WTF is this?

[Tim754]

Years of formulating and testing of modified SL125 pipes has led to this being the best all round system.
 or perhaps it was just shite hanging around the shed I cut and shut for looks.....

[Tim754]

Check out these old SL125  "racing" bits from some bloke called Pops Yoshimura or something like that. What would he/they know, No fancy pants foofer valves or malignant  fungus growths on their pipes... .


 Look at the prices......

1974/80 advertisement....

[Daryl Jones]

I'm definitely in the market for at least 3 Yoshimura racing crankshafts at $27.50 ea.
The other bits look reasonably priced as well, (Japanese recession/stagflation?)

I 'lost' the ball cage from one of my (44 y.o.) main bearings at the Masters Games.

Here is my MX/Flat Track pipe. 
It has a small "finger proof" baffle as an "effective silencer."
It has also Passed an MQ sound test at Nudgee flat track.



The expanded chamber pipe was built for the CMX Nationals at Barrabool in 1996, Cherabah in 1994, and so preceded FMF's patents by a fair bit.

Cheers, DJ

[chrisdespo]

looking at taht KTM thingy makes me think we need to keep the same volume of pipe so if we make this thing we wont have a pipe sticking to far out past the engine where it runs past the cylinder and everyone will think its a real trick bit but more just to stop the rider getting burnt anymore than buying a KTM. well really thats my thinking.and no diagrams explaining the suck squeeze bang and blow.. Lol

[Daryl Jones]

......and no diagrams explaining the suck squeeze bang and blow.. Lol

Chris, you could be right about the flattened chamber for leg clearance, with the side benefit of a "trick" look.

Don't discount, and Lol, the Ricardo diagram, though.

Look closely and you will (could/should) see that it is a 2-Stroke cycle (with valves).

The whole process is completed in 1 revolution, not 2.

Cheers, Daryl.

[davidmc]

Most of the power bombs and cans, create a longer header volume at low RPM giving better performance down low while retaining the benefit of a short header at higher RPM.

This KTM example is looking more like a 2 stroke chamber; which is a little more complex and more effective.

 The same theory applies to 2 strokes and 4 strokes,
 all about timming pressure pulses at the speed of sound to assist extraction of spent gasses and cylinder filling of fresh charge,
 only happens half as often with a 4 stroke for the same RPM, so gains will be half of a 2 strokes gain of the same capacity.

In theory there should be no reason a 2 stroke style chamber wont work on a 4 stroke if you dont mind the volume of the chamber.

[Rusty]

Fark, it is a 4-choke pipe ..... any old crap pipe laying around that is the right diameter will do the trick.

[GMC]

I had a theory many years back that some four strokes may benefit from an expansion chamber.
Never got a chance to try anything so it was just one of many untried theories that bounce around inside my head.
Maybe KTM came up with the same idea?

You should start wearing an alfoil hat to thwart any future industrial espionage. 

I have such a hat but the voices inside my head told me not to put the hat on
Do you think it's too late?

Performance 4-strokes have a considerable overlap period where both the inlet & exhaust are open at the same time.
Just like a 2-stroke.

As far as full expansion chambers for 4-strokes goes, I spent a fair bit of time with Lotus's Sim program, designing pipes for the mighty SL100/125.

In my attempts, the full expansion chamber did not ever produce a higher output than a tuned length pipe.

What's your definition of a "full chamber"?
As the valve overlap is only short compared to the open ports of a 2 stroke I had imagined a chamber being short and small for a 4 stroke not unlike the KTM version.

Also on the KTM above the pipe does appear to be an expansion chamber, as in an open chamber like a 2 stroke.
The FMF power bombs have a tube running all the way through the 'bulb' with just a 10mm hole into the cavity part.
So the big question is what's inside this KTM pipe?

[Lozza]

That hole would 'disappear' over a certain rpm, there would be the similar in the KTM bulge, the 4T engine has no benefit from an expansion chamber when a simple step will do the same thing

[Daryl Jones]

"What's your definition of a "full chamber"?
As the valve overlap is only short compared to the open ports of a 2 stroke I had imagined a chamber being short and small for a 4 stroke not unlike the KTM version".

My definition of a "full chamber" was a Two-stroke type chamber with header, diffuser, centre section, baffle cone, and restricted exit (stinger) to maintain a working pressure in the pipe.

Lengths varied between about 300mm to 1.2m. & max diam 100mm (the practical limits for fitting on a SL100/125 VMX bike).

Yes, the transfer (overlap) is shorter but the time span from exhaust valve open to exhaust valve closed is a bit longer
Exhaust pulse strength & duration is different, as the exhaust valve 'window' is smaller and more restricted and after initial "blow-down" the
exhaust flow is provided by the rising piston.

What I was looking for, was a long tapered diffuser to create & maintain a long duration negative pulse from ExVO to IVO.
This negative pulse was also intended to draw fresh charge into the cylinder and into the exhaust port at IVO.
As with a Two-stroke, it was then the job of the baffle cone to generate a pressure pulse to push the escaped charge back into the cylinder before EXVC.
Not too dissimilar to a reverse cone megaphone, but with pressure to create more sound wave strength.
(In the vacuum of space, no-one can hear you scream).

Where things get more complicated is during the remainder of the induction & compression strokes  (c. 360deg.).
The exhaust pipe is only communicating with the back of the closed exhaust valve.
Waves are bouncing and reflecting and inverting, up and down the pipe.
Pressure is bleeding out, with no new pulse to top it up.
AND, If we produce a positive pressure wave at next EXVO, we have taken a big step backwards.

I only tried what I could at the time, within the limits of my understanding and the capabilities of a "free" down-loaded Sim program.

THE ANSWER is still Out There.  Put on your tin-foil hats, and get into it.

cheers, DJ

[Daryl Jones]

The FMF power bombs have a tube running all the way through the 'bulb' with just a 10mm hole into the cavity part.

That hole would 'disappear' over a certain rpm, there would be the similar in the KTM bulge

My old SR500 had a chamber, off the side of the pipe, under the engine, the communication hole was also about 10mm.
Back in those days, it was generally considered that this was to smooth-out the exhaust note, by reducing the strength of the pulses.
Of course, we cut them off, to make the bikes FASTER (noisier).

Cheers, DJ.

[pokey]

Sounds like one of these




 or is it more like one of these?

[Daryl Jones]

"Now hold still. This is gonna hurt"

Oww Ouch!   Nice one Pokey