High CR pistons and squish heads chat…

jseng1 · Nov 15, 2021

Fast Eddie said:
Unless we’re talking domes designed to very closely match the hemisphere of the combustion chamber, they won’t affect squish.

The easiest squish to get on a Norton twin is by using the flat stepped area. Yes the stock head creates turbulence, but it tightening up the gap in the stepped area gets a nice bit of squish too.

Personally, I am convinced of the benefits of squish due to running a tight squish (approx 0.040”) in my 920, which runs 11:1 cr, without issue, on pump fuel.

The domed piston below has the tight squish band close to .040" (fine tune from there). The dome is only 5mm tall. The combustion chamber is approx 22mm deep. There is still plenty of room for fuel efficiency. When you have 15mm high domes as on a Triumph - how much fuel efficency are you losing then? Do Triumphs get poor fuel milage due to unburned fuel? - be honest. Yes the small round combustion chamber is best. But don't forget that the highest HP 2 valve motorcycle engine is the HD XR 750 with pistons that are highly domed as shown in the photo at below.

High dome HD XR750 piston

There is a problem with jacking up the crown for more compression and keeping the top flat - the crown gets thicker as the pockets get deeper ending up with a heavy crank breaking piston that inhibits flow by shrowding the valves with those deep pockets such as shown with the Omega short stroke piston and its huge dome trying to make compression with the large fully hemisphered dome of the Norton factory short stroke as shown below.

All things considered a slight dome is the best option if you're trying to get more compression because you:
1 keep weight down
2 avoid deep pockets
3 improve gas flow by avoiding valve shrouding

Compare the Omega piston to the powermax piston in post #4. The powermax is much lighter with shallower pockets (in a smaller combustion chamber) and both have about the same compression.

Fast Eddies large displacement 920 gets 11:1 with no dome at all - but actually a dish as shown below. With the tight squish band this is an ideal situation. You just can't achieve this with small a displacement Norton using the same head. So you have to do the best that you can for a given engine size.

See flat top 850 high compression below with tight squish band. Having a higher displacement the 850 flat top piston achieves over 10.2:1 compression.

Iain.b40 · Nov 16, 2021

The standard Norton Commando cylinder head and piston is a good design. The inlet ports are short which reduces friction losses and heat transfer to the mixture therefore denser=more power) and importantly they diverge and effectively 'twist' into the combustion chamber. This creates swirl of the air and fuel molecules on the inlet stroke helping to mix molecules. On the compression stroke the air and fuel molecules continue to swirl due to the hemispheres being offset to the cylinders which is evident from the concentric to the bore step/recess/squish band Norton engineers put in the cylinder head all those years ago. It's there to provide running clearance for the piston. Reducing that clearance does two things; raise compression ratio a bit and makes it difficult for the flame front to burn the air and fuel mix trapped in that pocket. The standard compression ratio (8.5 to 1) and combustion chamber design provides enough squeezing (squish) and swirl to thoroughly mix the air and fuel molecules for good combustion.

"Unless we’re talking domes designed to very closely match the hemisphere of the combustion chamber, they won’t affect squish." I think you have misunderstood the purpose of squish and how it is achieved.

Madnorton · Nov 16, 2021

A dedicated forge die for a particular piston will enable valve pocket depth and piston crown height can be overcome as you can forge in the material in the precise area to machine the pockets, reducing the weight under the crown. The problem occurs when you use a forging for other piston type as you have to machine what you have.

Fast Eddie · Nov 16, 2021

Iain, good points ref the swirl, Norman White is big on maximising that when he flows heads.

I’m not sure what you mean about my squish confusion? I think perhaps we’re mixing domes designed to raise CR, with squish as is normally achieved in a Norton by using the inbuilt flat stepped area in this conversation.

I agree Iain the standard 8:1 (ish) set up is fine for standard applications. But this thread is enquiring about how to achieve 11:1 in a 750 for racing, with a standard type head.

As Jim points out above, by playing with deck height and piston protrusion, an 850 can achieve a .040” squish gap with a flat top piston, and this should give around 10.2-10.5:1 CR (depending on valve seat cut, previous skimming, etc). 850 pistons therefore only need a very shallow dome to achieve 11:1CR unless choosing to not minimise the squish gap, in which case the deck height will be lower, and the required dome higher.

A 920 running a .040” squish needs a dished piston to keep the CR ‘down’ to 11:1.

The question in this thread was, what does the dome look like to achieve 11:1CR in a 750. Only Jim has actually shown what that looks like, at least when using his pistons (thanks Jim).

