Squish, mystery or not?

[best]

James, excellent thread!

Can I add some things?

So how much squish (thickness) is enough?
A 2 stroke motor doing 10,000 rpm is a mighty flexible thing.
The main and rod bearings have a surprising amount of clearance in them and the rod will actually stretch at these speeds as it accelerates to nearly 4000 ft/min (45mph!) then stops and goes the other way over 300 times in one second. Things stretch enough so that if you don't allow enough clearance they will collide and the piston will become a hammer and pound things to pieces. The bigger the motor and the more rpm involved the more clearance is needed, so take heed of James 0.050" measurement as a minimum. Read more at:

Gordon Jenkins 2 stroke Tuners Guide


James, your drawings, which as simple as they were, are excellent for illustrating your points. I hope you don't mind me modifying a few of them to expand on a couple points.

First off is the jetting effect of the air/fuel mixture shooting out of the squish zone. It creates a donut (toroidal) shaped "tornado" in the chamber of the head. This keeps fuel in suspension and keeps the mixture mixed so there are no lean or rich spots.

Below, the spark ignites the mixture, the same toroidal (donut) shaped "tornado" carries the flame up and over the roof of the chamber, sparing the piston from some of the heat. While the flame swirls in circles, the pressure emmenates out from the core of the flame toward the ends of the cylinder. The quenched (cooled) squish areas prevent the pressure waves from creating detonation spots at the ends of the cylinder. If the squish thickness is too much, detonation can happen at the ends of the chamber (against the cylinder walls, usually on the hot exhaust side).

This picture below goes back to where the intake mixture is purging the exhaust gasses out of the cylinder. A complicated swirl takes place that chases the exhaust gasses out of the cylinder. The better a job it does, the less exhaust gasses left and the more intake charge to create power. You can see how the deep chamber creates a hiding place for exhaust gasses. The Blaster is even worse than this picture. It is a deep cone that holds a lot of the exhaust remnants.

[

Below you can see how a shallow chamber is designed for good cylinder clearing. There is a balance. Too shallow and you do not get good torroidal swirl. Too deep and you do not get good clearing.

Also notice there is less squish area (distance from the cylinder walls) in this picture below. This is better for high rpm work too. It takes energy to make the swirl, so the more squish area you have, the more it took from your available power output if it is not contributing to better burn. More squish area promotes toroidal swirl at even low rpm. At high rpm less swirl is needed. The 50% of head area rule is an average. Lower rpm motor may want more than 50% squish area, Higher rpm motors will want less than 50% squish area.

It is all a balance. The squish and the head shape make a big difference in your power.

 

[sicivicdude]

[

It is all a balance. The squish and the head shape make a big difference in your power.

Steve as simple as your drawings are, they are excellent for illustrating your points.... but let's delve a little further....

The picture above shows the secondary transfer ports as having an angled port roof but anyone who's played with one knows that's not how they really are. One would wonder why that's the case but the answer is quite simple, high angle port roofs tune the charge to a higher rpm range but there's actually more than one reason!

The boost port is aimed upwards. In your drawing above it shows all of the ports pushing charge upwards towards the head but if they were all pointed up, the charges would collide and render the speed of the charge useless.

The trick is, to match the portjob to the head rechamber so that the secondary transfer ports purge the middle part of the cylinder (the bulk) and the boost port (being that it's located in the middle of the cylinder at the back is perfectly situated for the task) purge the head. Reading porting manuals it's suggested to use a "visual aid" to guide the work that involves the boost port to ensure that the roof angle of that port is pointed directly towards the inside lip the edge of the combustion chamber to point the incoming charge in to purge that area too. This effect is obviously much more effective when the charge speeds are high enough to carry through but not so high to de-laminate and mix excessively with the outgoing charge.

The optimal head shape is really efficient at one rpm range when combined with the portjob. The target then should be the tuned range you're aiming for with the pipe/carb/portjob/intended use.

 

[best]

Steve as simple as your drawings are, they are excellent for illustrating your points.... but let's delve a little further....

The picture above shows the secondary transfer ports as having an angled port roof but anyone who's played with one knows that's not how they really are. One would wonder why that's the case but the answer is quite simple, high angle port roofs tune the charge to a higher rpm range but there's actually more than one reason!

The boost port is aimed upwards. In your drawing above it shows all of the ports pushing charge upwards towards the head but if they were all pointed up, the charges would collide and render the speed of the charge useless.

The trick is, to match the portjob to the head rechamber so that the secondary transfer ports purge the middle part of the cylinder (the bulk) and the boost port (being that it's located in the middle of the cylinder at the back is perfectly situated for the task) purge the head. Reading porting manuals it's suggested to use a "visual aid" to guide the work that involves the boost port to ensure that the roof angle of that port is pointed directly towards the inside lip the edge of the combustion chamber to point the incoming charge in to purge that area too. This effect is obviously much more effective when the charge speeds are high enough to carry through but not so high to de-laminate and mix excessively with the outgoing charge.

