expansion chambers GT 550

[0ldbiker83]

I am building a GT550 cafe racer looking for a set expansion chambers used or new any suggestions or which brand is better

[tz375]

I don't believe that anyone has dyno tested any of them, let alone different ones to offer objective comments, so the answers will be largely subjective. In fact pipe makes rarely offer dyno charts or HP claims and in this litigious day and age, I doubt that they would even make claims for improved performance.

So, what is available?

New, there are JEMCo out of TX and Higgspeed from the UK. I didn't check to see if Jim Lomas makes any for that bike.

Used, we had Bassani, J&R, and probably others.

As to what works, Allspeeds used to make more or less stock power. J&R typically peaked hard, early and signed off equally fast. JEMCo seem to be well liked and look OK and seem to work. Higgspeed pipes make a sound like corn popping at idle which some like and others of us hate.

[jabcb]

Best depends on the relative importance of performance vs. "coolness".

An option is the OMAR/Jemco 3-into-1 exhaust. http://omarsdtr.com/Ex550suz.html

Examples:

[JFISHSOLEVIBE]

Correct me if I'm wrong but aren't 3-1's on these triple 2 strokes counteractive? Seeing as each cylinder and crank are individually isolated, I don't see how a single collector would benefit the performance?

[tz375]

Good question. Old style collectors were designed to create a single step change in pipe diameter which would generate a return wave to the cylinder on a 4 stroke. The same would be true on a two stroke with a huge sudden change in diameter. With a modern merge collector, the diameter change is more gradual and properly sized, it can work as the front end of an expansion chamber.

So that's not all bad on a two stroke triple.

The problem is really the returning waves from the baffle cone which will tend to arrive at the wrong time for other cylinders and will tend to choke the pipe. BUT, two waves of different amplitude, of differing sign and traveling in different directions meet, they temporarily cancel out and then pass through each other as if they were the only wave in action.

So a positive wave traveling down a pipe meets a negative wave traveling back up the pipe. As they pass each other, teh net pressure at that instant is the net of the + and - waves, and they continue in their original; direction. So they basically pass through each other.

If both waves travel in the same direction, at the same frequency and and amplitude but one is + and one is -, in that case they more or less cancel each other out.

But we don't have a static wave generator and the wave shapes are irregular and with wildly varying amplitude, so the interaction is likely to be more complex.

The real issue then is what shape will a pipe be to handle the exhaust gas volume from three cylinders and yet still be at the correct angles to generate pulses (waves) of the appropriate amplitude and duration to effectively scavenge and fill the cylinder. It's that flow versus wave dynamics conflict that reduces power in a well designed 3 into one pipe and why so many bad pipe designs don't work at all well.

[Craig380]

I love that quote from Walter Kaaden, the pioneer of expansion chambers on 2-strokes, about pipe design: "You will know when you have arrived at the correct design, because it will then be impossible to fit it to the machine without it burning the rider, dragging on the ground, or forcing the relocation of at least one major component."

[jabcb]

Am no expert, but I think the 3-into-1 gives up some top end performance and adds to the "coolness".
Low & mid rpm performance is fine but those pressure waves you guys talk about depress top end performance.

I wouldn't put 3-into-1s on a bike like the GT380. It doesn't have much low end power & you need that higher rpm performance.

I expect the Omar 3-into-1s to be fine on a GT750 as long as you have some respect for the speed limits in the US. Have them on order so I will find out this summer.

Don't know what it does to real-world performance of a GT550. Maybe some forum member knows.

------------------

Interesting bit of history on Walter Kaaden.
"Kaaden used an oscilloscope to examine the resonance in the exhaust system and devised profiles to maximise the engine's efficiency. The net result of this development programme was that by 1954, Kaaden's two stroke 125cc racing engine was producing 13 bhp, more than 100 bhp/litre. This engine was further developed to produce 25 bhp at 10,800rev/min."

http://en.wikipedia.org/wiki/Walter_Kaaden

[Vintageman]

Here is my 2 cents. about 3 into 1.

Look at snowmobile designs from 90s into early 00s (triple no more in sleds). single pipe version. Also Jet Skis. The header pipe is short, but well designed, and a single regular looking fat old chamber section follows closely. So this I think fixes timing delays as best as can be expected for single chamber serving 3 120 degree phase exhaust pulses


GT380 (only) ZenShin looks like I like above and cones/belly awesome IMO

http://www.motoparts-online.com/apps/we ... ow/2951142" onclick="window.open(this.href);return false;

See similar ones in Japan on smaller kawa triples.

Heh!. How come no one from Japan on this board. No one at all (google translate is all we need, sort a)

[tz375]

Waves coming back at the wrong time and bumping into other waves isn't the issue, but waves traveling up the "wrong" pipe are.

For example, number 1 opens its exhaust port and a wave starts to travel down the pipe. As that wave passes down the first cones, it reflects a partial low pressure wave back up the pipe and if it's timed just right, it helps to "suck" gas out of the cylinder. The rest of teh first wave continues down the pipe and when it reaches the baffle cone, it sends a stuffing wave back up the pipe. That stuffing wave is supposed to arrive just before the port closes and stuffs some of the clean mixture back in.

The problem arrives when the pipe is resonating perfectly, because a triple creates overlap between cylinders and the exhaust stuffing wave arrives when one of the other cylinders is sending its gas down the pipe. That means that some of teh hot gas gets pushed back into the chamber instead of allowing it to escape.

The longer the exhaust port duration, the greater is the overlap between cylinders, so a 3 into 1 tends to work better with a mild motor. Below peak revs, it's not an issue because the pipe is out of resonance and is working semi efficiently. As it gets closer to peak, the pulses are stronger and interfere more with each other, so a 3 into 1 tends to stifle top end.

One solution might be to design the pipe to resonate at say 1000 rpm higher than the real peak, so that the pipe is never fully in tune and also never fully interfering. And of course those waves are not simple sine waves. They are far more complex than that and there is more than one wave in a pipe at any time and more than one rotation of gas in the pipe at any time.

Most sled pipes use short headers and some split the center pipe to get a 3-2-1 effect - presumably to make the collector less of an abrupt change in cross section. And some 2 into 1 sled pipes are simply enormous.

[nateryan]

yeah but for that kinda money you can buy 2 sets of jemco pipes.

ive done a lot of reading on the subject lately and im pretty much coming up with jemco and highspeed being so similar that the only differences youll find is in price. jemco's are still cheaper by $200 and less shipping if you are in the us. but what do i know, im probly just bias since i grew up in houston.