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absofrigginlutely nothing actually....

I am alway's listening to people say this~N~that about cc sizes and have seen heads with 60 something cc chambers go for a certain $ amount and other heads with a certain cc size go for other amounts... and hear talk of heads with 170+ cc chambers and heads with 50 something chambers and was idly thinking bout the whole overview and how it is relevant to anything performance wise... I see the following possible and corrrect me if I err:

Generic engine with 200cc heads and a dome piston can make X amount of compression with any old X rated cam.

Another Generic engine with 50cc heads and a flat top piston, can make the very same X amount of compression with the same X rated cam.

So what makes Either heads particularly better or worse than any other?

(NOT taking into account the valve sizes because we know that they can be larger than stock and then cantering them will enable making them even larger than that... so adjust the thinking to a standard size int and exh valve and Focus on what is a better thing and possibly Why...)

FE

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<font size=-1>[ This Message was edited by: FEandGoingBroke on 4/13/06 1:34am ]</font>
 

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The trend is smaller chambers with mirror chamber dish in the piston. Lots of turbulence, lots of quench, good detonation resistance. I think you're confusing intake port volume (the "200cc, 180cc, 218cc", etc) with combustion chamber volume. A certain amount of intake port volume is necessary to support a certain level of power production. Chambers are worked to promote quench, swirl and flame travel. The size is almost secondary and you can design the piston to arrive at the compression ratio desired.
 

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On 2006-04-12 10:35, ckelly wrote:
The trend is smaller chambers with mirror chamber dish in the piston.
This is what I've heard/read as well. Take three engines with identicle compression. One achieves it by a the largest (of the three) combustion chamber and a dome piston. Second one with a smaller combustion chamber and a flattop piston. The third by a small combustion chamber and a piston with a dish that is the mirror image of the combustion chamber.

Three will make the most power followed by two and then last is number one. For the reasons ckelly stated.

What kind of head has a 200cc combustion chamber? This would have to be a huge displacement motor!!!



<font size=-1>[ This Message was edited by: 289nate on 4/13/06 2:06am ]</font>
 

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Chamber volume differences on an otherwise identicle head are more for helping to achieve a desired compression when you already have a shortblock and are not changing the pistons. Maybe a 58cc chamber will bump the compression enough that you won't be able to run on pump gas. Then go with the 61cc option. The opposite if 61cc will make you drop too much compression.
 

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On 2006-04-12 10:58, 289nate wrote:
On 2006-04-12 10:35, ckelly wrote:
The trend is smaller chambers with mirror chamber dish in the piston.
This is what I've heard/read as well. Take three engines with identicle compression. One achieves it by a the largest (of the three) combustion chamber and a dome piston. Second one with a smaller combustion chamber and a flattop piston. The third by a small combustion chamber and a piston with a dish that is the mirror image of the combustion chamber.

Three will make the most power followed by two and then last is number one. For the reasons ckelly stated.

What kind of head has a 200cc combustion chamber? This would have to be a huge displacement motor!!!



<font size=-1>[ This Message was edited by: 289nate on 4/13/06 2:06am ]</font>
You are right that generally the best design for a sombustion chamber is a dish piston and a small chamber in the head (assuming the same compression ratio, valves, etc.) Same concept as the old Hemi motors. Physics says that the closer to a sphere we can make the combustion chamber with the spark as close to the center as possible will burn the fuel most efficiently.

But lets face it if I already have a short block and want compression ratio X, I'll find a head that comes close to what I need. Or if I have a nice set of heads already set up, I'll find pistons with as much dish or dome as I need to get the compression ratio I want.

While a domed piston makes the shape of the combustion chamber less efficient, it may overcome that loss by producing higher compression (again assuming all other parts the same)

If I wanted to build an engine from teh block up not reusing any parts I already had, then I'mm probably better to use a small dished or flat top piston and a smaller combustion chamber if it will get me very close to the same compression ratio as an equivalent head with a larger chamber and a domed piston.

Or I can just do it the easy way, using a dished piston to create an efficient combustion chamber shape and use a turbo or supercharger to produce to compression I want.

All this is assuming that I don't have much of a budget to stick to. Back in the real world we do the best we can with what we can afford and dont always have the time/money to wait until we can buy the perfectly matched parts and then build it the way we want it.
 

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I suppose most people don't do it this way, but if I were to build an engine for performance (even a milder driver,) I wouldn't just take Shortblock A and slap Head B on there and call it good. You can make power with it, but not as much as you could if you spent the time to research the combo.

If I don't have enough to build it right, I'd wait until I do.

There are benefits and drawbacks to each shape of combustion shape - heart/closed/fastburn vs. open/D-shaped vs. hemispherical - Yeah, hemi's can extract more power because of their design - but they don't generate the turbulence of a fast-burn closed chamber, and so can be more prone to detonation.

To more answer the question, the actual size has little do do with power potential - the piston can just be made to take up that extra room. The shape, however, has been constantly evolving both to make things more turbulent in the chamber and to extract the most force out of each firing. Its a compromise.

Some chambers are big (especially the pent-roof chambers in multi-valve heads) because to squeeze all the valves (or large valves) in there it has to be large. The piston can take up this extra room.
 

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I certainly agree that we mostly have to make do with what we have, because a lot of that theory-to-pratice stuff costs money. Just depends on what you want. But you'd be suprised how much power is in the details.

I've told it elsewhere, but it applies here - we re-did the 302 this winter. Zero decked the short block and took 1 cc off the heads for a total of 60cc. Put in fresh top and middle rings and fresh bearings. That's it. Everything else went back in as is - cam, carb, etc. Car picked up .2 and 2 MPH, after 3 months at the track we're at .3 and 3 MPH. That's about 20~30HP. The compression ratio went from 9.0 to 9.5. On paper, that would make very little difference by its self. But the engine really liked the change and responded way beyond what the slide rule might have said.

The extra power cost $700.
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<font size=-1>[ This Message was edited by: ckelly on 4/13/06 9:23am ]</font>
 

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Fazle is right that a spherical chamber can lead to detonation easier, I was simply talking about the most power. Detontation can be avoided by higher octane or timing curve, but depending on what you are trying to do is not the best overall solution for a lot of people.
 
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