ESE's works engine tuner

[Frits Overmars]

My 50 has a very sharp crackle that no muffler is able to quieten. Everyone comments on it, although the bike still registers an acceptable dB reading on the track. The engine turns out 11.8HP at the wheel at 13,000RPM and the nozzle has a 14.75mm throat. The tailpipe is 19mm ID. Is it possible that my de Laval nozzle is forcing the exhaust gases to supersonic speed at the outlet? Any other suggestions?

The outflow may become sonic during a small part of each revolution but that should not create what you are describing. A 14,75 mm end cone restriction should be good for over 22 crankshaft-HP, so no problem there. Other noise-contributing factors can be a high exhaust timing, a late ignition timing and a low compression ratio.
And of course an absorbing-type silencer that is not absorbing because its stuffing is either too tightly packed, saturated with oil, or gone with the wind.

[jonny quest]

Is this what you call 'bowling' Jonny?
Attachment 332547

Exactly, Frits. I guess I don't know a more common term, if there is one?

TZ, I haven't modified one yet. But have researched them. I've heard they don't respond well to mods. I probably would have started with 192-194 ex 128 trans total first just to see if it responded.

Wouldn't hurt at this point to "bowl" out ex now. Does A port closest to ex have a good pronounced angle towards C?

[TZ350]

Does A port closest to ex have a good pronounced angle towards C?

Yes......

I have a Honda RS125 NF4 cylinder to look at and the bowel of the RS and NSR look much the same but the NSR has a discontinuity where the power valve retreats into the cylinder and the RS has a much thinner bridge than the NSR.

[husaberg]

Yes......

I have a Honda RS125 NF4 cylinder to look at and the bowel of the RS and NSR look much the same but the NSR has a discontinuity where the power valve retreats into the cylinder and the RS has a much thinner bridge than the NSR.

The RS PV has a longer pivot to the swinging Gate so it stays closer to the port thoughout its arc.
i have some pics of the ports plus some pics of the kit cylinders, Fits has posted pics of the RS kit pipes as well "READ KTM"

[TZ350]

i have some pics of the ports plus some pics of the kit cylinders, Fits has posted pics of the RS kit pipes as well "READ KTM"

I would love to the pictures if you can find them easily.

[husaberg]

I would love to the pictures if you can find them easily.

bottom of page outlines mods to std rs cylinders
https://www.kiwibiker.co.nz/forums/a...id=4841&page=3
kit pipa aka derbi ktm
https://www.kiwibiker.co.nz/forums/a...hmentid=310144
RS port map
http://bonavolta.ch/hobby/en/moto/rs125r.htm


NSR mc28


MC18

http://www.2t-racelab.com/2012/03/mc...rt-layout.html

NSR125 drawings
http://thetrxproject.blogspot.co.nz/...-finished.html

the kit pics will either be on my hard drive or in the rs125 album
they are rough cast there is also ics of the NRS500 cylinders in the NSR500 album

[TZ350]

Thanks for the pictures Husa, interesting that the RS and NSR cylinders have pretty much the same transfer and exhaust timing.



If the modified NSR cylinder (red line) had more over rev it would probably be useable. I am not sure why it stops so abruptly, the fueling is on the money so the 24mm throttle body restricting it maybe.

The crankcase volume is much bigger than would be usual so may be the inlet timing and the 24mm throttle body are restricting refilling the case volume.

But with the way it "Pops" at the end of a dyno pull I still suspect exhaust blowdown issues.



The rpm range from 6-13k is looking clean and the fueling is pretty good too.

Looking forward to getting the throttling issue sorted.

[wax]

I have a question for our two resident experts

What are the pros and cons of cylinder reed versus case reed.
I always figured the case reed has less restriction in front of it and it doesn't have to travel as far before it get to the combustion chamber

[F5 Dave]

And change direction, not aimed so directly into tublent crank action.
Potentially less change of section.

[wax]

And change direction, not aimed so directly into tublent crank action.

so your saying cylinder reed is better ? doesn't it have to go into the crank case before it goes back up the transfers ?
The case reed does not have to do this

[F5 Dave]

Erm, no the case looks neater to flow. And if done right less section change with large reed.

[wax]

Erm, no the case looks neater to flow. And if done right less section change with large reed.

yeah thats what i was thinking, When you said the turbulent crank i thought you were talking about case reed

[Grumph]

And change direction, not aimed so directly into tublent crank action.
Potentially less change of section.

So is there now anything to choose between what I'll call a barrel reed - with Boyesen's, and a case reed with shields over the rotating crank ?

[F5 Dave]

Um. I was kinda indicating case always. Barrel is more of a compromise but still seem to work ok.

Shields etc Wobbly has covered a little. Well out of my pay grade. Rotation of crank can interfere enough so it's a nasty environment.

[dark art]

One big advantage of case reed engines is the better piston support on the inlet side