High CR pistons and squish heads chat…

I still do not know why, besides that some milling etc work is needed, nobody ever or perhaps not too many bothered to do some conical squish heads and pistons?
I so far, had in engineering and fabricating/modifying heads and pistons within the last years (although on non commando engines) very good experiences and terrific reviews from users of my head packages.
If combustion chamber shape versus squish angle is correct, sufficiently high compression ratio (considering cam timing) will even permit to cut away on valve cut outs and ease combustion scavenging.
Or at least reweld and reshape the combustion chamber a hint more oval (O-shape or egg shape) without impeding flow.

Kind regards Christian

Fast Eddie said:

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?

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The piston is in contact with the bore and therefore the barrel via the rings. The barrel is also bolted to the hot cylinder head and the hot combustion gases also heat the cylinder bore/barrel.

Iain.b40 said:

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

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The piston top is much hotter than the head.
Mahle states that their piston tops will see up to 300 c or 600 f.
The head shouldn't be anywhere near that or it will suffer badly.
The quench heads are enough to make the difference.
The word quench is a verb meaning " to rapidly cool down".
Glen

Iain.b40 said:

The piston is in contact with the bore and therefore the barrel via the rings. The barrel is also bolted to the hot cylinder head and the hot combustion gases also heat the cylinder bore/barrel.

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I do not agree that all heat transfer from piston to barrels is entirely through the piston rings.

The contact area is just too small. Especially on single ring racing pistons.

You’re coming up with a lot of arguments against this and that Iain, which is fine. But I would therefore genuinely like to know your thoughts on how a squish band / quench area works, how is it, in your opinion, that it helps so much in reducing knock, even with a raised CR?

Hudriwudri said:

I still do not know why, besides that some milling etc work is needed, nobody ever or perhaps not too many bothered to do some conical squish heads and pistons?
I so far, had in engineering and fabricating/modifying heads and pistons within the last years (although on non commando engines) very good experiences and terrific reviews from users of my head packages.
If combustion chamber shape versus squish angle is correct, sufficiently high compression ratio (considering cam timing) will even permit to cut away on valve cut outs and ease combustion scavenging.
Or at least reweld and reshape the combustion chamber a hint more oval (O-shape or egg shape) without impeding flow.

Kind regards Christian

Click to expand...

This has been done quite a lot with the Vincent Hemi head.
One experimenter showed me his melted piston collection, sort of a giant cardboard egg crate filled with holed and melted pistons.
Eventually he figured out just how far to push things and built a very quick bike that has held together.
It's not close to the performance one gets from the Prince heads with big flat squish zones and late Manx shaped combustion chambers. There may be other factors at work there
With the Norton, you already have quite a nice flat squish area to make use of, as Nigel has done.

Glen

I’m in no way trying to suggest that any of this is optimal! Just my attempt to use whats already there, as Glen said, but get increased benefit from it.

This gives an old school, carburated, 920 motor running 11:1 CR on pump fuel with no issues at all.

These are the 81mm JS pistons, with a different dish to what Jim showed earlier. You can also see the piston protrusion:


This is the head, with the ‘squish’ area opened out to accommodate the 81mm bore and the rest of the combustion chamber left stock. As you can see, the bore increases the size of the squish band considerably, more would of course be better, but to achieve this without any complex welding etc is great in my book:

@worntorn
Hum I'm not quite sure if you complety understand what I meant (no critic at all intended!).
I'm not saying conical pistons and leaving original squish areas untouched.
I'm suggesting a combustion chamber shape that doesnt show horizontal squish areas but inclined angled surfaces with matching areas on the piston. That way one gets way higher compression, could get also rid of the valve pockets, better scavenging and during higher rpm's the mixture gets where it should be close to the spark plug for less spark advance.
Or at least to try to some hemishaped piston domes with hemispherical squish areas on the side.
Unfortunately I think (and am sure that would be also according to findings of the jolly good Irish prof) that the area of the original squish is quite to small for sufficient Msv (mean squish velocity). Admittedly better than no squish or a brick on the head, but ther usee are better choices than "better than a brick on the head".

