Combustion chamber size

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bigjoe1
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Combustion chamber size

Post by bigjoe1 »

Do any of you have a strong opinion about a large chamber and a flat top, as opposed to a smaller chamber and a reverse dome piston ?? Case in point== High performance street and strip 406 or 434 Chevy== Afr heads that come with 80 cc chambers to be used with flat top pistons , or 65 cc heads, to be used with a reverse dome piston. With the same ( or very nearly the same ) compression, will both setups make about the same torque and HP. These both will be in the 650 to 700 HP regime. I will say this. I have used the big chambers with flat tops with great success, but I would like to hear if anyone is doing the other method. These are meant to be run on 91 octane pump gas, by the way.


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Re: Combustion chamber size

Post by governor »

Most of the high dollar 9to1 circle track motors were reverse dome and made some very good HP, but they were swisting the snot out of them and they made a lot of heat.

Arn't a lot of the Cup motors reverse dome?

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Re: Combustion chamber size

Post by Rizzle »

Do the different CC heads have different sized quench pads?

How is the spark plug location changed between the two setups?
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Re: Combustion chamber size

Post by alan johnson »

if the reverse dome piston weighs less thats my choice.with these modern chamber's i don't think one size would have
anything on another.
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Re: Combustion chamber size

Post by Orr89rocz »

Wouldnt the charge be hotter inside the cylinder more with a small combustion chamber head and dish piston? VS the flat top and charge more inside the head's chamber? Thinking you'd beable to get away with more compression with a larger cc head/flattop vs a dish/small cc head because you can cool the charge more in the head since the aluminum disipates heat more to the cooling water than Iron would. OR atleast beable to run abit more timing...whether that makes a difference or not i'm not sure but usually more timing is more power to a point.

Concept is used on blower/nitrous cars..use higher quench heights to get the combustion out of the bore and more into the head side
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Re: Combustion chamber size

Post by Unkl Ian »

Would the quench area be the same for both ?
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Re: Combustion chamber size

Post by PackardV8 »

Would the quench area be the same for both ?
Quench area is usually greater with small chamber/cup in piston.

I note most OEMs are going to a small chamber head. I'm told this is for improving squish/quench, pressure recovery and spark plug location.

Interestingly, back in the day, Gen I Ketterings and Gen II Kettering SBC were pure wedge heads, with flat tops and all the chamber in the head. Next came the Heron head, Westlake head, Chevy 348"-409", Lincoln 430"; all used essentially flat heads with all the combustion chamber in the piston or in the cylinder. Then, the trend went to semi-hemi-twisted-wedge, many using domed pistons and back to the compromise of the LS--style small chamber wedge.

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Re: Combustion chamber size

Post by JoePorting »

I'd go with the big chamber/flat top piston. I remember flow testing the AFR 227 65cc and 75cc heads. The 75cc heads had about a 3 to 5 CFM improvement which should translate to another 8 to 10 HP. Not alot, but better then nothing. In another story, when I flat milled a set of 65 cc AFR 227 heads .050", I lost around 15 CFM across the board. ouch!!! So I don't think you can lose with big chambers, but can lose alot with ultra small chambers.
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Re: Combustion chamber size

Post by pdq67 »

I look at it this way and no I don't have any proof!

Imho, the tightest chamber you can use along with a true flat-top piston does well until you run up against the tightest chamber you can run AND a matching chamber outline dished piston to give you the same CR.

And even then, I would go flat-top.

This is way old emissions study stuff here that is driven by octane/timing burn.

If you want to look deep into this, this is why Ed's, (aka, racer1320), BBC ran so good with highly worked over semi-closed -215's.

It's known that the old -206 BB heads with a true "bath-tub" chamber can produce power after being de-shrouded, but they are "dirty" smog-wise. And the power produced is due to timing differences vs the later open chamber heads at a given CR!!

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Re: Combustion chamber size

Post by ClassicComp »

the $3500+ Ed had into those heads are why they worked well.
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Re: Combustion chamber size

Post by JBV-HEADS »

If the chamber is the only item changed, then the reverse dome (horan) chamber has the most potential. It needs to be cut as close as possible to the shape of the chamber in the head. Any areas that do not match will create, for a lack of a better word, dead areas in the technology. The Horan chamber has been ( around WW-1) and still is the most efficient chamber design known for rpms under 7500-8000 for most applications. In many applications the turbulence in the air/fuel mixture above 6500rpm starts to eat into the squish turbulence, and flame front directional advantages the Horan has. Again on many applications somewhere around 7500rpm there will start to exist an exhaust flushing problem. All of these can be controlled and advanced with centered ignition, centered chambers along the pin, equal area and distance on both sides of the pin and shapes. It’s still used, but controlled improvements are limited, in NASCAR today. For like to like comparisons, the reverse dome cut right, will produce more power if there is a compression rule. And your dyno will pick it up. It’s above dyno error. Good luck,

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Re: Combustion chamber size

Post by David Redszus »

The shortest burn path will produce the smallest burn angle and therefore the highest combustion pressure. The best combustion chamber would be a sphere with the spark source in the center. The next best would be a dished (reverse dome) piston since the flame path is shorter and less chamber surface area is available to quench the flame front.
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Re: Combustion chamber size

Post by torquefan »

David Redszus wrote:The shortest burn path will produce the smallest burn angle and therefore the highest combustion pressure. The best combustion chamber would be a sphere with the spark source in the center. The next best would be a dished (reverse dome) piston since the flame path is shorter and less chamber surface area is available to quench the flame front.
question for you, david: since we are applying pressure to the top of the piston to turn the crank, wouldn't you want to use the entire piston top since that pressure is expressed as psi? iow; how much combustion pressure would be developed with a 15:1 compression open chamber headed engine with a 4" bore and flat tops where the combustion pattern is evident for approximately 3.9" vs a 15:1 engine with a smaller closed chamber head and a 4" piston with a 3.5 diameter dish and no evidence of burn outside of the dish?

assuming (i know, bad word) no flow irregularities for the closed chamber and adequate combustion for the open chamber.

could that be why the singh grooves work?
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Re: Combustion chamber size

Post by Unkl Ian »

torquefan wrote:
since we are applying pressure to the top of the piston to turn the crank,
wouldn't you want to use the entire piston top since that pressure is expressed as psi?


The burn lasts long enough that it will act on the full diameter.
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Re: Combustion chamber size

Post by DrillDawg »

Would it make any sense to use a small pressure recovery type of chamber for better intake flow and position the dish on the exhaust side of the piston to help move the gases closer to the exhaust port?

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