squash, quench?

[88bluestar]

What is more ideal for a tight quench/squash in regards to deck height, and chamber size?
Is it better to reduce the chamber size of the head or should it be less deck?
Looking to produce as tight a quench as possible, .039-.045

Was thinking a 0 deck, stock replacement fel pro gasket 4.125-.039 thickness, and 72cc chamber.

On a USRA hobby stock engine, 360 max, 2 bbl, 9.5:1 comp max

[rebelyell]

no need to zero deck ... deck only enough to achieve optimal quench ... leave some meat on decks in case you NEED to deck in future

gen 1 sbc

composite
GM 14096405 about 0.028"
Mahle 5746 about 0.026"

shim
FP 1094 about 0.015"
FP 7733SH1 about 0.016"

[mag2555]

The shallower the chamber the sooner for any given amount of valve lift will have the valve become more unshrouded for better flow conditions.

With that being said depending on how much valve lift you do run a shallow chamber can make for a need to port/ roll back the lifter valley side of the wall of a valve notches in the pistons , so yes this can be called piston porting if you like.

Once I get my piston top to head deck clearance with a head gasket in place to .045” ( steel rod ) I never fight for any more most times , but in terms maxing out combustion efficiency and the extra power it can make it can be a very good thing to to make the distance from the top of the ring to the face of the block deck as little as possible!

Getting the piston .010” to .015” up out of the hole and using a thicker gasket will always make more power if you do a comparison test that maintains the same compression ratio.

[88bluestar]

This will all be steel rod, cast or forged crank. Which piston material has the least amount of expansion?
I was hoping for around .040 ideally to maximize what little I can with such restrictive rules.

[rebelyell]

our local rules prohibit piston above deck YMMV


in general:
more expansion -------------- less brittle

2618
4032
cast
hypereutectic cast

less expansion ------------- more brittle

[mag2555]

Well then part of the picking process for you pistons should be from the manufacturer who can get the top ring groove as close to the top of the piston as they feel safe with for the level of power this motor will make.

Heat conductivity will be a big factor for this type of ring position so a Hyper type piston gets ruled out right then a there!

Don’t worry so much about the piston material, but the weight of it and the pin.
Since this type of motor will not make over 600 hp and not need to rev passed 8K and hold together you can max out the lightness of these parts and allow the motor to rev up faster which is in some cases more helpful then 8 more hp .

[hoodeng]

Without buying into which clearance is optimal { i use .035"-.040" in what i do} Some years ago i read that squish and quench were not exactly the same as far as manufacturers intent for that area of the combustion chamber, although dimensionally and location is/are the same.

Squish is to activate the combustion process, quench is to have a cooler area in the combustion chamber, even though the results are the same.

Cheers.

[88bluestar]

How much can most chevy open chamber heads be surfaced?

[F-BIRD'88]

Generally, GM says .040".. I have milled the 305 heads right down to the edge of the spark plug boss.
On these and the similar open chamber the spark plug boss location creates the limit.

[88bluestar]

Thank you F-BIRD'88.