Vader

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Quench area is the space between the piston and the flat portion of the head. This feature can do two things.

First, it is the last area of the cylinder to get compressed on the compression stroke, so that the gasses in that portion of the chamber are forced rapidly into the open (valve) area of the chamber. This can produce a little more effective compression ratio. Also, since the area tends to have a greater exposure of cooled metal to the gasses, can allow a little higher mechanical compression ratio since the last bit of gasses being compressed is cooled better to prevent dieseling/preignition.

Second, on the power stroke, it effectively is a "zero" space from the standpoint of flame travel. This helps control the combustion rate and pattern, and helps keep the chamber temperature slightly lower as the mixture burns. The result is more power and less pollution from a more complete burn at lower RPMs.

Because of these two factors, a tighter quench area on street heads can allow a little higher mechanical compression while burning street fuels.

Many engines without a quench area run just fine (or sometimes better). Open-chambered big blocks don't have any quench area, and run just fine. With dished pistons, the SBC is similar. Even though the flame kernel starts in the spark plug area and travels to the opposite side a little more slowly because it has to "turn a corner" around the flat area, it could essentially be thought of as an open chamber because the flat area isn't tight enough to stop the flame kernel like a flat-top piston could.

With a dished piston, the final compression event is not the same either, so the extra little force of compression doesn't occur. Even at the same mechanical compression ratios, an engine with a tighter quench are will produce a little more actual compression pressure at lower RPMs (street speeds). At higher RPM, there isn't a lot of advantage as far as I'm concerned, and may be a slight disadvantage in slower flame propagation.. And open chambers on BBCs again seem to do quite well at higher RPMs. Full hemispherical chambers do pretty well, too.

For applications using dished pistons, there is no effective quench area. In that case, I'd stick with the head gasket that seals the best and provides the clearances you need. Forget about maintaining the "0.045" quench area, because you just won't have one. I've always been a little scared of solid metal or very thin head gaskets, since I like the coolant to stay out of the chambers and oil, too.

Mark A Shields

Thanks Vader, you da man.

I'm goin with the 50cc head and a .040 gasket giving me a 10.3:1 compression.:hail:

V8Astro Captain

iTrader: (1)

:hail: