Quench Clearance and Deck-Height
Although a quench-type chamber is not very desirable from a performance standpoint, nearly all production cylinder heads have quench areas. Actually, the term “quench” is a misleading description. The quench portion of the chamber is that area where a flat portion of the head and a matching flat portion of the piston close together at top-dead center, significantly reducing the volume of the chamber.
From the name, it sounds as if the quench area is supposed to restrict combustion, but in fact, the quench design is used to enhance combustion in a chamber that has a relatively large diameter. When the piston and head quench surfaces come together during the compression-ignition changeover, the combustion gases in this area are pushed over into the open part of the chamber, creating turbulence that improves the combustion process.
For maximum performance with a quench-type cylinder head, the quench clearance should be as tight as possible, but there must be sufficient clearance between the two surfaces to prevent any contact at top-dead center.
The desired quench clearance is actually a figure selected as random by the engine builder, and once it is selected, the builder must calculate a corresponding piston deck-height (the distance from the deck of the piston to the deck of the block). The total quench clearance will include the piston deck-height and the compressed thickness of the head gasket (the thickness after the cylinder head is clamped down with the recommended fastener torque). Therefore, the thickness of the compressed gasket – which is normally supplied by the gasket manufacturer – is subtracted from the quench clearance, and the resulting dimension is the required deck-height.
It is hard to recommend a desired quench clearance that will be correct for all engines, but I think 0.038-inch of clearance between the quench surfaces is an absolute minimum in an engine with a 4.00-inch bore and about 0.007-inch of piston-to-wall clearance. However, if the piston clearance is greater, say 0.009- to 0.010-inch, this may not be enough. A safe figure for all-around performance is 0.040-inch. Anything up to about 0.045-inch is probably okay, and I wouldn’t worry about a little extra clearance as long as it was close to these recommendations.
But remember, if you let the quench clearance open too much – to 0.060-inch or more – you’re going to lose power. This much opening at the quench will allow excessive amounts of the intake charge to remain in the quench area as the piston reaches top-dead center, and especially if the piston has a high dome to obstruct the spread of the flame front, these gases may not be properly combusted during the ignition phase. This significantly reduces combustion efficiency.
I’ve also heard there are some builders who claim an engine will make more power if you let the piston “smack the heads a little.” I think this sort of talk is mechanical nonsense, and I would not recommend it under any circumstances. When you disassemble an engine for inspection, always check the piston and head decks on the right bank for signs of contact. Because of the crank rotation and piston sway in a conventional V8, the pistons in the right bank tend to show signs of contact before those on the left, and if you see signs of contact, I think you should open the quench clearance a little bit or you’re going to bread some parts.
Another important thing to watch in the quench area is to make certain that when the piston is at top-dead center the quench area is exposed to the open part of the chamber. If the quench decks are not parallel or angled slightly toward the valves, the quench area may be closed off from the rest of the chamber as the piston dwells near the top of the stroke, and gases trapped in the quench will not be combusted properly, this can lead to secondary combustion and detonation problems.
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