Squish band?

fiatfan · Aug 9, 2016

Is the lowered "ring" around the combustion chambers on an 850 head referred to as a squish band? And if so, what good does it do? If not, why is it there?

comnoz · Aug 9, 2016

fiatfan said:
Is the lowered "ring" around the combustion chambers on an 850 head referred to as a squish band? And if so, what good does it do? If not, why is it there?

It is there basically as a leftover from smaller bore engines that had the same cup and valve angles.

On a stock low compression engine it really does nothing - there is too much clearance.

On a high compression or race engine it can be reduced to the point [.020 to .030] where it will make turbulence that will speed up combustion and make the engine more tolerant of lower octane fuels or make a bit more horsepower with high octane fuels. Jim

lcrken · Aug 9, 2016

comnoz said:
fiatfan said:

Is the lowered "ring" around the combustion chambers on an 850 head referred to as a squish band? And if so, what good does it do? If not, why is it there?

Click to expand...

It is there basically as a leftover from smaller bore engines that had the same cup and valve angles.

On a stock low compression engine it really does nothing - there is too much clearance.

On a high compression or race engine it can be reduced to the point [.020 to .030] where it will make turbulence that will speed up combustion and make the engine more tolerant of lower octane fuels or make a bit more horsepower with high octane fuels. Jim

.020" -.030"? With all due respect, you're scaring me, Jim. With stock rods and crank, I've always used .040" as the magic number. I've seen imprints from the pistons touching the head at .035" (although that particular engine didn't seem to have suffered any damage from it). Maybe with a billet crank and steel rods, but I still remember tearing down Martin Adam's Commonwealth Norton after it broke the cases, and seeing serious piston-to-head impacts at .020" squish clearance, with a Nourish crank and titanium rods. You've got more experience than I have at this, but that just looks too close to me.

Ken

comnoz · Aug 9, 2016

lcrken said:
comnoz said:

fiatfan said:

Is the lowered "ring" around the combustion chambers on an 850 head referred to as a squish band? And if so, what good does it do? If not, why is it there?

Click to expand...

It is there basically as a leftover from smaller bore engines that had the same cup and valve angles.

On a stock low compression engine it really does nothing - there is too much clearance.

On a high compression or race engine it can be reduced to the point [.020 to .030] where it will make turbulence that will speed up combustion and make the engine more tolerant of lower octane fuels or make a bit more horsepower with high octane fuels. Jim

Click to expand...

.020" -.030"? With all due respect, you're scaring me, Jim. With stock rods and crank, I've always used .040" as the magic number. I've seen imprints from the pistons touching the head at .035" (although that particular engine didn't seem to have suffered any damage from it). Maybe with a billet crank and steel rods, but I still remember tearing down Martin Adam's Commonwealth Norton after it broke the cases, and seeing serious piston-to-head impacts at .020" squish clearance, with a Nourish crank and titanium rods. You've got more experience than I have at this, but that just looks too close to me.

Ken

No question, I would not run that tight a clearance with aluminum rods. But then I would not build a high performance motor with aluminum rods either. Steel Carrillos, an aluminum barrel and full length pistons run fine at .020. Jim

Holmeslice · Aug 9, 2016

comnoz said:
No question, I would not run that tight a clearance with aluminum rods. But then I would not build a high performance motor with aluminum rods either. Steel Carrillos, an aluminum barrel and full length pistons run fine at .020. Jim

He he,

I remember one teardown we had a bit of carbon around the squish relief in my piston and Jim and I were mulling it over. Wasted HP!
So we tightened it up even more. I run a very tight squish with Carrillos, alloy barrels, and your wonderful pistons, Ken. And it all goes well!

Fast Eddie · Aug 9, 2016

I run JS 10.5:1 pistons and steel rods. These have a raised crown and allow a tight squish band. I am running at +/- .030 clearance.

I do not think it would be practicable to run 10.5:1 CR on a street bike, using pump fuel on an old Brit engine like this WITHOUT using the squish band. It allows a better flame path, better combustion and cooler running (according to the theory).

In practice, the theory seems proven as mine runs fine, on pump fuel.

I have done a lot with squished heads on Triumphs in the past. This involves a lot of work, a lot of welding up of the head (which can then soften it) and machining of head and pistons etc. I ran Triumphs with 11:1 CR on super unleaded pump fuel without issue. A stock Triumph can have pinking at just over 7:1!

Most modern engines use some kind of squish band, it is definitely good practice. But in a stock Cdo with low CR there is nothing to be gained or lost IMHO. Start tuning though, and it seems a shame not to use whats already there, being able to achieve high CR and a tight squish on an old engine like these with NO fancy welding or machining is a huge plus IMHO.

acotrel · Aug 10, 2016

I think the standard squish band in a commando engine must be effective. Triumph pistons out of an old motor are usually pretty coked up on the side of the crown away from the plug. I've not seen the inside of many old commando engines, however in those that I have seen the carbon has been evenly distributed across the crown of the piston. There must be some value in the Norton squish band because for similar tuning effort, the Norton is always faster than the same size Triumph, and it cannot be due to the more downwards directed inlet port, because Bonneville heads also have downdraft on the inlet.
As far as tightening up the squish band is concerned, I understand that the Manx uses 30 thou - with a roller big end.

Fast Eddie · Aug 11, 2016

acotrel said:
I think the standard squish band in a commando engine must be effective. Triumph pistons out of an old motor are usually pretty coked up on the side of the crown away from the plug. I've not seen the inside of many old commando engines, however in those that I have seen the carbon has been evenly distributed across the crown of the piston. There must be some value in the Norton squish band because for similar tuning effort, the Norton is always faster than the same size Triumph, and it cannot be due to the more downwards directed inlet port, because Bonneville heads also have downdraft on the inlet.
As far as tightening up the squish band is concerned, I understand that the Manx uses 30 thou - with a roller big end.

