Squish Ratios and Angles, Combustion Chamber Volume and Shape

[Slotracer577]

For clarity, the 50% we’re talking about is band radius/ combustion chamber radius? The band should not exceed 50%?

Seems like Squish band volume/combustion chamber volume would follow a similar rule but not as balanced. 30sqb/70cbc maybe?

The 50% is based on area. Ie the squish band area is 50% of the bore area.
If it gets too high, the squish velocity gets too high and you can get detonation in the squish band. It also can increase loads on the main bearings. If you look at old ie 70’s air cooled 2-st in motorcycles they often had wider squish bands. This would lead to detonation. The fix was narrower squish band width.
You can keep the same compression by having a shallower chamber and narrow squish band. Easier to do on engines with a separate head than a saw with a combined head and cylinder.

 

[Ketchup]

The piston acts as the valves in these engines, adding to the complexity.

The crown architecture aids in directing the incoming charge and outgoing exhaust.

It’s fascinating stuff. The piston wash is tough to decipher. Below is the piston wash on one of the best runners I have ever built. Makes no sense, as it looks SO wrong.

View attachment 271542

Yeah. WTF?
Makes me rethink piston wash. @Bigmac mentioned a tight squish reducing carbon buildup in textured squish bands. Is the clean part of the wash pattern developing during the highest velocity of transfer flow, or at the point of ignition where the charge is the least diluted? Bothish, I guess. I hadn’t considered the ignition circumstances as significant.

@drf256, could your transfers be aimed high on that jug? So many questions there. Saw model?

 

[Slotracer577]

What is the purpose of a squish band versus no band? To take up space?

Does a couple of things, one is it’s easier to make compression with no piston in the way, two is getting all the burnable gasses into a small area. This makes the burn quicker as the flame doesn’t have to travel as far to burn the mixture. Note that flame speed is a variable than can be influenced by fuel type and how well it’s mixed with air. Also by squeezing the gasses into the chamber the turbulence mixes the fuel and air. Tighter squish mixes better, I assume this is due to the higher velocity as the gasses enter the chamber.

it also can have less surface area. This helps by reducing heat transfer into the head, allowing what heat we have to do more work. Look at a hemi head on a v-8. Absolutely horrible for surface area. The only reason they work well is that they have a lot of area for valves. That’s why they are used in dragsters, lots of valve area to get gas in and out. Funny thing, top fuel engines are one of few engines with the exhaust valves larger than the normal 1.4/1 intake to exhaust. Turns out a lot of oxygen comes in as fuel and has to get out of the cylinder.

 

[Wisconsin Welder]

Don't know if it'll make it into a lathe yet this weekend, really curious about this, so I cut most of the top of this evening.


With a sawzall............

Had the sawzall a pair of dogs i feel this would have gone faster LOL

Left a little bit for the lathe to true up.

 

[Bigmac]

Don't know if it'll make it into a lathe yet this weekend, really curious about this, so I cut most of the top of this evening.


With a sawzall............

Had the sawzall a pair of dogs i feel this would have gone faster LOL

Left a little bit for the lathe to true up.


View attachment 271732

What are you going to try first? Or are you going to may several and try them back to back?

 

[Wisconsin Welder]

What are you going to try first? Or are you going to may several and try them back to back?

I have more book learnin' to do, and more measuring I would think, first thing I would like to try would be to optimize all the variables I can control to favor top end power. I would think this would affect chamber CC's, squish %, squish angle etc. and see what happens.

Eventually I would also like to make a pair of other heads that are from opposite ends of the spectrum and run them back to back.

 

[Bigmac]

An narrow band and shadow combustion chamber, i feel is the best for higher rpm, at least on pipes motors. The narrower band has a shorter path to the exhaust port and the shadow chamber (all things relative) has a fast burn. I don’t calculate the squish %, I test band width and cc’s, there are a lot of variables, and you can get lost in them if you want, chamber depths, combustion chamber radius, band step to the chamber, and lot’s of stuff that can make you crazy! I have tested parallel bands and 1-3° of taper in the band even ran a few that had 1-2° of negative taper, and parallel is optimal, a degree or 2 was not a power killer and burn was still good, didn’t notice any extreme detonation issues. The interesting thing for me, I do more playing with atv and Mx stuff, but the tighter squish helps power and tuning ease under peak power, .010 of extra squish(as long as compression stays the same) doesn’t effect peak hp but will make the motor lazy in comparison, even though there won’t be much of a chance on the dyno, and this is all reed valve/tuned pipe combo’s.

