[Diy_efi] Combustion chamber question

[JMSMAUTOMOTIVE at cs.com]

You have asked a good question, however the answer is not  a simple one! 12:1 
compression (static) on pump gas is impressive, factor in the dynamic 
copression ratio (almost certanly the volumetric eff. of said motor is over 
100%, so at high rpm's the compressoin ratio is even more)and it is even more 
impressive. a hemi head is a good head for flow, especially when its a two 
vale head compared to a two valve wedge head, the valves unshroud themselves 
as they open. however, the old (chrysler) hemi head was not optimal for 
controling pre-ignition.one reason (among others) is a little thing we call 
quench, basically a flat area(s) of the combustion chamber that, when the 
piston aproaches tdc, literally squirts the air fuel miture caught between it 
and the corrasponding flat top of the piston into the center of the 
combustion chamber at high veocity and creats all kids of swirl/turbulance 
etc. this controlls pre-ignition in several ways, it helps to cool, or 
controll any hot spots, it also makes it more difficult for an unwanted flame 
front to propagate due to the high rate of sqirl/turbulance. the quench 
measurement is generally ~.040 clearance between the head and piston. they 
can do other things to controll pre-ignition such as cool the head before the 
block,when chevy "reverse cooled" should read "properly cooled" the small 
block and fitted it w/aluminunm heads(better heat transfer) they had no 
trouble geting 10:1(more is surelt possible with proper machining and a 
cooler t-stat, stock is 195degr.) the honda 600 head (i dont know exactly the 
layout)is most likely a pent roof design, kind of a shallow hemi head with 
flat sides for quench. some engine designers are able to controll 
pre-ignition with nothing more than a high swirl port(s). i am by no means an 
expert, and i could have miss stated some things, like i said there is a lot 
more to it, but we build 10:1 compression engines here every day with no 
trouble(we are at 4600 ft). and the most we have done was a 383sbc. that had 
11.5:1. these are real measured ratios, we cc'ed the chambers etc. and set 
the quench to the right spec and the motors love it. joe

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