I've never looked inside a 750, but in a stock 850 there is no way there is any squish effect from the 'left over from a smaller combustion chamber' material in the Commando head. The gap between it and the piston is far too great.

When a squish band is working properly, there is little or no combustion taking place in the squish area, so there will be little or no carbon build up there.

fiatfan · Aug 11, 2016

Thanks everyone for your view on this. So it seems on a stock engine, which I will have apart from a slightly higher CR, the squish band is just...there..... :? Which is fine, but a bit strange. But the important thing is that it doesn't do any harm, so I'll just leave that, and move on :arrow:
Tommy

Fast Eddie · Aug 11, 2016

fiatfan said:
Thanks everyone for your view on this. So it seems on a stock engine, which I will have apart from a slightly higher CR, the squish band is just...there..... :? Which is fine, but a bit strange. But the important thing is that it doesn't do any harm, so I'll just leave that, and move on :arrow:
Tommy

That's what 99.9% of all Commando owners have ever done Tommy, so I think you're safe!

worntorn · Aug 11, 2016

Think of it this way- having that little flat there allows a bunch of macho guys to discuss in detail just how tight their squish is!
What a great forum this is :mrgreen:

fiatfan · Aug 11, 2016

worntorn said:
Think of it this way- having that little flat there allows a bunch of macho guys to discuss in detail just how tight their squish is!
What a great forum this is :mrgreen:

+1!!! :lol:
Tommy

Fast Eddie · Aug 11, 2016

worntorn said:
Think of it this way- having that little flat there allows a bunch of macho guys to discuss in detail just how tight their squish is!
What a great forum this is :mrgreen:

You're only jealous...!

worntorn · Aug 11, 2016

Yeah, well maybe.

I'm ashamed to say that my squish is pretty......loose.

Gle

acotrel · Aug 11, 2016

There is a very big difference in the way the coke builds up in a Triumph cylinder head when compared with a Norton. I knew one guy who fitted additional plugs to the other side of the Triumph combustion chamber, however I never knew how he would know if he had a duff plug. If the piston is coked up on the side of the piston crown which is away from the plug, the motor cannot be operating at it's full potential ? The part of the piston crown which experiences the most heat is what limits how lean you can jet the carburation. So the mixture is never right in a Triumph motor unless twin plugs or a squish band is fitted. I don't believe the Commando motor has this problem.

Fast Eddie · Aug 11, 2016

worntorn said:
Yeah, well maybe.

I'm ashamed to say that my squish is pretty......loose.

Gle

Not in your modern Triumph I would wager.

Or your TPV equipped monster Egli?

Fast Eddie · Aug 11, 2016

acotrel said:
There is a very big difference in the way the coke builds up in a Triumph cylinder head when compared with a Norton. I knew one guy who fitted additional plugs to the other side of the Triumph combustion chamber, however I never knew how he would know if he had a duff plug. If the piston is coked up on the side of the piston crown which is away from the plug, the motor cannot be operating at it's full potential ? The part of the piston crown which experiences the most heat is what limits how lean you can jet the carburation. So the mixture is never right in a Triumph motor unless twin plugs or a squish band is fitted. I don't believe the Commando motor has this problem.

I agree with you Alan. And the higher you go with the CR in a Triumph the worse it gets due to the high domed pistons. I got over 11:1CR using modified stock T140 pistons in squished heads, which only have a very slight dome. When I ran 11:1 powermax pistons (without squished heads) it was a very fussy motor and needed good race fuel to work.

The shallow combustion chamber of the Norton is better IMHO. But the trade off is limited space for valve sizes.

The shallow combustion chamber on a Norton tweaked to utilize the (already built in) squish band is even better, in my book at least.

ewgoforth · Aug 12, 2016

acotrel said:
There is a very big difference in the way the coke builds up in a Triumph cylinder head when compared with a Norton. I knew one guy who fitted additional plugs to the other side of the Triumph combustion chamber, however I never knew how he would know if he had a duff plug. If the piston is coked up on the side of the piston crown which is away from the plug, the motor cannot be operating at it's full potential ? The part of the piston crown which experiences the most heat is what limits how lean you can jet the carburation. So the mixture is never right in a Triumph motor unless twin plugs or a squish band is fitted. I don't believe the Commando motor has this problem.

The Norton also runs optimally with about ten degrees less advance than a Triumph. That's also an indicator of a better combustion chamber and head design.

Dances with Shrapnel · Aug 12, 2016

Now that most of this has been flushed out, it would be interesting to see what improvements would be realized by improving the squish (quench) area on an otherwise stock Commando.

Let's say one did not bump the compression ratio up but only managed to achieve an optimal squish area. From what I have read, one can overdo the amount of squish area. I suspect an enhanced or optimal squish would reduce the incidence of detonation and allow for less timing advance which is always a good thing for power.

For street applications on a Commando, this is really an academic exercise yet I would find it interesting to see the net results.

Nater_Potater · Aug 12, 2016

Dances with Shrapnel said:
I suspect an enhanced or optimal squish would reduce the incidence of detonation and allow for less timing advance which is always a good thing for power.

Absolutely! Small block Chebbies vs. small block Mopars run around 38' total vs. 34' total for best power. The mopar has both more centrally located valves and spark plug, leading to a more compact/more efficient combustion chamber, allowing either higher compression or less of a thirst for high-test.

Of course, there's always a downside; as pointed out earlier by Fast Eddie, there is little or no burning taking place in the squish area, with the associated unburned fuel finding its way into the atmosphere. Heavens!

Nathan

Squish band?

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