Piston squish

[ChargerST]

I know this has been talked about before but I still have some questions left. I'm trying to get the best squish properties for my engine (Mopar 440 based stroker, 4.25 stroke, 4.34 bore, TF240 closed chamber heads).

Ideal squish distance? most people seem to favor 0.040 piston/head distance no matter what bore size. Some people say that bore size DOES play a role when choosing squish distance - any truth to that?
Bill Grumpy Jenkins observed best power output @ 0.060 distance - closer would result in pumping losses. I assume Bill was mainly interested in WOT performance so this may not necessarily be the optimal distance for a street/strip build.

Should you aim for parallel piston/head surfaces or is a slight taper (e.g. 2 degrees) beneficial? Intuitively I'd say if you have a large quench area a taper towards the piston center would reduce the chance of a trapped charge in a large quench area.

What are your opinions about this?

Thanks,
Stefan

[mag2555]

Jenkins found power with a .060" due to his tall piston dome limiting flame travel so that will not apply to your motor.

In short if your not running the motor in a form of competition where every last 2 hp must be found then .020" to .040" in the hole is more then fine regardless of your Bore size, stroke length and combustion chamber shape.

To me among the many factors the come into play here is the valve lift used and the way the deck clearance and VTP clearance can effect high lift flow.
With most motors running .600" lift or more a dished piston , or atleast a good diameter Intake valve notch will allow better flow and the resultant greater HP.

Also remember that a piston will grow 005" taller when it's at normally seen temps with EGT numbers in a normal range.

To me the last word would be to call Ray Barton racing engines, if anyone could hone in better on the specifics of your motor Ray would be the guy!

[digger]

The theory seems to be generate a squish velocity which depends on both the % of the bore area that is effective and the clearance. If you have a large squish area % the clearance is probably less critical in theory

[pcnsd]

I have been a taper squish guy since reading this:

https://www.spyderchat.com/1zzfe.pdf

The whole thing is good. Taper squish starts at page 5

[DanE1]

We used a tight tapered quench area in our racing two strokes. Worked best.

[Steve.k]

On a 438 ford we dynoed both ways. On a particular setup cause of block being over cut we ended up with a .055 squish (head gasket sizing)as compared to usual .040. On dyno absolutely no difference in hp for that setup. On a engine wound a little tighter you may see gains however with tighter squish.

[Bazman]

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[Bazman]

As per above not sure there is much power difference but it definitely can help raise the detonation threshold. I'm running around 032" on my LS3, and anything I do would want to be under 40. We've even run 45 on an alloy rod Pro Mod motor (494 mopar wedge), that engine saw 9500rpm and ran regular 6's and never touched the head. Not sure I'd recommend anyone "try that at home" but we got away with that back in the 90's with GRP rods, Arias block and Indy heads

But you have a hemi not a wedge head. Back in the 90' I used to deal with and chat with Ray Barton quite a bit. He had special pistons made and hand finished the hemi piston domes to give a custom tight squish. I'll just leave that there. That guy is the best there is on NA race 426 style motors

[David Redszus]

What is the purpose of squish?

Squish is the rapid movement of mixture forced by the diminishing volume between the piston and head.
The parameters that affect squish are squish area ratio, vertical clearance, and piston velocity.
Squish is a velocity expressed in units of meters/second. Maximum squish velocity occurs at approx 10 deg BTC and ATC.

A larger squish area ratio, reduced clearance and increased piston speed will result in an increase in squish velocity.
The squish velocity is added to the laminar flame speed to determine the ignition advance required to properly position the combustion pressure peak angle.

A change in squish will require a corresponding change in ignition advance angle. The greater the squish velocity, the less ignition timing required.