Daniel Jones
Gearhead Posts: 790
From: St. Louis, MO
Registered: AUG 99
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posted 02-28-2000 03:23 PM
 
There were two basic combustion chamber designs - closed (also
referred to as quench) and open. The closed chamber heads have
smaller combustion chambers (62 to 66 cc's for 4V quench, 55 to 59
cc's for Aussie 2V quench), characterized by a large flat area (the
quench area) opposite the sparkplug. As the piston travels towards
the chamber, mixture in the quench area gets compressed more quickly
than the mixture in the rest of the chamber. This squirts the
mixture out of the closing gap between the piston & head, toward
the plug, with high velocity. The resulting swirl/mixing homogenizes
the mixture better right around the plug, the burn is more complete
and more power results. The quench effect also tends to cool hots
spots, averaging temperatures in the chamber. Note that to optimize
the effect, the piston should be a flat top (in the quench area) and
needs to get fairly close to the chamber, within 0.040" (static)
according to the Keith Black Pistons tech page. When computing this
distance, remember to include the compressed thickness of the head
gasket and the deck clearance (the distance your piston is down in
the bore at TDC). Cleveland quench heads are rather resistant to
pre-ignition and can tolerate 11:1 compression ratios with a typical
performance cam grind. Quench heads also require less total spark
(an important measure of chamber efficiency) than open chamber heads.Open chamber heads were introduced for emissions reasons and to
lower compression ratios to match the lower octane fuel of the
early 1970's. One undesirable consequence of the quench chamber
is that the flame front is prematurely extinguished as it travels
across the chamber, resulting in unburned mixture and increased
emissions. Open chamber heads, lacking the quench area, do not
have this problem so they burn cleaner. They also shroud the valves
less, increasing flow a bit but not nearly enough to offset the
loss of the quench effect. Open chamber heads are also less
tolerant of high combustion ratios with 10:1 (requires a pop up
piston to offset the larger chamber volme) being the suggested
maximum. They also require more spark lead since the flame front
has to travel across the larger chamber). Vizard claims open
chamber heads respond well to multiple spark ignitions (e.g. MSD 6),
requiring less ignition lead. The bottom line is that quench heads are the best pick for high
performance applications. Both 2V (Aussie 302C) and 4V ('70 and
'71) heads are available with quench chambers. Dan Jones
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