I have been reading about this lately and have a question...

If you use, for example, a Keith Black 1470 dished piston isn't your effective "quench are" gone so to speak?

Just wondering how the quench is affected by a "standard" dish and not a D-shaped dish. I understand that with a D-shaped dish you will still have a quench area.

CG

 

the quench area will be bigger but those pistons are not made for racing anyway. they're made for low compression and retarded timing (low power) engines

 

The quench area is the flat spot around the edges of a dish piston.

 

Your quench isn't gone, but it is definitely less than a flat top or D-shape.

 

So maintaining the factory 0.04" quench is not really necessary for a nominal performance build with 9 ~ 9.5 compression?

CG

 

Factory quench was more around 0.08" or so. And those pistons are also a reduced height.

 

On what engines?

 

The KB 1470's have a compression height of 1.54" (stock is 1.56"), so if you deck your block 0.02" you are back to OE specs with a 0.025" deck clearance and 0.015" head gasket, which equals a quench of 0.04".

CG

 

I thought all of them, but maybe I'm about to learn yet more about the small block chevy.

I know the L31 Vortec is around that quench, or so I thought.

 

The 2 350 91 roller blocks I had the opportunity to mesure had .025 piston to deck, but I could not mesure the gasket precisely.

 

The Vortec truck motors use "full face" dished pistons. Just the outer rim is up around deck height, the whole center part of the piston face is the dish. Not much "quench area" with them. The outer rim is at a typical ~.025" down from the deck surface, but the dish in the center is probably more like .080-.090. Much too far away to give you any meaningful quench effect benefits.

Please note that while a good tight quench area is good for performance (becuase of the mixture motion it gives as the piston passes TDC) it's not good for emissions compliance. It gives an area where unburnt A/F mix can hide out and escape proper combustion.

It's one of those areas where the goals of a performance engine builder and the goals of a factory engineer are a little different.

 

Thx Damon. That is the kind of info I was looking for.

CG

 

I thought they used flat-top pistons with a .050" head gasket?

Now I'm confused. I thought the whole "fast-burn" chamber thing was created by heart shape and the quench area. Why have that huge flat area in the first place, wouldn't a hemisphere chamber work better in this case?

 

I dont think having a tight quench area neccesarily means poor emmisions. It helps speed combustion in the volume not under quench and that has positive emissions effects. But I dont really know what the trends are these days.

All the 80's/90's engines Ive torn apart with factory gaskets had the .015" shims. That Put quench at .015 + .025 piston down = .040" Right where everyone seems to reccomend before getting nervous about piston to head clearance (LS1 startup piston slap problems come to mind)

I actually switched back to shims on my motors to tighten up quench after running into detonation problems. With a dynamic compression ratio of more than 7.5 or so, my engines always got very sensitive to detonation. Higher DCR motors tend to need all the help they can get to be robust (tolerant of non optimal timing) at such a high specific output. A tight quench helps to homogenize the mix during the burn making for predictable timing requirements and A/F ratios helping the motor be more tolerant of changes in ambient conditions and throttle settings.

Nothing wrong with a full dish if you dont care about every last bit of power that displacement could yeild in "off design" (not WOT at torque peak) operation. Heck, Hemis are great engines in "on design" operation (usually WOT upper rpm's) but they tend to be peaky in "off design" operation. To a certain extent fuel injection can correct for this as can 3D timing maps but now we are talking about stuff I havent really had much experience with.

Do some searches on quench around here and you should find some interesting stuff. Thirdgen is a good source for engine design philosophy stuff.

 

No, pistons are dished in a Vortec. Only way to get compression down at the truck's rated 9.1:1 with 64cc Vortec heads.

You are thinking of the ZZ4 engine that uses flattops, 58cc chamber heads and big 'ol fat .050+ thik head gasket to hit it's rated 10:1 compression ratio. The LT series of motors also uses flattops and specs out around 10.5:1 but they heads are specific to the LT motor (i.e. they're not Vortecs).