 

[Bigmac]

That's some savage squish! Is over my head lol

It’s been fun to test(not my concept) and it’s showing promise! I feel the inverted band increases velocity/mixing , even at high squish clearances. It also seems to have more low end with less compression, and seems to have a larger tuning window. The short story on the theory is the dish piston is supposed to trap more transfer charges and not have the fresh carve roll off the traditional dome piston and out the exhaust port. My thinking is it also creates a transfer charge that has literally nothing in its way as the piston edge opens the ports, and the negative space of the dish piston creates more of a rolling charge across the piston top. I think it helps scavenging, without losing as much out the exhaust port, it’s interesting to think about, and the motor has an explosive low end hit off the bottom and still revs well, I have a bunch of other things I would like to try, set up wise, but it’s running too good to really want to try anything else! Lol

 

[drf256]

Yeah. WTF?
Makes me rethink piston wash. @Bigmac mentioned a tight squish reducing carbon buildup in textured squish bands. Is the clean part of the wash pattern developing during the highest velocity of transfer flow, or at the point of ignition where the charge is the least diluted? Bothish, I guess. I hadn’t considered the ignition circumstances as significant.

@drf256, could your transfers be aimed high on that jug? So many questions there. Saw model?

That’s Dall’s 036. It did well on the dyno for a 60cc saw. The transfers, are indeed, pointed a bit upwards.

 

[Stump Shot]

The piston acts as the valves in these engines, adding to the complexity.

The crown architecture aids in directing the incoming charge and outgoing exhaust.

It’s fascinating stuff. The piston wash is tough to decipher. Below is the piston wash on one of the best runners I have ever built. Makes no sense, as it looks SO wrong.

View attachment 271542

I don't think it looks wrong to run extremely well, how is it on fuel efficiency?

 

[Moparmyway]

I don't think it looks wrong to run extremely well, how is it on fuel efficiency?

Mostly it looks like power is going out the exhaust........ but, it could be that with the smaller chamber and larger/tighter squish, it’s burning efficiently enough for the leaving exhaust/transferr mixx to keep it cool enough to not carbon up ?

Twas said that this saw doesn’t suck ........ means it’s using the combustion process efficiently imposed on it by ( @drf256 ) Docs mods !!!!

 

[Moparmyway]

The transfers, are indeed, pointed a bit upwards.

Dual uppers
What part of the roof(s) point up, primary, secondary, both ?

 

[Deets066]

Mostly it looks like power is going out the exhaust........

Or it’s using fuel to push spent charge out. Which is good

 

[drf256]

I don't think it looks wrong to run extremely well, how is it on fuel efficiency?

Not sure on fuel efficiency. The best I have done on that model, and probably the 3rd one I ever did.

I can certainly say I was surprised when I looked back at my work. Learned a few things, one of of which was pretty don’t equal fast with porting. I didn’t dare touch anything.

FWIW, the uppers opened exhaust side first.

 

[drf256]

Don't know if it'll make it into a lathe yet this weekend, really curious about this, so I cut most of the top of this evening.


With a sawzall............

Had the sawzall a pair of dogs i feel this would have gone faster LOL

Left a little bit for the lathe to true up.


View attachment 271732

I used a Portoband to cut mine off. Can’t do it all on the lathe, too time consuming.

 

[srcarr52]

I used a Portoband to cut mine off. Can’t do it all on the lathe, too time consuming.

Agreed! Too much time and too easy to catch a fin and destroy it. I use the drop band saw to rough them in.

 

[Deets066]

I break the fins off with a hammer

 

[Lightning Performance]

Pictures or it didn't happen.

I'll document this one and clay imprint the important bit. A cut-away tells a thousand tales I can not.

Read up a bit more on this during last nights insomniacs theater. Seems things wif a corn can like flat bands as youz already know. This thing here couldn't be farther from that. This design could achieve less than two percent waste. It does have a very distinctive port window angle that Doc eluded to. Wouldn't know how to qualify such little losses in such a tiny area. Cutting a piston could be interesting here. Grunt is on the menu.

Lunch time now... then to finish this thread and get back to the grind with this crap weather and more coming Wednesday.

This topic has intrigued me. Not certain if the OP is questioning the squish band being slightly angled and getting looser as it approaches bore or just angling both.

Cutting both the piston crown and the band at an angle makes more sense to me. It will allow one to get more % of band to bore and also increase surface area of the entire compression area to possible allow more cooling from combustion. Creates more surface area in the entire cylinder.

Cutting the piston crown is what is find the most interesting. Think about the angle of the transfer roof opening into a square edge of a piston crown vs an angle that nearly matched to the roof.

You'll will get more port area faster and possibly more overall port "volume" during the open window time is my take from your statement. That piston dome is a game changer imo.

 

[Mastermind]

I'll document this one and clay imprint the important bit. A cut-away tells a thousand tales I can not.

Read up a bit more on this during last nights insomniacs theater. Seems things wif a corn can like flat bands as youz already know. This thing here couldn't be farther from that. This design could achieve less than two percent waste. It does have a very distinctive port window angle that Doc eluded to. Wouldn't know how to qualify such little losses in such a tiny area. Cutting a piston could be interesting here. Grunt is on the menu.

Lunch time now... then to finish this thread and get back to the grind with this crap weather and more coming Wednesday.


You'll will get more port area faster and possibly more overall port "volume" during the open window time is my take from your statement. That piston dome is a game changer imo.

So......no pictures then. Got it.

 

[Deets066]

So......no pictures then. Got it.