On 5/29/04 3:19 PM, "Al Hatten" <alhatten@xxxxxxxxxxxxxx> wrote:
> Actually, as far as 2 strokers go, large diameter pipes give more bottom end
> torque and small pipes give more high rpm torque. When I rode a Greeves
> trial s bike it had what was called a "blooy" pipe. It looked like a
> megaphone but gave gobs of power on the bottom end. By contrast, the Yamaha
> Ascot scrambler was almost uncontrollable. It had a small, about 3/4"
> diameter stinger for a pipe. It would double its horsepower over a short rpm
> range, usually when you were sideways in a corner. It was what we called
> "peaky".
>
> Al Hatten
> '93 Virago XV-1100
> VOC #7745
> Napa,CA - Wine Country
>
Al, good observation, you are right in what you saw, but you misunderstood
what you were seeing. Actually the phenomena is not at 2-stroke thing it is
the result of 'tuning' the exhaust and it isn't a function of pipe diameter
but effective length. Here is how it works. First a rule: When a wave meets
a change in the medium it is traveling in some is reflected. Consider the
obvious case of a sound wave traveling from the source and hitting a wall.
The medium is air and then the medium is plaster. When its hits the wall
some is reflected and some goes into the plaster. The simplest case is a
straight pipe with no muffler or anything between the valve and the end of
the pipe. When the exhaust valve opens the cylinder dumps hot air into the
exhaust manifold. The pressure leaving the cylinder is higher than ambient
air pressure (positive). When the pulse reaches the end of a straight pipe
the medium changes from a restricted pipe to the open atmosphere and some of
the energy bounces back down the pipe toward the valve. Behind this positive
pulse is a negative pulse. If you run the engine at the right speed you can
get positive pulses arriving at the open valve and the engine doesn't
scavenge very well and HP drops. At a different RPM the negative pulse will
arrive just as the valve opens and the cylinder will scavenger better than
it would into ambient pressure and HP increases. The shorter the pipe the
higher the RPMs at which 'tuning' occurs. A flare in the pipe flattens or
widens the power band which is very narrow with simple straight pipe.
Surprisingly it takes a lot of work to develop and intake and exhaust system
that doesn't tune and provides at wide power band.
If you want to make a screamer you can take an engine, tune it for a very
narrow power band, and put a 50 speed transmission on the vehicle so you can
drive it at more than one speed. <g>
--
Doug
Tir na nOg
[Tir na nOg is a mythical place where no one ever gets old. It is the land
of the forever young in an Irish legend.]
Copyright ) 2003 Douglas W. St.Clair
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