The optimal head shape is really efficient at one rpm range when combined with the portjob. The target then should be the tuned range you're aiming for with the pipe/carb/portjob/intended use.

You are absolutely right. Especially on the job of the boost port. The boost port kind of aims and centers the whole flow. Backbone.
My point was to illustrate the charge sweeping by the cylinder head and the effect chamber shape had on clearing the remaining exhaust.
The above drawing is very misleading in what is happening at the transfer ports, it it more like this:

(I don't know where this drawing came from, found it on my computer)

But even that is a bit misleading due to drawing a 3 dimensional event in 2D.
We are straying from what squish does, but I think you are right to delve into, especially if something is a bit murky and misleading.
The ports are not right either in my previous drawings. The exhaust would be completely open if the transfers are. Oh well...
From straight on the intake charge flow would look like this:

Again, photo credit to Sicivicdude for the original which I modified.

The intake charge comes straight out of the side transfers toward the intake side, running along the floor of the piston crown.
It collides in the center of the piston crown and spills up, because it has no other place to go and guided by the little boost port above the intake.
The intake charge sweeps past the cylinder head, washes out the rest of the cylinder and chases the remnants of the exhaust out the exhaust port.
Big complicated swirl that requires carefully aimed transfer ports to get it right.
All I have ever done with transfer ports is clean up the cylinder liner mismatch and set them all to the same height while keeping the original roof angle.
Looking at the centering of the carbon on the top of the piston can tell you if the transfers are balanced.

A bit off topic, but I notice two of the transfer ports have a bit of a "pocket" in them. A little cave at the corner.
This is different from the DT200 cylinder I have been using as a guide. Anyone here ever fill this in with epoxy?

 

[sicivicdude]

I understand your diagram was simplified to make a point about head shape So were mine... I:I

The combustion bowl size and shape DO need to have attention paid to them while tuning for rpm but the efficiency of the squish band and combustion chamber versus scavenging are kind of conflicting requirements unless the transfer and boost port are included in the design.

BTW, good drawings! I:I

 

[little3mil]

great read here

 

[sporty982000]

i get the idea. but is there some specs for the blaster to go buy, in making our own or having one done at local shop.

So when we take it in, they dont go huh and we have numbers and ect to give to them ?


thanks

Sporty

 

[sicivicdude]

There are some general specs but the problem is....

Head volume is dependent on trapped volume. Trapped volume is dependent on exhaust port duration. You see I've been fairly mum on any actual hard numbers because in order to maximize the head shape and volume, you need to design it for each application!

Any "numbers" I give for a ported engine will be too tight for a stock port engine. Any numbers for a stock port engine are going to be loose on a ported motor.

 

[best]

There are some general specs but the problem is....

Head volume is dependent on trapped volume. Trapped volume is dependent on exhaust port duration. You see I've been fairly mum on any actual hard numbers because in order to maximize the head shape and volume, you need to design it for each application!

Any "numbers" I give for a ported engine will be too tight for a stock port engine. Any numbers for a stock port engine are going to be loose on a ported motor.

You are right SiCivicDude.
If we put it in terms of 4 strokes that we are more familiar with, a wilder cam works better with more compression, and a tame cam cannot stand as much compression. So it is with 2 strokes. Wild porting (and high rpm) likes a small combustion chamber, whereas mild porting will need more chamber volume.

I will be bolder than James and suggest around 20cc or less for a wildly ported high rpm engine, and 20cc to 30cc for a lower rpm or torque ported Blaster engine, but as he suggests it is never as simple as all that. The stock head is a remarkably small chamber (22-23cc) because it uses a large squish gap (0.100"-0.130" or so). It is impossible to get a proper squish gap on a stock head due to detonation. Cut the chamber larger (25-28cc)and you will get more power (nearly double!) when you correct the squish gap t 0.050" or so!

Also, the head volume figure gets messed up by where you put the piston (or the cylinder jug height). Drop the jug and you can run a 28cc head and make power. Raise the jug and you can run 18cc with no detonation. So in the end, as usual, SiCivicDude is right. A cc figure is useless unless you figure in your jug height, piston height, and squish gap.

Incidentally, raising or lowering the jug has the same effect on where the powerband is. We lowered the jug (removed the base gasket and only used sealer) and with a modified head, made an engine with fat low end power. It was almost unstallable. Sliding an extra base gasket under the jug will make an engine a screamer IF your head is cut to match.

Do you want a place to start?
The green line figure in this drawing is a pretty good place to start, but use the red squish area values.
You will have to shim your cylinder to get the right squish gap.
I do believe Ken O'Conner can deliver a similar head for much less than $100.
This head is based on the DT200 32hp engine.

