Ralph, I have taken squish measurements on my:

DA150 - .055
3W150 - .030
3W157 - .015

Now I DO KNOW that my DA150 is an absolute gas hog in that after 18 minutes on a 50 oz. tank I better be lookin for terra firma.

My 3W150 made a deadstick at 27 minutes once on my 50oz tank.

The 157 seems stingy too, but I don't know how stingy.

What exactly are the affects of these vastly different squish measurements? I'm assuming that the port timing is probably close to the same, but I haven't put a degree wheel on them to check. Let's assume the port timing is the same between all 3 motors.

Rich Sornborger

Don't know, I only make ignitions.
Our race twin 289 is such a tractor it just takes a 28 pitch prop to go fast, but I do set the compression distance at .022 and intake duration at 140 degrees and clean up the roughness is the ports....
FastRat Porting will know, maybe he will see this..
.055 is weedeater territory, maybe DA does it this way for torque at lower rpm ?
I made a new set of really high compression heads for my first Zenoah 445 twin, it went slower..[8|
Welded up some higher compression piston tops for the 289 racer, the compression was too high and the centers of the glow plugs melted out....
Made a high compression head for a 125 Maloney, same result, lost 1000 rpm...
Only good result I ever had was with a GT80, put in another transfer port, good for 200 rpm, hardly worth the trouble...

In general, the tighter the squish clearance the better. But, it can't be so tight that when the rod or crank stretches at high rpm that the piston smacks the cylinder head. The purpose of a tight squish is to set up some turbulent flow from the squish band towards the combustions chamber space near the plug. This helps to completely burn the charge. Also, the volume of gas trapped in the squish area gets cooled by the cylinder head, so this volume doesn't get burned and can contribute to a slight loss of power. You might see about 5% power loss (according to Gordon Jennings) with a larger squish clearance. My 3W-106 has .017 and .018" squish clearance with one base gasket. With two gaskets, the clearance is .034". I tried running it on two gaskets and lost a little power (100-200 rpm), although this changed the port timing too, so I can't attribute the effect only to the increased squish clearance.

SuperTigre was the king of the "squish band of the month " club. Simply amazing, the different setups done as running changes .
Having tried them as they appeared and swapped em when possible - I found the clearance to be less of an issue - but compared against ratio of band to chamber, it changed.
OS used more band area to chamber than initial ST engines and typically the bandshad very slight taper upwards
On the teensy engines - Cox etc., the chambers also went into trumpet and some racing stuf into double bubble.
It all looked like a clustr f--- to me.
There never appeared to be a real "This is the best" configuration as the fuel used and rpm really determined which worked best .
- the pressure in the cylinder can be increased easily by using tuned stuff which will pack the combustion more. So the ideal head configuration changes again-

Ok, lets help the technically challanged.
What is a squish band?
What is squish?

The squish is the distance from the top of the piston at TDC to the top of the cylinder...Less squish, more compression..
All cylinders have a dome at the top, sometimes a hemi, sometimes not..There is a narrow band, usually about 1/4 to 3/8 inch, around the top of the cylinder where the dome starts..This band is where the squish is measured...
There were a few engines that had a flat combustion chamber, with the top of the piston machined out for the combustion chamber..Some diesels, I think...
Squish is measured by putting a piece of solder into the spark plug hole and squishing it flat by rotating the crank...

Use a piece of piano wire for accurate measurement
(do this only on some one elses engine)

Dick....

....why would you recommend using hard piano wire ?

FBD.

The devil made me do it

.....that rascal. [X(]

So let me see if I have this straight.
Squish is the distance from the piston lip and the top of the cylinder at TDC.
On some engines this distance changes as you progress across the piston (curved cylinder and flat piston top) on other engines the distance can be fairly constant. (curved piston and cylinder). If you measure at the spark plug hole, is this considered the worse case condition?

Let's start over --
What RCIGN was explaining was simply clearance between closest possible point between piston and top of combustion chamber -
The shape of the chamber is quite varied-but most are symmetrical being flat / slightly inclined,outer portion with a raised dome in the centre.
Don't use piano wire - that's the hard way to do it (rimshot)
He shoves the lead wire into the hole and angles it to one side
then hand twists the crank to bring the piston up, smashing the wire flat. remove and measure thickness of the tiny lead wire.
It is a very time accepted practice.
Some elderly designs used a wedge shaped chamber (similar to those in auto engines) and you had to feel for the "low spot. Others had weird shapes with slots for piston baffles - and the pistons-depending on who designed the thing, have been all kinds of shapes -concave /convex, center dished , domed -etc..
Most current stuff - squish band and center dome

Dick, you forgot to add that you should take the measurement parallel with the wristpin so as not to rock the piston inside the cylinder.

I pull the cylinder off the engine and lay a piece of solder across the middle of the piston parallel to the wrist pin - no rocking motion. I put a piece of tape over the center of the solder to hold it in place. Reassemble the engine, turn it over, disassemble to remove and measure the solder.

I also forgot to mention the solder size -.035 is likely OK (circuit board size) - Radio Shackup has solder in different dia and lead content - best is highest lead - anymore since I stopped futzing with the friggen glow engines, I just run stock clearances . On the glow engines it is far more critical in getting proper deck height so that pipe setup works .