06/26/2003

Reference:  John D Humpries book "Automotive Supercharging and Turbochargine
Manual - A Technical Guide"  Hayes ISBN 0-85429-880-0

The buchi exhaust is a tuned exhaust - where all the major pulses return to
the exhaust port preferably during exhaust blowdown and thus their effect is
canceled.  This methodology is limited to 3 ports or less at 240 degrees or
more.  

Birrman patented a number of pulse convertor designs in the forties and
fifties.  Petak improved these design's but most texts still refer to them as
either pulse convertor designs or birrman exhausts.

Not all systems could be buchi'd - but the need for scavenging was
overwhelming.  What Birrman did was develop fluidic diodes ( before there was
such a term ).  Picture a wye - with two inlets and one outlet.  Both sides
are equal angles to the outlet.  Now add a center divided that such that the
wye entries run into a plate down the middle that's cut off just above the
exit tube so that each wye sees about 50 percent runner size as it slips into
the outlet.

The high velocity exhaust blowdown gasses shoot past this restriction with
glee, but it seriously slows down the returning pulses.  In fact, much of the
anti-reversionary devices are basic implementations of fluidic devices - back
all the way to kadenacy in the 30's.

What Petak did was take buchi rules runners and stretch them long enough so
that the returning pulses occurred at the port after the exhaust valve closed.
Acoustically tuned again - but this term to insure the pulses did not
interfere.

He then joined each buchi runner with a birrman pulse convertor and into a
stagnation chamber.  The stagnation chamber was designed to convert the pulsed
energy into constant pressure energy and drive the turbo thusly.

For those familiar with Smith - think of his stepped pipe and its rules.
Basically the step ( i.e. collector ) should be at least 2.5 times the area of
the pipe and at least twelve to 15 cylinder volumes in size and no more than
3.5 to 4.0 times the area of the pipe.

Every pulse fires into the collector and gets a long run before its reflected.
With the same energy spread over a larger area, there is less energy reflected
back up the pipe.  The long run ( volume induced ) stretches the short high
pressure pulse into a long low pressure pulse.  That's Smith.

Then, you can stuff the pipe into the collector a significant distance.  The
further the pipe goes into the collector, up to about 1/3 the collectors
length, the lower the returning pressure pulse.  Draw a long cylinder.  Draw
some sign waves.  You will quickly note that resonant waves peak at the ends
or center, but around 1/4 to 1/3 of the way thru is mostly a low pressure
zone.

If you were to run 2 or more pipes firing out of phase into a large collector
and the runner pipes were properly acoustically tuned, you would have a basic
birrman system.

A tri-wye 270 segment, extended long enough, firing into one side of a large
low restriction dual entry truck cat is not that bad as a scavenging exhaust
for a turbo.  Two entries, two pipes - 270 buchi in the pipes, long enough so
that during the power range, they don't interfere and viola.

Could do the same with 360 4 cylinder, or 240 6 cylinder spacing.  Note that
as in the buchi - equal length not needed, as long as the acoustics are happy.

Cats are wunnerfull.  I can get dual entry single exit 3" piped stainless
steel low restriction truck cats for usually under $150 or less.  Works great
as a muffler - without much restriction.  Try pricing a stainless steel, 3
inch muffler lately.

Note for the rpm happy.  Normally as long as the runner is long enough, we are
not concerned about the length - because anything longer than x gives us the
desired scavenge.  But, if the x was tweaked so as to return the positive
pressure pulses in the vicinity of exhaust closing and this was matched
towards mid range detonation peak, you would get a significant power drop at
this point, allowing you to coast thru with higher boost and compression.
Looking at Baka Bombers exhausts, I sometimes wonder if the Japanese Engineers
( I respect much of the Japanese engineering and culture and I need to keep my
vitriol aimed at those who deserve it - the ones with Kanji Stickers on the
window ) may be doing exactly this sort of thing and Tuner Boy "cleans up" the
exhaust run and can't finger out why its so touchy and wants to detonate.
Hope he blows his heads off.

Side note to self:  Same setup NA would be almost zero back pressure exhaust
and scavenging might be doable street.