ESE's works engine tuner

[wax]

Hey guys thanks in advance for your thoughts
Im working on a new head for our vintage skis and I have finished the drawing except for the combustion chamber shape.
From the reading on here from people who are clearly in the know I will be using a toroid shape chamber. My question however is there a magic number when it comes to squish band ratio.
Or what is the effect on the power of having the squish band ratio to much or to little.
Please note im not talking about clearance its the ratio of squish to combustion chamber I am after.
Cheers

[wobbly]

Squish width is dependent upon how much piston clearance you have.
I have found that a calculated MSV of 38M/Sec gives the beat result in most cases.
To achieve this number you start with the clearance and keep increasing the width until the velocity is correct.
What is your bore/stroke/clearance/com.

[wax]

Squish width is dependent upon how much piston clearance you have.
I have found that a calculated MSV of 38M/Sec gives the beat result in most cases.
To achieve this number you start with the clearance and keep increasing the width until the velocity is correct.
What is your bore/stroke/clearance/com.

Thanks Wobbly
my bore is 75mm
The stroke is 60mm
My projected squish clearance is .9 mm
The engine will rev to about 7500

[wobbly]

With those numbers I get 57% SAR = 13mm wide.

The effect of excessive MSV is that in effect the added chamber turbulence is the same as adding advance,the result being lack of overev.
The excess volume trapped in the squish also burns too late in the cycle to create as much power as the energy could do if it was done correctly.

[lodgernz]

With those numbers I get 57% SAR = 13mm wide.

The effect of excessive MSV is that in effect the added chamber turbulence is the same as adding advance,the result being lack of overev.
The excess volume trapped in the squish also burns too late in the cycle to create as much power as the energy could do if it was done correctly.

Interesting. That sounds like there's a formula. True Wobbly? Or are you simming it?

[wobbly]

There are plenty of calculators available on line, but the best rule is to drop the clearance to the point that the piston will just clip the head
when overeved, thus the clearance will be zero in effect.
In that big bore short stroke ski engine only reving to 7500, the safe limit would for sure be closer to 0.8 mm, and then the width can be reduced to around 10mm.
This gives the same MSV, but way less mixture is trapped and not contributing to making power.

[F5 Dave]

So knowing nothing is skis one would expect over-rev would be king. I'd guess single gear and you have several props maybe adjust final drive so it matches what that prop can turn at without cavitating. But with it all being a guess how rough it will be having over-rev would make it more tolerant.

[wax]

There are plenty of calculators available on line, but the best rule is to drop the clearance to the point that the piston will just clip the head
when overeved, thus the clearance will be zero in effect.
In that big bore short stroke ski engine only reving to 7500, the safe limit would for sure be closer to 0.8 mm, and then the width can be reduced to around 10mm.
This gives the same MSV, but way less mixture is trapped and not contributing to making power.

Thanks alot wobbly. i really appreciate it

[JanBros]

Hey guys thanks in advance for your thoughts
Im working on a new head for our vintage skis and I have finished the drawing except for the combustion chamber shape.
From the reading on here from people who are clearly in the know I will be using a toroid shape chamber. My question however is there a magic number when it comes to squish band ratio.
Or what is the effect on the power of having the squish band ratio to much or to little.
Please note im not talking about clearance its the ratio of squish to combustion chamber I am after.
Cheers

no exhangeable combustion chambers? seems like a missed opportuniy.
I made one for a KR1S :

http://kr-1s.co.uk/forum/viewtopic.php?f=2&t=11307

[wax]

no exhangeable combustion chambers? seems like a missed opportuniy.
I made one for a KR1S :

http://kr-1s.co.uk/forum/viewtopic.php?f=2&t=11307

It looks great
Whats the crush you ran on the domes to make them seal properly

[wax]

Ok well this is the combustion chamber finished
Thoughts and constructive criticism is most welcomed

[Frits Overmars]

RSW and RSA ducts were the same. Roof angles did not change since 1995.

What did change, were the leading directional angles of the A-transfers. Aiming them more away from the exhaust port proved beneficial.

... it becomes clear that all the work done by hand in the R&D dept to match the Blowdown flow numbers with the transfers gave a highly synergistic scenario.

That hand work in the Aprilia Racing Dept. included modifying every dimension of every port in 0,25 mm steps over a 10 year period; not something you are likely to equal by yourself during a winter season... It would take the likes of Honda to put in a similar effort. But Honda could not manage.
So in a way you might say that it was Jan Thiel who drove Honda to ban two-strokes from GP racing .

There are plenty of calculators available on line, but the best rule is to drop the clearance to the point that the piston will just clip the head when overeved, thus the clearance will be zero in effect.

Indeed, there are a lot of Mean Squish Velocity calculators available on the internet, but I haven't yet seen a single one that takes the dynamic stretch of a high-revving engine into account, and it makes a lot of difference whether your squish clareance is 0,6 mm or 0,06 mm. As a rule of thumb that stretch will be about 0,1% of the stroke, assuming that your crankshaft is sound. I agree with Wobbly: reduce the squish clearance until you are going to suspect that the piston may be kissing the head. Then there will be zero clearance and zero mixture in the squish band. And you won't get detonation if there is nothing there to detonate .

[wax]

What did change, were the leading directional angles of the A-transfers. Aiming them more away from the exhaust port proved beneficial.


That hand work in the Aprilia Racing Dept. included modifying every dimension of every port in 0,25 mm steps over a 10 year period; not something you are likely to equal by yourself during a winter season... It would take the likes of Honda to put in a similar effort. But Honda could not manage.
So in a way you might say that it was Jan Thiel who drove Honda to ban two-strokes from GP racing .

Indeed, there are a lot of Mean Squish Velocity calculators available on the internet, but I haven't yet seen a single one that takes the dynamic stretch of a high-revving engine into account, and it makes a lot of difference whether your squish clareance is 0,6 mm or 0,06 mm. As a rule of thumb that stretch will be about 0,1% of the stroke, assuming that your crankshaft is sound. I agree with Wobbly: reduce the squish clearance until you are going to suspect that the piston may be kissing the head. Then then will be zero clearance and zero volume in the squish band. And you won't get detonation if there is nothing there to detonate .

Thanks Frits
I have told people about that before. That the squish being at zero at high rpm and they say it causes deto. I always reply " whats going to deto"
I have set the head for .9 of a mm in clearance and my eyes are about to fall out of my head from to many hours drawing hahaha.

[lohring]

As a rule of thumb that stretch will be about 0,1% of the stroke, assuming that your crankshaft is sound. I agree with Wobbly: reduce the squish clearance until you are going to suspect that the piston may be kissing the head. Then there will be zero clearance and zero mixture in the squish band. And you won't get detonation if there is nothing there to detonate .

We did that test on a 26 cc racing engine that developed peak power at around 18,000 to 19,000 rpm. We ran the over rev to 23,000 rpm and found that the piston hit the head button with .05 mm clearance. since the stroke was 28 mm that's about .2% of the stroke.

The engine had 4 main ball bearings and caged needle bearings on both ends of the rod. The big end bearing may have had its clearance increased a little to help it live.

Lohring Miller

[Frits Overmars]

Experience will of course overrule rules-of-thumb, Lohring.
I've witnessed Kreidler engines with reputedly stiff crankshafts, but with angular contact bearings, that needed at least 0,7 mm clearance for their 39,7 mm stroke;
I've also tested Rotax engines with a 54,5 mm stroke and 0,4 mm squish clearance, that merrily revved past 14000 rpm without the piston touching the head.