Squish Bands


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Squish or Quench Bands are a feature found in Harley engines from 1984 on with the advent of the Evolution engine. More specifically, they involve a radical change to the combustion chamber shape from previous hemi-head configurations. The Beach Boys glorified the hemi's in their Sixties music since it was the leading engine technology of the time. This change was a tremendous move forward for the Factory. I would even venture to say that it is probably the most important design change in decades.

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V2 Squish bands allowed the aftermarket to develop the "bathtub" or "kidney" shape combustion chambers for hi-performance Evos. This later became the evolving shape for the Twin Cam 88. Yeah! So what's so special about all this? These designs produced more horsepower with less gas. Furthermore this extra horsepower is produced with fewer emissions. Why?A higher percentage of the air/gas mixture is burned. Why? Turbulence. The air/gas mixture is mixed better for a higher percentage of burn. Let's back up a bit since I'm getting ahead of myself.

Combustion Chambers

1) Hemi-heads:

For the uninitiated, the celebrated hemi-heads like the Shovel, Ironhead XL, Pans and Knucks have rounded hemispherical shaped combustion chambers. The piston top or dome is also vertically rounded and fits into the hemi-head. This system isn’t very good for flame travel or a high percentage of burn. The air and the gas enter the chamber with lots of "fat" and "lean" spots or globules. A fat spot has too much gas and not enough air to burn effectively. The lean spots have the opposite problem. There is no mechanism for mixing the fat and lean spots to encourage a nice controlled burn. The poor old spark plug goes off trying its best to ignite this improper mixture.The domed piston getting in the way of the flame travel compounds this tough job. How is the spark plug supposed to do a good ignition job on the other side of the chamber with that piston dome in the way? Now, the reader can see the distinct advantage of dual plugging these old motors. A more efficient burn develops. This ignites a higher percentage of the gas, which creates more horsepower with what is already there! However the "fat" and "lean" spots are still not addressed. Of the different head designs we will discuss, the hemi-head is most prone to detonation, the great engine destroyer. Proper level octane in the gas is crucial to prevent this.

2) D Shaped Heads

The Evos have a “D” shaped combustion chamber which is shaped like the letter D. Inside the D is a rounded chamber that is suddenly cut off. The rest of the circle is flat and even with the surrounding gasket surface. A flat top piston squeezes up against this flat “squish band”, squishing the air/gas mixture turbulently into the smaller D chamber promoting better flame travel and a much higher percentage of burn. This is the main reason that the Evo is faster than previous engines such as the Shovelhead. The "D" configuration helps reduce detonation especially when compared to the hemi-heads. This shape is used on all Evo Big Twins from 1984 to 1999. Evo Sportsters utilize the same type of flat top piston with either a shallower hemispherical chamber or a bathtubbed one.

3) Bathtub Heads:

Aftermarket Evo heads such as Edelbrock, Rev-Tech and STD or ones modified by the likes of Jerry Branch and shops like mine incorporate these generic designs that resemble the shape of a bathtub or a kidney. If you want to double the horsepower at the rear wheel of a stock displacement Evo Big Twin, this is the way to go. Bathtubbed heads that incorporate bigger intake valves and raised intake and exhaust ports to promote increased air flow along with a support team of parts such as cam and carburetion can accomplish this goal without sacrificing reliability. It shou

 

Is it absolutely crucial to run the HTCC piston? Or is there something out there (even -aftermarket ) that can produce good / or similar numbers, it�s playin hell finding a set roany helps appreciated, Thanks

 

The closer you can set the piston to the cylinder head w/o contact during operating temps, the more turbulent chamber you'll have.
FWIW, we did not run a cylinder head gasket on our 108" AHDRA Mod Class bike, as we machined the pistons, and combustion chambers at the exact same angle, and built the squish into that precision process.
Scott