Mr Cox

I was just wondering if anyone has any numbers of typical squish band "heights" for these small engines?

I haven't played around with this before but I always thought that on a high performance engine you want the squish band to be as tight as possible without touching the piston?

I've had a closer look at the TT GP-07, and from what I can tell the squish band "gap" would be about 0.1 mm without any head gasket and the standard gasket is 0.2 mm so that would make the gap 0.3 mm, sounds really high to me...?

BMatthews

It probably is but it's more important to get the right overall compresion ratio than to have just the right squish band.

If you don't mind playing with the head then lathe off the riser so just the gasket is the gap producer and see how much more you get.

I know that I've also see some info that says the squish band is sometimes better with a very small angle to the center so the squish tends to squirt the mixture towards tne center more completly. I'm not sure this would help much on our small size squishes. It was more for larger engines like motorcycles where the size of the band is longer and the mixture may get confused about which way to go. But again it's something worth trying if you're after that last couple of RPM.

But as I remember from all the reading on engines from the past magazines when they wrote up stuff like this the compression to nitro ratio is far more important. And that ratio is also dependent on the RPM you're running. Toothpick props on speed engines will support more compression ratio than "normal" props where the earlier ignition point will kick back on the slower revving more.

combatpigg

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High performance engines have pretty tight gaps, off the top of my head, .010". The squishband directs the flow of hot expanding gas away from the sidewalls of the cylinder and along the top of the piston. At Delphi Forums there is a group of C/L speed racers who can give detailed info about 1/2A combustion chamber design. Most of them are with the North American Speed Society [NASS]

BMatthews

I understood that the idea of squish bands is to shoot the gases into a smaller and more efficient hemisperical combustion chamber. It's a "fix" for when the piston is large enough that it'll be too far for the flame front to travel at ignition. It also has the effect of adding some turbulence to the mixture as it compresses over that last little bit. All of this happening before the ignition event so it's not really to keep the flame from the cylinder walls so much as it is to concentrate the flame "ball" into a volume with the least surface area contact with the head, piston and walls.

Japanman

I read a very interesting piece about the development of the squish band. It was found that to work, the squish band needs to be tighter than 0.6mm.
What the writer found was that using a squishband allows a higher total compression ratio in spark engines, as it squishes in some of the gases that have been "cooled" to the temperature of the cylinder and head, and would be in an area to thin to burn- it was reasoned that gases within a certain distance of the clinder and piston did not burn, because the metal cooled the gasses too much and stopped combustion.

So after the ignition takes place and combustion is well under way, a volume of these unburnt gases are injected into the main combustion area, which is shaped to have the best compromise between low surface area <closest to a sphere> and a shape able to be well scavanged by the rapidly moving gas front without causing turbulation of it.

This happens at almost exactly TDC, and the gases injected probably ignite very rapidly, making pressure above the piston just when you want it. So there is the advantage, me thinks, and why it works for glow too, seeing as ignition should have already taken place.

Ahh... what a bore I am at times! A squishband of about 2mm width and a running height of less than 0.5mm and you should be good. If you are feeling flash you could put in a taper of about 1 degree on it, but there may be very little difference.

...Did I say something someone else hasn`t already?

J.M

Mr Cox

Thanks, no worry for any repitition, it doesn't hurt to hear these things several times.

Ok, 2mm wide and less than 0.5 mm gap, I will aim for that.

GrahamC

I have been doing a bit of reading through some old magazines and news letters. Some of the old North American Speed Society (NASS) newsletters has had some details on squish band for the CS/CZ 049 racing engine and I have started to compile a short list of some of the details:

CS/CZ 049 twin ball bearing rear exhaust racing engine

5/16" hemi shape, .058" - .062" plug depth, .003" deck height High Nitro (Greg Settle)



5/16" hemi shape, .075" plug depth, .004" deck height, 72.5% Nitro (.006" deck height 40% nitro on pipe) (Walt Gifford)

(.290/.395 chamber to bore for squish band size)



3/8" hemi shape .055" plug depth (049) .065" (061) .005" deck height (Galbreath)




Cox 049 mouse race (Gibeault)

properly set up cylinder with 4 to 6 head shims using Cox head (usually 5) 40-60% nitro


So, you can see there is quite a range but generally speaking - more nitro smaller deck height.

As for the typical Cox engine, a lot depends on whether you have the cylinder set to the proper height - top of piston level with plug seat in the cylinder and what type of head you are using. The #325 standard head has no squish band and the #1702 high compression head has some. A common mod done by the racing types years ago was to machine the bottom side of the #1702 to give a bit more squish band (and then adjust compression/squish thickness with head shims)

On one of my Cox 049 (sorta like a black widow) I use the Galbreath head with 4 to 5 head shims (plus or minus 1 depending on weather conditions) using 25% nitro with the piston top set level to the plug/head seat.

cheers, Graham

Toad

I run .006 on my modified Norvels on 40%. thats right in there with walt Gifford, I modify the galbreath head to where I have .045 plug above the piston and my combustion chamber is set with a .312 ball endmill, the squish I use is angled 2 deg and is aprox .030 wide from the seating surface on the cyl to the combustion chamber. All these numbers vary a bit from one engine to another.
I think the squishband area is important if you have the room for it.

All this is what I had to do to use a nelson plug in the norvels

Mr Cox

Thanks for those detailed numbers. I can see that I will have to make a few different heads if I want to sort this out.
The standard head doesn't seem to be too far off then, accept for the regular plug...