Squish Zones - Is there an optimum clearance?

[Neal H]

Hi Guy,

Something I have been thinking about recently is squish zones. My understanding of this is that they are there to confine the "explosion" in the combustion chamber to promote a fast and efficient burn, and also to induce a degree of turbulance with the incoming inlet charge as the piston rises to TDC and the inlet charge is forced away from the squish zone. I am sure there are other reasons in addition to these, I just can't recall them.

My questions being, is there an optimum clearance, from piston to head for these zones? Should there just be ample clearance so the piston will not hit the head, or should there be a gap between the two for some reason that I can't think of.

Any advice would be much appreciated,

Thanks,

Neal.

[Guy Croft]

Forget squish clearance as such. That stuff has to be computer modelled. And well-prepped race engines don't need a lot of squish-induced turbulence anyway. Too much achieves nothing on an engine that has plenty of inlet-deveopled turbulence to encourage fast burn and just robs power.

The trouble with squish is the subject gets a 'head of steam' like a few other topics that come up in discussion here and there is no simple answer. Everyone wants to know the 'optimum' and no such optimum exists. No-one I know of has conducted accurate bench dyno tests on varied squish clearance, far less published qualitative results.

The determining feature is really how close you can run the piston to the head. This has to be based on experience and the longer the stroke & rod, the longer the crank and the heavier the reciprocating parts - so the more you need. In the case of yur engines (that I know of, V6 and 4 cyl 16v VW) that needs to be 0.5mm (0.020") and not_less.


G