . I know chamber and piston shape also influence power production , and combining these qualitites would help make great power . It was my understanding that a football shaped fuel kernel was ideal , having the edges equal distance from the spark plug . Great thread , and my .02 .

DavidNJ

And I've talked to Larry on the phone. The dish isn't really dish, its a reverse dome making the as-close-as-possible spherical chamber. Other than unshrouding a deep set valve, the advantage was being able to control the radius at the floor of the chamber, rather than have the opening there to the quench area.

If the quench was tight (you've may have read my question posts asking about sub-.030 quench on 7k CI blocks and sub-.025 on aluminum blocks) and the chamber well shaped the benefit is smaller.

And the pistons are harder to make. Virtually always custom. Some standards, like Yates heads, have reverse domes available. Others were custom. Diamond suggested sending them a head.

And finally, several people and piston companies--with far more experience than me--say that the dished piston adds weight even with lightening machining.

It would seem the benefit would come after beneficial spark plug location, top ring land depth, and overall piston ring sealing.

Diamond and Manley have new for '06 short pin (2.25-2.3") pistons. Diamond will add lightening to a standard piston without a custom surcharge. It seems you would need a pretty extreme engine (competitive racing, everything else done) to justify the cost and time of the reverse dome.

SStrokerAce

iTrader: (2)

David, I'm well aware of revearse concave dome pistons... that's what I was referring too. Hence the Windmer reference.

Bret

DavidNJ

However, the posted said 'dished' piston. LS1 dished pistons look more like this:


than a reverse dome.

white2001s10

iTrader: (4)

I agree with this.

Steve Bryant

iTrader: (1)

Although in theory symetrical, mirror image combustion chambers and piston crown topography is worth considering, I'm really wondering about the practical aspects of commonly available production/aftermarket parts. Consider the 243 versus the 317 production heads. They are for all the world variations of the same part except for a few CC's of combustion chamber volume. Now, you are going to buy off the shelf pistons (JE, Diamond, Wiseco, Mahle, anybodies) for a production LS2 block with a 4.000" bore and you have the choice of dished (not reverse dome) pistons in increments of 2 CC's. Which combination would you go for and why?

Steve

v8pwr

iTrader: (19)

Which piston manufacturer would you choose to make a set of pistons for heads fairly common like LS6's ? Who would provide good turnaround on a custom set without costing so much . I know it costs money to play , how much would a set of custom pistons run ? Can someone give a breakdown of the initial cost then how much the options such as horizontal gas ports , lightening , custom ring sizes , and whatever else they can do . Is there a company that is better to deal with than others , why ? Any benefit to using one manufacturer over another such as A is affordable but slow , B is expensive but can do it all , C has the best tolerences . Thanks for any info .

RednGold86Z

[QUOTE=v8pwr] I would prefer to have more surface area on the piston versus the head . I know it's a small difference but it's still psi , more area = more force . QUOTE]

This is quite wrong. Even though there is more "surface area" it's not "normal to" the cylinder axis, thus it does nothing to push the piston down, and only forces opposed surfaces apart. If you want a better picture in your head, just imagine a piston with 100 bumps like a golf ball. It won't feel any more pressure in the downward direction than a flat top piston. The ONLY way to get more usable surface area is to increase the bore.

If I were building an engine, I'd go with as flat of a piston as possible, with the lowest mass, and most of the chamber in the head. The flat surface will promote flame travel much better. Not all combustion happens during TDC. The flat design also tends towards lighter pistons.

Jessica

This is what all the Winston Nextel Sprint Cup engine builders do (Exept for the ones getting lapped)

SStrokerAce

iTrader: (2)

Steve,

If that's what you are shooting for then the chamber shapes and compression ratios you are looking at are not going to fit into the small chamber dish setup. When you do that, especially on something that doesn't start off with a super small chamber like a 243 casting, you are welding the heads AND making custom pistons.

In that case you are better off going with the best chamber design you can get your hands on. The 243 over the other OEM castings, or something like the Dart chamber and milling that down to size and then running a flat top piston.

Bret

SStrokerAce

iTrader: (2)

Diamond does nice custom work, Greg Anderson uses them so they are good enoug for me.

JE can get you anything you want, along with CP as well.

To get the rest of the info you are looking for call up each company OR work with someone who sells those parts and they should be able to get you a quote on what you want.

Bret

SStrokerAce

iTrader: (2)

Good examples of the small chamber and reverse dome setup. All of those chambers were welded up to get them that small.

Bret

Richiec77

iTrader: (3)

I've also noticed that the Valves are also pretty much unshroaded (sp?). That would help also.

I understand the reason for a spherical chamber is do to the fact that any object that burns goes from solid/liquid to a gasous state. That means there will be a change in pressure and it'll move in an Omni-directional way. Hence the Sphere being the untimate "perfect" idea.

How is the Football shape better? Is it due to the sake of the chamber design due to inline valves?

SStrokerAce

iTrader: (2)

The football shape is a result of the valve placement, yes.

BTW good quote by the old man in your sig, words to live by!

Bret