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Flat top -small chamber VS dish piston - smaller chamber

General engine tech -- Drag Racing to Circle Track

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levisnteeshirt
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Flat top -small chamber VS dish piston - smaller chamber

Post by levisnteeshirt »

the combo i'm looking at , the dish piston is lighter , about 30 grams ,

was wondering if the benefit of keeping a tight quench , with maybe a groove in it , would be more of a benefit than a lighter piston with the dish piston

expect 75-7800 rpm range
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Post by Big Rat Attack »

I would go with the flat top piston slightly larger combuston chamber.The 30 gram lighter piston would be just a little easier at high rpms on your rod bolts and rods in general might make a hair difference in how quick the engine reaches its power band. the flat top piston would be smoother for a better burn across the piston the bigger chamber would probably be a help at higher rpm, even at low lift on the valve the bigger chamber may unshroud the valves better.
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Post by F1Fever »

I would go dish, for the "rounder" chamber and lighter piston personally...
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Post by David Redszus »

The ideal combustion chamber shape would be a sphere; technically called a bomb. It would also have a central point ignition which would provide the shortest flame travel path, quickest burn, and highest combustion pressure.

A hemi seeks to approach this ideal shape.

A small diameter combustion chamber with a dished piston is a step in the right direction provided there is enough squish area to produce a high squish velocity.
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Post by PackardV8 »

Most current OEM designs are small shallow chamber and dished piston.

thnx, jack vines
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Post by W. Tripp »

David Redszus wrote:The ideal combustion chamber shape would be a sphere; technically called a bomb. It would also have a central point ignition which would provide the shortest flame travel path, quickest burn, and highest combustion pressure.

A hemi seeks to approach this ideal shape.

A small diameter combustion chamber with a dished piston is a step in the right direction provided there is enough squish area to produce a high squish velocity.
In this ideal shape, how important is sparkplug to piston clearance?

I seem to find that keeping the spark kernal within a given distance from the piston at ignition through TDC can make a measureable gain. And there seems to be a difference between what a hot CDI and a good inductive spark want for best results.

This seems to indicate a donut shape is better than a stepped hemi (hemi with squish) shape.
Perhaps I am just tilting at windmills and misunderstanding my results. :wink:
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Post by kirkwoodken »

Quote:This seems to indicate a donut shape is better than a stepped hemi (hemi with squish) shape.
Perhaps I am just tilting at windmills and misunderstanding my results. Unquote!


The squish band combustion chamber works great in two strokes, and is probably the ultimate, needing very little ignition advance. The problem is getting valves in its restricted size.
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Post by W. Tripp »

Ken,
I am talking about a half donut shaped piston dome, not a full donut including chamber.
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Post by kirkwoodken »

Maybe the designer of the sleeve valve engine was on the right track. No poppet valve to hinder air flow. Can use any shape combustion chamber for best burn control.
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Post by rmcomprandy »

Reguardless of it's CC size, the SHORTER chamber will usually make more power as the valve gets to see the wide open spaces of the bore at a lower valve lift.
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Re: Flat top -small chamber VS dish piston - smaller chamber

Post by pdq67 »

ICON makes a stepped flat-top eye-brow piston. If you want to look into them??

At least in 4.155" dia.

Say, a 50 cc chamber and ..... Gary, (aka, GOSFAST), can hook you up.

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Re: Flat top -small chamber VS dish piston - smaller chamber

Post by mag2555 »

While on the subject of Pistons let it also be mentioned that running a piston with the ring pack as close to the deck of the piston as possible without burn thru's helps to make greater levels of power due to having less fuel on the cylinder walls along with the benefit of having the top part of the walls ware less.
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Re: Flat top -small chamber VS dish piston - smaller chamber

Post by brentry »

mag2555 wrote: Fri Sep 11, 2020 6:03 am While on the subject of Pistons let it also be mentioned that running a piston with the ring pack as close to the deck of the piston as possible without burn thru's helps to make greater levels of power due to having less fuel on the cylinder walls along with the benefit of having the top part of the walls ware less.

Probably less chance for detno to.
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Re: Flat top -small chamber VS dish piston - smaller chamber

Post by My427stang »

Dish and smaller chamber, up until the point you cannot support airflow or compression numbers you need.

Early Ford 352 heads are like this, untouched are lucky to flow 210 cfm, despite the intake port being almost identical to the later larger chamber that flowed 240-ish.

However, if the chamber is a good one, not just small, always like a dish or matching cup if the numbers allow.
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Re: Flat top -small chamber VS dish piston - smaller chamber

Post by houser45 »

It’s all going to be hinged upon whether the larger chamber size allows the flow that the head wants, if the dish set up shrouds the valve flow because of the small chamber vs. A little more optimize shaping under the intake valve on the flat top the flat top version may make more power, I wouldn’t worry about the 30 g of weight savings. Some instances in shrouding the valve too far would hurt the head so it’s all gonna come down to testing. Opinion wise I would probably go with the flat top
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