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Thinking behind de-squishing

Engine Building Fundamentals

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Hi Andre,

I'm totally confused by this topic.

I was of the understanding that the idea behind squish was that by forcing the end gasses into the center of the combustion chamber and leaving no room for a combustible mixture in the squish area it prevented two converging flame fronts from forming?

However, Prodrive would desquish WRC heads and these old articles from JUN referring to SR20 heads says the same.

Removing squish area (archive.org)


What are your thoughts on the theory behind de-squishing?


True competitors spread disinformation constantly.

The only way to really know will be to do your own development and testing.

My thoughts are - it depends.

What you are calling "squish" is also called "quench" - the same area of the combustion chamber doing two different things.with some engines, especially if the piston-head clearance falls into the 'critical' distance the head and piston surfaces don't pull enough heat out of (quench) the compressed gas in that area and compression ignition can occur due to the pressure front from the ignited flame front. This is the common detonation problem, but there can also be a problem with pre-ignition where there is an area where the fuel-air mixture is compressed to the point where it can spontaneously ignite, even before the spark plug fires.

Squish is used to promote turbulence, and better mixing, of the fuel-air mix, especially at light throttle, and isn't quite so critical in an engine running under full power/boost and by removing it it removes a potential problem area. IMO, the mixing is important but some port arrangements are designed to 'tumble' the charge as it passes into the chamber to aid mixing.

As David said, if an engine builder finds an advantage, in a competitive sport, they are likely to lie about it - no point in going to all that trouble only to give that advantage away.

Unless you've a strong suspicion this is your problem, I would advise you to leave it alone - the gains, if any, may be hard to measure and you may actually hurt performance if you get it wrong. Remember, these people have big development budgets.

Something you may also find interesting is the "Singh groove" concept, which was a topic of much discussion a few years back, this is one of the better discussions on it - https://www.speed-talk.com/forum/viewtopic.php?t=4069 Seems to be snake-oil, but just as some swear at it, some swear by it.

We usually reply within 12hrs (often sooner)

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