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Discussion Starter · #1 ·
The squish band is one of the factors affecting engine power. Other factors: carb size, engine compression, and ignition timing.

Squish Band Width
The width affects the length of time of combustion. The air/fuel under the squish band burns later than the bulk of the mix in the dome. So the more mix is guarded there, the longer the combustion time. The wider the band, the longer the combustion duration. That duration needs to match the time duration between TDC and port opening which can be called the power stroke duration (PSD).

POWER STROKE DURATION
The higher the port and the higher the RPM, the shorter you need to make the combustion pressure duration with a skinnier squish band. If the combustion pressure duration (CPD) is too long then pressure is lost out the exhaust port which should of been used to push down the piston and rotate the crank. If CPD is too short then you have the bulk of the pressure happening when the angle between con rod and crank is not very good so less power contributes to crank rotation which lowers engine power.

Squish Velocity
The wider the squish band and smaller the squish clearance, the faster the squish velocity. It's been said that less than .8mm squish clearance messes with the needed boundary layer of air/fuel over the metal, so don't go less than that. Otherwise the only obstacle is to not have the velocity so high that it causes detonation or overly scatters the combustion flame which lowers power. In one paper about squish bands I saw that squish velocity differences didn't have a big influence on the power output. But the area of squish band does.

How To Test
You can install a thicker head gasket to increase the squish clearance to also increase the squish volume but if that results in less power then you'll know your engine wanted a shorter combustion duration, not more, and so you can lathe down the head or cylinder mating surface as long as you don't reduce the squish clearance to less than .8mm.

Needed Squish Band Width
You need a squish velocity calculator to juggle the width, clearance, and velocity. But the main idea here is that the narrower the band, the less air/fuel is guarded under the squish band to burn later. Typical for high duration exhaust port engines (over 180 duration) is 45% to 50% squish area. Typical for low duration exhaust ports (under 180*) is around 60% squish area.

Graph Analysis
Below is a graph demonstrating what I believe happens with a wide squish band, how it lengthens the combustion to better match a low duration exhaust port. Notice how the narrow band graph lowers to 9% at 80*ATDC which matches a 200* exhaust port duration. And the wide squish band graph is at 9% at 90*ATDC which matches a 180* exhaust port duration. I didn't guess at the crank force graphs. I'm pretty good at math and I did lots of it to come up with those derived values from the PSI graph. So the idea is to match the combustion pressure duration with the power stroke duration. Long exhaust pressure time matches a long time from TDC to port opening. Adding all the numbers of the graphs it is revealed that the long duration graph gave 10% more total crank power. That happens because the more PSI exists at later crank degrees, the more power is transferred to the crank due to the angles involved between con rod and crank.
Rectangle Slope Plot Font Triangle



% Squish Area
It's easy to find the % of bore area the squish band occupies. The main formula is Area = 3.14 x (diameter/2)x(diameter/2)
So figure the center non-squished area and subtract it from the whole bore area. Then divide that result by the bore area. It will give the % as a fraction such as .45 which is 45%.

Feedback From Others On This Subject:
Luke V:
"on a 125cc CVT engine I didn't get over 30hp. I talked with Helmut, a former TM Racing factory tuner, and he told me to make the squish band narrow, just like you see in the TM ICC engines. I did what he told me, first run 36 HP and with some fine tuning 39.7hp. All peak powers were at 13k so that didn't change. 10 years back I did scooter racing engines. Somehow I always came to a narrow squish band thru testing. Engines ran 14.2k rpm and 20-25hp. Exhaust durations were always between 192 and 198."

Me: "Luke, when you changed to a narrow band did the exhaust note soften? I'm thinking that with a long combustion pulse that there's more pressure when the exhaust port opens which would cause more exhaust noise."
Luke: "engine sounded smoother"

Michael W: "I have found the squish clearance is very much related to pipe temp. Looser squish is higher pipe temp/cooler piston. less risk. Tighter squish is cooler pipe and hotter piston. More risk."

Matching Squish Band to Piston
For zero squish angle between piston and head the DIY way is to glue rough sandpaper to a piston top. Then insert the wrist pin and circlips and bend a thick wire (such as clothes hangar) around the wrist pin and twist the two wires together in a clockwise motion and stick that into a drill chuk and use the drill to rotate the piston while pressing it onto the squish band. That will make the head conform to the piston so you can reduce the clearance with less possibility of contact at WOT when the piston rises even higher due to extra heat expansion of the connecting rod.

Squish Band Velocity and % Area
 

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Discussion Starter · #2 ·
Doing further study with my squish velocity calculator I'm finding how to choose the squish characteristics by the exhaust port duration. First it is best to try to get the lateral squish area % of bore to the right %. Then change the squish clearance to get the volume under the squish band at 5*ATDC correct.
Rectangle Slope Plot Font Parallel
 

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Discussion Starter · #3 ·
Squish Effects
Here's an additional way of thinking about squish effects. The time till peak pressure is greatly influenced by the squish velocity, and the remaining pressure time is greatly influenced by the squish area % (squish ratio) and squish volume.
Rectangle Slope Triangle Plot Line
 
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