My understanding of squish bands is that they are NOT supposed to allow the fuel to burn in the squish gap. The whole idea is to concentrate the burn into a smaller, controlled combustion chamber. Which is why a high CR with effective squish is usually able to resist knock more than a lower CR motor without.

I mention .040” squish gap because IMHO it’s a good number for a Norton. Some HRC engines were set up with zero cold clearance, clearly this is a bit hardcore for us! Many production car engines that run a squish use as much as .060” as at that figure, they still get enough squish bennefit, whilst allowing for manufacturing tolerances, future head skins, etc, but it’s on the edge of actually functioning as a squish. In my experience, at anything less than .030” you‘re risking the piston kissing the head in a Norton (depending on crank flex, piston weight, revs used, etc). Above .060” and the benefits of squish diminish as you can see carbon build up in the squish area. So keeping it between .040” and .050” is a good safe zone in my book.

jseng1 · Nov 16, 2021

My friend Ely Schless was a successfull Triumph racer back in the day and he used to spend a lot of time perfecting his squish band which was at near contact at the outside periphery of the piston and gradually opened up with more clearance toward the center of the combustion chamber. After all the racing parts were in this is where he spent most of his time getting more power. It basically shot compressed air/fuel charge towards the chamber center and the turbulence of the swirling mixture (efficient burning) was the claimed source of added power. The less efficient combustion chamber design of the ultra high pent roof dome Triumph piston needs all the help it can get. The Norton chamber is a much better design and has a lot going for it from the start.

A big clue to combustion chamber efficiency is the ignition timing - generally the more efficient the chamber the less ignition timing advance. That's why the Triumph needs more ignition advance than the Norton.

BTW I think Ely Schless road the last (classic type) Triumph to win (or place?) a National Superbike race (in Florida in the 70s or 80s). Ely also claims to have build the prototype electric car that was then purchased by Elon Musk and eventually turned into the Tesla. He makes electric trials bikes now.

Iain.b40 · Nov 16, 2021

Fast Eddie said:
Iain, good points ref the swirl, Norman White is big on maximising that when he flows heads.

I’m not sure what you mean about my squish confusion? I think perhaps we’re mixing domes designed to raise CR, with squish as is normally achieved in a Norton by using the inbuilt flat stepped area in this conversation.

I agree Iain the standard 8:1 (ish) set up is fine for standard applications. But this thread is enquiring about how to achieve 11:1 in a 750 for racing, with a standard type head.

As Jim points out above, by playing with deck height and piston protrusion, an 850 can achieve a .040” squish gap with a flat top piston, and this should give around 10.2-10.5:1 CR (depending on valve seat cut, previous skimming, etc). 850 pistons therefore only need a very shallow dome to achieve 11:1CR unless choosing to not minimise the squish gap, in which case the deck height will be lower, and the required dome higher.

A 920 running a .040” squish needs a dished piston to keep the CR ‘down’ to 11:1.

The question in this thread was, what does the dome look like to achieve 11:1CR in a 750. Only Jim has actually shown what that looks like, at least when using his pistons (thanks Jim).

My understanding of squish bands is that they are NOT supposed to allow the fuel to burn in the squish gap. The whole idea is to concentrate the burn into a smaller, controlled combustion chamber. Which is why a high CR with effective squish is usually able to resist knock more than a lower CR motor without.

I mention .040” squish gap because IMHO it’s a good number for a Norton. Some HRC engines were set up with zero cold clearance, clearly this is a bit hardcore for us! Many production car engines that run a squish use as much as .060” as at that figure, they still get enough squish bennefit, whilst allowing for manufacturing tolerances, future head skins, etc, but it’s on the edge of actually functioning as a squish. In my experience, at anything less than .030” you‘re risking the piston kissing the head in a Norton (depending on crank flex, piston weight, revs used, etc). Above .060” and the benefits of squish diminish as you can see carbon build up in the squish area. So keeping it between .040” and .050” is a good safe zone in my book.

Eddie, Jseng mentioned squish

Fast Eddie said:
Iain, good points ref the swirl, Norman White is big on maximising that when he flows heads.

I’m not sure what you mean about my squish confusion? I think perhaps we’re mixing domes designed to raise CR, with squish as is normally achieved in a Norton by using the inbuilt flat stepped area in this conversation.

I agree Iain the standard 8:1 (ish) set up is fine for standard applications. But this thread is enquiring about how to achieve 11:1 in a 750 for racing, with a standard type head.

