Building your own racing Expansion Chambers (RD/RZ 350 YPVS)
Why should I even bother making them
The chambers is the single most important thing when it comes to tuning the 2-stroke engine. Without a pair of really good chambers you will never get much power from you RD 350 YPVS.
How do the exhaust work
This is something I won't deepen in too to much. It's very complex, and if
you want to know more I can recommend you to start with reading A. Graham Bell , Two
Stroke Performance tuning, ISBN 0-85429-329-9. It's a good book to start with if you want
to understand how a two-stroke engine (including exhaust) works.
Very simplified explained, a good expansion chamber works like this:
|Piston just passed top of exhaust-port. A strong pressure wave sucks out burnt gases from cylinder.|
|Fresh mixture has now filled the cylinder, and the exhaust sucks out even more fresh mixture. Thanks to this, the 175 cc cylinder gets as much 250 cc fresh mixture sucked out of the crankcase.|
|The pressure wave has now been reflected at the end of the chamber, and perfectly timed pushes all fresh mixture back into the cylinder just as the piston closes the exhaust-port.|
To start with, you need a good design of the chambers you are about to
make. This is, as I see it, the absolutely most important thing to get a good
result. What a good design is, depends on what kind of power you want, what tune
the rest of the engine have, etc.
It's many factors that decides how the chambers should be designed, for example carbs, transfer ports shape and height, exhaustport shape and height and more. If you for example raises the exhaustport, you will move the powerband to higher rpm's, and by lengthening the chambers you will move the powerband to lower rpm's.
So what's the difference between a good and a bad chamber. Simplified the good chamber works as described in the figures above giving lots of power over a wide rpm-range.
The bad designed chamber have a weak pressure wave that don't suck the charge out of the cylinder, and then don't push much fresh charge back into the cylinder, producing less horsepower, wasteing fuel and polluting more.
Most aftermarket pipes are designed to work well on a original/mildly tuned engine. If you want much power from your RD/RZ (>70bhp), you need for example bigger carbs and a porting job. With this change, most standard aftermarket chambers design don't work so well anymore. What you need now is a chamber design that fits your specific tuned engine. Here you can see some examples of possible chamber designs:
As you can see, they differ quite a lot from each other. But on a race tuned RZ 350
they would both work much better than the original RZ pipes. But what you want is a pair
of OPTIMAL designed pipes for your engine. You can see the design of the
SRW race pipes at the SRW team page (under construction), optimally
designed for the SRW race-tune.
One last thing regarding design, computer software can help you with the design, but don't expect superdesign from the software. The software uses mathematical formulas that are approximations of reality, and the software made for home-computers uses even more simplified formulas because good simulation of expansion chambers requires much more computer-power than you home-PC can offer.
The first thing you need to do is buy some steel (I don't recommend using
stainless steel, because it cracks more easily). I think 0,7 mm thick sheet-metal is a
good choice. It is light and strong enough. You can use 1 mm thick metal as well, but than
your chambers will be quit heavy and it's harder to work with thicker metal. About 2 m^2
of metal is required for a pair of chambers.
Now, it's time to calculate how to cut the metal, so you can cut the "flat cones" that will later become your chambers. SRW has made an Excel-document that does all the calculus for you. Mail Rickard [mail-adress on startpage] and he'll send you a copy :-).
When you have done your cutting, it' time to fold the cones. Just do like
This might seem like an easy job, but if you want good result it isn't. First off, you need do be very careful when you do the cutting, getting really nice "flat cones". Next is the folding. Here it's very important that you get the cones circular. If the cones get a little oval, the pressure wave will loose some of it's energy meaning less horsepower. So make sure that all cones are 100% circular - let this job take all the time it requires. The easiest way to do this is welding the "flat cone" and then making it circular. When it's done you got something like this :
More to come when I get some spare time..........
Constructive comments are always welcome.
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