 

[N8tsNastyBlasty]

any of you guys see the "turbo cut" squish on modified cylinder heads? A guy on ebay had one for sale awhile back had some crazy swirled grooves on the squish that was supposed to direct scavenge and accelerate the charge? Im currently employed @ a machine shop as a machinist apprentice and im considering making fixtures and programming to do as much cylinder and head modification as i can...all portwork, case work, cr125 reed cages clearancing and heads ect... where can i find the programming to do all the trig for me on the cylinder combustion chamber? Nice work on here fellas ,props !!!

 

[best]

any of you guys see the "turbo cut" squish on modified cylinder heads? A guy on ebay had one for sale awhile back had some crazy swirled grooves on the squish that was supposed to direct scavenge and accelerate the charge? Im currently employed @ a machine shop as a machinist apprentice and im considering making fixtures and programming to do as much cylinder and head modification as i can...all portwork, case work, cr125 reed cages clearancing and heads ect... where can i find the programming to do all the trig for me on the cylinder combustion chamber? Nice work on here fellas ,props !!!

I'd like to sound all knowledgeable and say the swirl cuts are pure hogwash, but I truly don't know, and see another possibility. I do know (and it is documented by several authors) that the closer the squish distance to more active the combustion turbulence (swirl) and combustion speed. While this action does improve combustion and power, as the squish distance closes under 1mm the process starts to consume increasingly significant amounts of power.

Mr Somender Singh has made a religious revival out of swirl cuts, and many intelligent engine builders are finding there is value to his work. There are varied theories, some about the effect of jets and swirls. I tend to think the reduced squish compression losses combined with still effective turbulence is the effect.

If this is the case, the cuts in the head do not have to be "swirled", but could be straight and work just as effectively. I have read the work of 2 stroke engine builders cutting 2, 3, 4, 6 and 8 straight radial cuts in close squish heads and noticing a power advantage from them. I haven't been bold enough the cut a good working head yet!

The gain is small in comparison to chamber reshaping, perhaps on the scale of a 10% gain I am lead to believe. Somender Singh has been doing this work for both power and economy gains, but seems tight lipped about details,like many builders, protecting his livelihood.

Steve

 

[Awk08]

"Hollywood Dirt Productions" were doing these swirl cuts everywhere...carb throat, intake, head and exhaust areas,
they are no longer in business.

anyone remember the "tornado" intake insert for auto's, supposedly to increase performance and milage, not sure they're around anymore either ???

i also know someone who has one of the H.D.P. swirled top ends, he says he will not be copying or incorporating the swirls into his proven current design ?

 

[sicivicdude]

I'd like to sound all knowledgeable and say the swirl cuts are pure hogwash, but I truly don't know, and see another possibility. I do know (and it is documented by several authors) that the closer the squish distance to more active the combustion turbulence (swirl) and combustion speed. While this action does improve combustion and power, as the squish distance closes under 1mm the process starts to consume increasingly significant amounts of power.

Mr Somender Singh has made a religious revival out of swirl cuts, and many intelligent engine builders are finding there is value to his work. There are varied theories, some about the effect of jets and swirls. I tend to think the reduced squish compression losses combined with still effective turbulence is the effect.

If this is the case, the cuts in the head do not have to be "swirled", but could be straight and work just as effectively. I have read the work of 2 stroke engine builders cutting 2, 3, 4, 6 and 8 straight radial cuts in close squish heads and noticing a power advantage from them. I haven't been bold enough the cut a good working head yet!

The gain is small in comparison to chamber reshaping, perhaps on the scale of a 10% gain I am lead to believe. Somender Singh has been doing this work for both power and economy gains, but seems tight lipped about details,like many builders, protecting his livelihood.

Steve

I've seen some of his work.

There is more to his story than meets the eye too..... You have to consider he's doing this work on a MUCH less than optimized vehicle to begin with and working with MUCH less than well refined fuel supplies.

Perhaps some of his power "gains" have more to do with working around the poor design he started with (the vehicle he started with) then actually finding some way to defeat physics. As such, I find it hard to believe that engineers at the likes of Honda, Toyota, GM, Ford, and Chrysler haven't at least pondered the thought and tested it to some degree. I doubt they'd be getting the fuel economy and power output they are currently getting out of the latest generation of cars if one could improve them 10% by taking a chisel to the head and making swirly patterns. They wouldn't be employed very long at all, would they?

My understanding is that HDP went under from extremely poor mismanagement and not because of the swirly designs. I also understand that a LOT of people swore by the swirly carbs, heads, and exhaust ports.

Personally, I don't see the value in it. My personal belief is that it probably disturbed the boundary layer causing more disturbance than a straight smooth shot...