As Jim points out above, by playing with deck height and piston protrusion, an 850 can achieve a .040” squish gap with a flat top piston, and this should give around 10.2-10.5:1 CR (depending on valve seat cut, previous skimming, etc). 850 pistons therefore only need a very shallow dome to achieve 11:1CR unless choosing to not minimise the squish gap, in which case the deck height will be lower, and the required dome higher.

A 920 running a .040” squish needs a dished piston to keep the CR ‘down’ to 11:1.

The question in this thread was, what does the dome look like to achieve 11:1CR in a 750. Only Jim has actually shown what that looks like, at least when using his pistons (thanks Jim).

My understanding of squish bands is that they are NOT supposed to allow the fuel to burn in the squish gap. The whole idea is to concentrate the burn into a smaller, controlled combustion chamber. Which is why a high CR with effective squish is usually able to resist knock more than a lower CR motor without.

I mention .040” squish gap because IMHO it’s a good number for a Norton. Some HRC engines were set up with zero cold clearance, clearly this is a bit hardcore for us! Many production car engines that run a squish use as much as .060” as at that figure, they still get enough squish bennefit, whilst allowing for manufacturing tolerances, future head skins, etc, but it’s on the edge of actually functioning as a squish. In my experience, at anything less than .030” you‘re risking the piston kissing the head in a Norton (depending on crank flex, piston weight, revs used, etc). Above .060” and the benefits of squish diminish as you can see carbon build up in the squish area. So keeping it between .040” and .050” is a good safe zone in my book.

Eddie, "You have a periphery squish band creating turbulance and an efficient reduced combustion chamber volume - tighter around the outside with the greatest volume in the center to concentrate flame propagation." Not my words but sounds good, but also technically incorrect. There is no difference between squish and compression. They both squeeze the air and fuel mixture into a smaller volume which is a definition of compression ratio. Efficiency comes from good mixing (swirl) of the air and fuel and complete combustion. Efficiency is relative in the case of internal combustion engines as they are effectively 'heat engines', they use the heat/expansion from combustion to provide power, they also require cooling systems to take away wasted heat and they also blow heat out through the exhaust system which accounts for efficiencies of around 30%. It doesn't make sense to say squish (compression) resists knock better than a lower compression ratio engine.

worntorn · Nov 16, 2021

My understanding of squish effect is that its main benefit is greater heat transfer from piston top to the head and away. That is it's most important role, allowing higher CR to work. The squish itself doesn't contribute much to power levels.
Drag racers in the US call them " quench heads" . The 351 Ford Cleveland engine is a good example of factory quench ( squish) head use. There is another more common version of that engine (Windsor) that often gets hotrodded but runs into detonation problems at around 10 to one CR.
The solution is to find some old Cleveland squish heads to fit, then CR can go even higher without detonation.
The quench effect allows one to run a high compression ratio without detonation.

Glen

Fast Eddie · Nov 16, 2021

Iain.b40 said:
Eddie, Jseng mentioned squish

Eddie, "You have a periphery squish band creating turbulance and an efficient reduced combustion chamber volume - tighter around the outside with the greatest volume in the center to concentrate flame propagation." Not my words but sounds good, but also technically incorrect. There is no difference between squish and compression. They both squeeze the air and fuel mixture into a smaller volume which is a definition of compression ratio. Efficiency comes from good mixing (swirl) of the air and fuel and complete combustion. Efficiency is relative in the case of internal combustion engines as they are effectively 'heat engines', they use the heat/expansion from combustion to provide power, they also require cooling systems to take away wasted heat and they also blow heat out through the exhaust system which accounts for efficiencies of around 30%. It doesn't make sense to say squish (compression) resists knock better than a lower compression ratio engine.

I think we’re disagreeing over semantics Iain. We’re using the same word for different things! And if we take the correct definition of the word ‘squish’, you are 100% correct, in that the rising piston ‘squishes‘ the gases into the smaller combustion chamber.

However, when I referred to squish, I meant squish band, also known as quench area (especially on t’other side of the pond, as mentioned by Glen) which is an area of the piston to head interface deliberately designed to allow little or no effective combustion to take place.

‘Squish band effect’ is a different thing to ‘compression ratio’. They’re inter related when building an engine of course, but are two different things.

ntst8 · Nov 16, 2021

I had thought the benefit of the squish band (as Eddie defines it) was improved combustion efficiency from the gases around the perimeter being forced into the centre as the piston approaches tdc, and so mixing things up a bit. Not that i have any training in these things.

Fast Eddie · Nov 16, 2021

ntst8 said:
I had thought the benefit of the squish band (as Eddie defines it) was improved combustion efficiency from the gases around the perimeter being forced into the centre as the piston approaches tdc, and so mixing things up a bit. Not that i have any training in these things.