Kind regards and greet'z from Italy

Christian

@Fast Eddie

I think the common idea of many that complex welding would be needed is to a large extent an urban myth dispersed by
1. Weld shops with antique machinery and lack of knowledge in setting up sophisticated modern inverter welders like a miller, Lincoln or fronts.
Or
2. Weld shops with a lack of experience in correct approaches and techniques.

Christian wrote
"Or at least to try to some hemishaped piston domes with hemispherical squish areas on the side."

That's what some hotrodders have done with their Hemi head Vincents- they had very expensive high compression pistons made with the lower portion of the piston dome matched to the combustion chamber for a 40-50 thou squish.
Results were mixed, lots of melted pistons created until the right shape was found thru trial and error.
It gave a few extra HP due to the high compression.
The ultimate hotrodder , T Prince, started with a totally new head design incorporating flat squish areas like many modern high performance very high compression auto engines. With that and various other modifications he increased power from 45 bhp stock to 165 bhp on dyno.
Hard to find fault with his result!

Glen

The link below clarified things a bit for me, assuming they know what they are talking about.

Some good info on how pistons dissapte heat.

In case you want to know what a 12.8/1 73mm Piston looks like here is the one from Sir Eddy's Rocket.

worntorn said:

The piston top is much hotter than the head.
Mahle states that their piston tops will see up to 300 c or 600 f.
The head shouldn't be anywhere near that or it will suffer badly.
The quench heads are enough to make the difference.
The word quench is a verb meaning " to rapidly cool down".
Glen

Click to expand...

I agree on the definition of quench but that definition does not apply in this context. Place your hand as close as you can without touching to a hot domestic radiator and feel the heat for 10 seconds. Now press your hand against the radiator and compare the difference in heat transfer. The piston top is not 'much' hotter than the cylinder head because the cylinder head combustion chamber is subjected to the same heat from combustion. There is a temperature gradient through the cylinder head but as for the piston getting cooled down via an intermittent small air gap by the hot cylinder head is nonsense.

Fast Eddie said:

I do not agree that all heat transfer from piston to barrels is entirely through the piston rings.

The contact area is just too small. Especially on single ring racing pistons.

Click to expand...

I explained previously there is not only heat transfer from the piston to barrel via the 3 piston rings but also through conduction from the piston to the con-rod and from the cylinder head to the barrel and that the piston is oil cooled from the underside of the crown. I didn't say all the heat transfer went through the rings. A simple demo you can do at home if you have central heating radiators. Place your hand as close as you can to a hot radiator without touching it for 10 or 20 seconds. Now press your hand against the radiator and feel the difference in heat transfer.

htown16 said:

Some good info on how pistons dissapte heat.

The Lives Of Pistons

Modern piston crowns and combustion chambers are nearly flat, resulting in minimum surface area, but that hasn’t always been the case.

www.cycleworld.com

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Interestingly, they talk about heat transfere from piston skirt to barrel occurring due to the thin film of oil, ie it’s not an air gap, it’s an oil filled gap.

Maybe that’s what happens with a squish band, perhaps the super-squashed charge is so dense in that tiny gap that it also adds heat transfere?

Iain.b40 said:

I explained previously there is not only heat transfer from the piston to barrel via the 3 piston rings but also through conduction from the piston to the con-rod and from the cylinder head to the barrel and that the piston is oil cooled from the underside of the crown. I didn't say all the heat transfer went through the rings. A simple demo you can do at home if you have central heating radiators. Place your hand as close as you can to a hot radiator without touching it for 10 or 20 seconds. Now press your hand against the radiator and feel the difference in heat transfer.

Click to expand...

Agreed, most piston cooling is via the engine oil, especially on a Norton that has an oil jet hole in the conrod.

However, in post #42 you answered my point about PISTON TO BARREL heat transfer by saying the piston is in contact with the barrel via the rings.

