ESE's works engine tuner

[emess]

I won some power by making the auxiliary ports bigger, but they were already too high I thought.
So I made sure the APC cylinders had lower auxiliary ports

Jan, why did you think the auxiliary ports were too high?

If you could not have a power valve would you make the aux ports open at the same time as the main exhaust?

Mick

[jamathi]

Jan, why did you think the auxiliary ports were too high?

If you could not have a power valve would you make the aux ports open at the same time as the main exhaust?

Mick

I already knew that too high auxiliary ports cost power.
So I would never make them as high as the main port!
Unless you make the main port lower, which I did for some time in previous engines (Jamathi, Bultaco).
But different heights are surely better!
And I don't consider a power valve that important....

[emess]

The standard KTM 50SX exhaust duct diameter is way too big

Frits, I'm curious, how do you work out the optimum duct diameter for an engine.

Mick

[emess]

I already knew that too high auxiliary ports cost power.
So I would never make them as high as the main port!
Unless you make the main port lower, which I did for some time in previous engines (Jamathi, Bultaco).
But different heights are surely better!
And I don't consider a power valve that important....

Thanks Jan

without a power valve what would you work on to improve the lower end of the powerband

Mick

[jamathi]

Thanks Jan

without a power valve what would you work on to improve the lower end of the powerband

Mick

The lower end of the power band is not the most important.
It is important that the engine revs freely, does not stop suddenly at high revs.
For acceleration the power after the gear change is most important.
This is what the riders feel most.
A lot of power at low revs can make that you can not open the throttle....
And usually does not improve lap times.

[shnaggs]

Frits, I'm curious, how do you work out the optimum duct diameter for an engine.

Mick

^^^^^ This

I tried making a half ass insert for a ktm 125. It was just a ring that was sandwiched by the pipe and it had a tongue that went up floor. Lost 5hp I can only guess that it closed off the duct too much, and/or it exaggerated the mid point area to outlet area ratio....which ever that should be???

[Frits Overmars]

Frits, I'm curious, how do you work out the optimum duct diameter for an engine.

Have you got a spare minute Mick?
In essence it's about Helmholtz resonance. You can find the basics via Wikipedia but don't get your hopes up; establishing the optimum diameters of a pipe is far more complicated than finding the correct lengths. That is why I developed a simple pipe concept that should get you in the ballpark without too much physics and math.

[emess]

I developed a simple pipe concept that should get you in the ball park without too much physics and math.

Thanks Frits, I had seen your pipe concept diagram and succesfully put it into practice with some help from Engmod but can't see how it works with Wobbly's 75% area rule. Are the two ideas compatible?

I love to understand the derivation of formulae and understand that is asking way too much but is it possible to say what Dx in the older version or X in the later version represents?

Mick

[Frits Overmars]

Thanks Frits, I had seen your pipe concept diagram and succesfully put it into practice with some help from Engmod but can't see how it works with Wobbly's 75% area rule. Are the two ideas compatible?

More or less. Wobbly starts with the total exhaust window area; I start with the exhaust blowdown area.

I love to understand the derivation of formulae and understand that is asking way too much but is it possible to say what Dx in the older version or X in the later version represents?

X is an empirical value to which the diameters are related. X represents the cumulative effect of quite a number of variables that do play a role in a running engine,
but whose values may be unknown to the beginning two-stroke tuner. I tried to concoct a simple exhaust concept, you know.

[emess]

Many thanks Frits, Jan and Wobbly for your answers and massive contributions to this site and for raising my understanding of 2-strokes and to TeeZee for this great site.

Mick

[diesel62]

Many thanks Frits, Jan and Wobbly for your answers and massive contributions to this site and for raising my understanding of 2-strokes and to TeeZee for this great site.

Mick

Second that ☺️

Sent from my SM-G900F using Tapatalk

[wobbly]

Frits is dead right,the duct exit should ideally be tied to the blowdown area, but I have carried on using the total effective Ex area ( that is the
chordal area X the down angle cosine ) as when you use the 75% reduction at the exit,EngMod invariably throws up a corresponding velocity
of 0.8 Mach at the spigot mount face..This has proven to be the magic number to shoot for in a huge range of builds.

And Jan is dead right as usual about the relative heights of the main and Aux ports.
A split of 2* to 4* opening angle is needed or you loose power.
This I believe is due to smearing of the exit wave amplitude - the main port opening first allows the biggest pressure delta to create a strong initial wave front
exiting into the duct.
Once this has occurred, the lower Aux ports simply bleed down the remaining blowdown pressure.
If the Aux are the same height,the much longer path length down the Aux duct causes the initial exiting flow to be smeared out in time, as well as a much lower
peak amplitude - result,less power.

[dutchpower]

So lots of blowdown area Frits

[Frits Overmars]

So lots of blowdown area Frits

Jan Thiel once suggested that you cannot have too much blowdown time.area. But you can have too much exhaust timing, you can have too much vicinity between the exhaust port and the A-transfers, you can have shortcircuiting between the auxiliary exhaust ports and the transfers via the gudgeon pin bores in the piston, and finally you can have port shapes that are too harsh on the piston ring.

[dutchpower]

Thanks Frits