<VV>Squish

[Padgett]

At 03:24 PM 10/24/2006, you wrote:
>But weren't the so-called squish heads fairly open?--I seem to recall that
>one of the more effective ways of reworking the heads was to mill down the
>surface facing the piston so as to reduce the depth of the squish so as to
>REALLY make it squish the combustionable gases and leave less of them to 
>form  smog.
>Scratching my head,


Hokey folks, "sqush" is a head designed so that most of the head is a flat 
area that almost touches the top of the piston. As the piston nears TDC, 
the air/fuel mixture is "squished" toward an open area generally near the 
valves. The "squish" volume is as near zero as possible and the bulk of the 
chamber area is in a much smaller area. This means the flame front does not 
need to propagate very far to reach all of the combustible mixture (does 
not need much advance).

OTOH an "open chamber" head is designed to unshroud the valves to allow the 
easiest passage of gasses into the head. This means less squish area and to 
keep the flame propagation length low you move the spark plug closer to the 
chanber (see Chevrolet "angle plug heads".

Now a sqush chamber also has a very fast pressure and temperature rise 
which is not so good for NOx. The open chamber head moderates the 
pressure/temperature peak at the expense of a longer flame path and takes 
longer to reach complete combustion hence needs more advance (though 24 
degrees is a whole bunch - an "open chamber" Pontiac head (also introed in 
1967) is all in at 38 degrees mechanical.

So to really make an open chamber head work, you need the plug at the 
center of the head (a la hemi). The Corvair could not move the plug 
location without some major changes so stayed the location best suited for 
a squish head and the valves did not need unshrouding.

Does that help ?

Padgett

ps if you put a timing light on #6, the timing mark is about 20 degrees 
ABDC with #1 at 14 degrees BTDC (phun things to do with white-out)