My point was, and is, that cannot be the only means of piston to barrel heat transfer. Which is a point made, and explained, in the article linked in post #51.

Christian, I know it’s not quite the exotic set up you have in mind (understatement!) but the humble old Triumph T140E had a bit of a conical squish band going on.

The E had a redesigned head and pistons compared to earlier Bonnies with a modest conical squish built in. Unfortunately, possibly due to factory tolerances, most bikes left the factory with the gap too big to allow the design to function as intended.

I had the head off my own T140E recently and took the opportunity to look at this. By fitting a thinner copper head gasket, and omitting the base gasket, I achieve a mild boost in CR and a functioning squish, proof of this is the total absence of knock, whereas previously it could be made to knock relatively easily (as do many Triumphs). some pics…

Fast Eddie said:

You’re coming up with a lot of arguments against this and that Iain, which is fine. But I would therefore genuinely like to know your thoughts on how a squish band / quench area works, how is it, in your opinion, that it helps so much in reducing knock, even with a raised CR?

Click to expand...

You make two assumptions; squish reduces knock (by raising the CR?) and that I agree it reduces knock...I don't. High octane fuel reduces knock and good mixing of the air fuel mix. I can see an advantage to 'squish' in a badly designed cylinder head e.g side valve or 4 valve pent roof engine with pent roof pistons but not on a Commando cylinder head as it already provides an unobstructed path for the flame front to spread. I explained previously the Commando head has a squish area because the cylinder head spheres are off set so as to produce swirl on the compression stroke which improves mixing of the air fuel molecules. Designing in an area where the fuel does not get burnt as you suggest is counter productive. Fitting an 850 head on a 920 barrel is one way of increasing the compression ratio.

Fast Eddie said:

Agreed, most piston cooling is via the engine oil, especially on a Norton that has an oil jet hole in the conrod.

However, in post #42 you answered my point about PISTON TO BARREL heat transfer by saying the piston is in contact with the barrel via the rings.

My point was, and is, that cannot be the only means of piston to barrel heat transfer. Which is a point made, and explained, in the article linked in post #51.

Click to expand...

So what's your problem? The vast majority of heat transfer from the piston is from conduction via the rings, con-rod and engine oil which is what I said previously. I didn't say MOST heat transfer is via the oil or via the rings it is a combination of the above.

Iain.b40 said:

You make two assumptions; squish reduces knock (by raising the CR?) and that I agree it reduces knock...I don't. High octane fuel reduces knock and good mixing of the air fuel mix. I can see an advantage to 'squish' in a badly designed cylinder head e.g side valve or 4 valve pent roof engine with pent roof pistons but not on a Commando cylinder head as it already provides an unobstructed path for the flame front to spread. I explained previously the Commando head has a squish area because the cylinder head spheres are off set so as to produce swirl on the compression stroke which improves mixing of the air fuel molecules. Designing in an area where the fuel does not get burnt as you suggest is counter productive. Fitting an 850 head on a 920 barrel is one way of increasing the compression ratio.

Click to expand...

Thanks. On that point then we should simply agree to disagree.

My opinion is that there is a huge amount of literature explaining the benefits of an effective squish band being mainly the ability to allow a higher compression and / or reduced tendency to knock. Also, as mentioned above, my own personal experience backs this up.

I tend to agree that the benefits are somewhat linked to a poor combustion chamber design, which is why older bikes and cars benefit so much perhaps. But, it’s also a factor built into many, many modern car and bike engines from performance engines in AMG Mercs to humble Toyotas.

My personal thinking is that it’s more popular in the US as they have a lot more bigger bore engines in their big V8 cars and V twin motorcycles. The NASCAR pushrod V8 world has no doubt had a huge influence in this too. A big bore presents challenges in terms of optimising burn, so effective squish bands probably have a greater impact in such motors.

An effective squish band, in the definition that I am using it, is one where the piston / head gap in the squish band is at, or under, .060”.