That’s my understanding too, to present a more ‘compact’ charge around the plug vs a rhin, wide, spread out charge (even of the same volume).

But believe Glens quench / cooling point is recognised as the main benefit.

Iain.b40 · Nov 16, 2021

worntorn said:
My understanding of squish effect is that its main benefit is greater heat transfer from piston top to the head and away. That is it's most important role, allowing higher CR to work. The squish itself doesn't contribute much to power levels.
Drag racers in the US call them " quench heads" . The 351 Ford Cleveland engine is a good example of factory quench ( squish) head use. There is another more common version of that engine (Windsor) that often gets hotrodded but runs into detonation problems at around 10 to one CR.
The solution is to find some old Cleveland squish heads to fit, then CR can go even higher without detonation.
The quench effect allows one to run a high compression ratio without detonation.

Glen

Unless the piston is touching the head there is going to be no heat transfer. Even if the piston was touching the head heat transfer would be minimal as both components would be of a similar temperature. Pistons are mainly oil cooled but there will be some heat transfer to the con-rod (also oil cooled) and via the rings to cylinder wall.

Iain.b40 · Nov 16, 2021

Fast Eddie said:
That’s my understanding too, to present a more ‘compact’ charge around the plug vs a rhin, wide, spread out charge (even of the same volume).

But believe Glens quench / cooling point is recognised as the main benefit.

Quench is the north American term for squish, nothing to do with cooling. Heat transfer between metal components only happens when they are in contact.

Fast Eddie · Nov 16, 2021

Iain.b40 said:
Quench is the north American term for squish, nothing to do with cooling. Heat transfer between metal components only happens when they are in contact.

Maybe you’re right about the lack of heat transfer?

Maybe the cooling benefits come from the lack of meaninful combustion taking place over a portion of the pistons surface area?

Iain.b40 · Nov 16, 2021

Fast Eddie said:
Maybe you’re right about the lack of heat transfer?

Maybe the cooling benefits come from the lack of meaninful combustion taking place over a portion of the pistons surface area?

Aluminium is a very good conductor of heat so heat at the central part of the piston from combustion quickly spreads to all of the piston. I'm definitely right about the lack of heat transfer. There's no contact and the combustion chamber is subjected to the same temperatures as the piston crown.

worntorn · Nov 16, 2021

Iain.b40 said:
Quench is the north American term for squish, nothing to do with cooling. Heat transfer between metal components only happens when they are in contact.

That's conduction.
Heat transfers in three ways, Convection, Conduction and Radiation.
Take a red hot piece of metal and place it 50 thou away from a cold piece of metal.
After a few minutes take the cold piece of metal away, but don't do it with bare hands!

Glen

Fast Eddie · Nov 16, 2021

Iain.b40 said:
Aluminium is a very good conductor of heat so heat at the central part of the piston from combustion quickly spreads to all of the piston. I'm definitely right about the lack of heat transfer. There's no contact and the combustion chamber is subjected to the same temperatures as the piston crown.

So, what’s your thinking on how / why a functioning squish band / quench area helps to allow reduced knock, with a higher CR?

Fast Eddie · Nov 16, 2021

worntorn said:
That's conduction.
Heat transfers in three ways, Convection, Conduction and Radiation.
Take a red hot piece of metal and place it 50 thou away from a cold piece of metal.
After a few minutes take the cold piece of metal away, but don't do it with bare hands!

Glen

My thoughts also, after all, a piston doesn’t touch the bore either, but there is still heat transfer through the barrel… I assume we’d all agree on that, as it’s well documented that an alloy barrel takes more heat away from a piston. Isn’t it?

worntorn · Nov 16, 2021

I could spend an hour or two to make a video of the non contact heat transfer, but we already know the outcome.
I have received many welding burns from moving metal items that were close to but not in contact with the item being welded.
You would think I would learn, leave the gloves on!

Glen

Iain.b40 · Nov 16, 2021

worntorn said:
That's conduction.
Heat transfers in three ways, Convection, Conduction and Radiation.
Take a red hot piece of metal and place it 50 thou away from a cold piece of metal.
After a few minutes take the cold piece of metal away, but don't do it with bare hands!

Glen

I agree regarding convection etc but the temperature difference between the cylinder head and the piston is minimal so heat transfer is minimal.

gortnipper · Nov 16, 2021

And that is how car radiators work... the water conducts heat away, the water pump and the fan provide convection, the radiator radiates the heat.

High CR pistons and squish heads chat…

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