Waver length tuning is simple on a 4 stroke, but in a 2 stroke back pressure comes into play. That is why the large diameter pipe works so well in a 2 stroke for low rpm torque and has the opposite effect in a 4 stroke. The loss of back pressure at low rpm allows the cylinder on a 2 stroke to get rid of spent gas faster, to the point that a lot of unburnt gas goes out the pipe. The object is not fuel efficiency here, only low end power. On a 4 stroke the loss of back pressure gives more high rpm torque, but wave length comes into play because of complex valve timing issues. That is when you see the "bowl of spaghetti" exhaust systems on formula racers. I am being simplistic here for the sake of discussion. I spent a whole semester at college in a class studying only exhaust and intake systems.
Al Hatten
'93 Virago XV-1100
VOC #7745
Napa,CA - Wine Country
On Sun, 30 May 2004 00:12:16 -0400, Douglas W. St.Clair wrote:
> On 5/29/04 3:19 PM, "Al Hatten" <alhatten@xxxxxxxxxxxxxx>
> wrote:
>
>
> 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>
___________________________________________________________________________
(un)subscribe info, archive: http://archives.itg.uiuc.edu/virago/
