Hi all,

I'm new to 2 stroke bikes but recently became hooked after trying an NSR 250 for the first time. Now I'm obsessed about owning one and losing sleep thinking about them (must be love )

Anyways, my question has to do with 2 stroke exhaust systems. I understand that 2 strokes use expansion chambers that are specially designed to use the resonance of exhaust waves to increase performance. I was looking at the different styles of pipes and noticed something on the GP style pipes (where both silencers are on the right side) that I don't understand. In the pictures the convergent, belly and divergent sections (the bulge part) are the same size on both cylinders. The skinny stinger section before the silencers however is significantly longer on the left side pipe. My question is, does the big difference in length between the two pipes cause any kind of change in performance or characteristics between the two cylinders. Do you need to compensate for the difference somehow like different jetting for each cylinder?

Here is a picture from Tyga Performance: link

Thanks in advance for your help!

hello, im not going to answer your question as such, but im fairley new to NSR's and from what i understand "With NSR's its not an issue" sorry that sounds horrid. But apparently the Honda exhaust is so good that even an after market system will get you maybe an extra 2hp at best, i wouldnt worry and just get a set you like the look of... if im wrong please correct me someone! sorry this dosent answer the technical part of your question on shape and length but for power i think im spot on....

stand back and get slapped RC46

Technically, it's the expansion chamber that produces the power characteristics of the engine, and anything happening after that has significantly less influence on the power delivery.

Matt from Tyga can give you the full run down as there's a helluva lot of scientific stuff involved with exhausts, but it's the chamber that you have to get right. That's why the aftermarket pipes need the bodywork spaced out - to achieve optimum power.
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Within reason, if the section of the pipe AFTER the "reverse cone" is the correct dimension and routed properly, it will not have too adverse an effect on power.

TYGA's first GP pipe wasn't quite right. If Matt remembers, I spotted a little discolouring of one of the pipes that "appeared" to me to be a bit of a restriction. I'm not sure if the pipe diameter was increased or the radius of the bend was increased, but whatever they did it solved the gas-flow problem! It was a tiny change that made the difference though!
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Andy.
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rc46 wrote:"With NSR's its not an issue" sorry that sounds horrid. But apparently the Honda exhaust is so good that even an after market system will get you maybe an extra 2hp at best,

Yep, the 21 pipes are superb and i believe the '18's are good to, I think Bill's seen something like 66bhp with them on his '21, the '28 pipes however are a proper restriction
Tyga got 12bhp with just a pipe change!
http://www.tyga-performance.com/images/MC28-Std-v-TYGA-Pipes-RO.gif
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Buy this book, it's great education. Has a whole chapter on expansion chambers.

http://www.amazon.com/exec/obidos/tg/detail/-/1859606199/qid=1124381875/sr=1-1/ref=sr_1_1/103-3899602-3791000?v=glance&s=books
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Thanks for all the great info guys. You guys make NSR World such an awesome tome of information.

Jeff SZ, I just ordered that book a couple days ago so I'm looking forward to reading it!

I am no expert either but as I understand it this would be a laymans explanation:

The first section is usually quite short and there is a point where it works best and making it longer/shorter can tune the revs and I believe as little as 1mm can alter where peak power is by around 100rpm. Maybe needed to alter this if the bike is tuned? High tune bikes (nsr/rgv etc) need it to taper right from the engine.

Most important part of pipe is the rear taper to "bounce" pressure back but front taper also has to be right to allow pressure wave to travel back properly. Not to sharp not to shallow.

The volume of the chamber effects torque, larger = torque lower down.

I think the length of the pipe after the chamber is not so important. Sure it does something, can't remember.

For these reasons the RGV pipes which go 2 up one side are NOT ideal. They sacrifice a little performance for cosmetics (oh well, I am a fashion victim then becasue I like them!). The best styles for performance are one each side where by the tuned lengths are identicle.


The steeper the rear taper I think means a narrower power-band but higher power? To a point...........like everything else it's all a compromise on a mass produced bike.

In the RGV world there are lots of pipes to choose and pipes from different manufactures give different characteristics, ie Arrow "seems" to suit a road stock bike giving good mid-range and peak but dies quickly after peak. A Lomas pipe seems to give less of a mid boost and peak by a hp or 2 but allows the engine to rev on much further which would seem to suit for further tuning work. Given this I think you need to choose pipe carefully.

If you are a racer the dream situation would be to have a pipe made to tune the enigne to give power exactly where and when you wanted it to suit your particular tune. A pipe like this would be then useless on say a stock bike. Because one pipe is good on one bike doesn't mean it will work on another if the tune is different.

Forinstance if you get a 300 conversion done on an RGV there is no pipe specifically made for the 300cc engine and it's characteristics. I am sure the 250 designed pipe must be a restriction to getting the best out of it. It's where I was going to look next before I sold the 300.

Of course having considered all this it needs to FIT the bike and look pretty good without restricting the rider in terms of ground clearance or burning his legs

Want to be an exhaust designer? Sounds like a real headache!

Anyway, just as I understand it, I am not saying I am 100% correct so if any experts are reading please educate! Exhausts = Magic
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http://www.rgv250.co.uk
http://www.thetuningworks.com

Pipe dreams eh?

Need to be good with a calculator, comfortable with numbers and have a good supply of headache tablets.

When you understand the basics of how a pipe works, fork out for TSR Software's NEWPIPE program. Saves a bunch of time and the tablets last longer.

The tail pipe has to be small enough diameter and long enough to generate the right pressure and heat in the pipe to get good torque. Too small or long and you'll over heat. Too big or short and you'll lose power.

The header controls bottom to mid torque.

The final cone controls the power spread and over rev mostly after peak torque.

The length of the pipe from piston face to the start of the tail pipe controls peak power rpm.

And so it goes on.

Read up on it. It's fascinating......or very boring, depending on your perversion.

Oh yeah.

250 pipes on the 300 will work, but they won't be the ultimate.

On our 250/300 conversion for example the port timing of the 300 is very similar to the 250, so as such the power curve using the same pipe is very similar in shape.