I've heard the same thing. It looks right but a horn alone won't optimize
the intake. I used to have a really good computer program for computing
optimal intake tract length, I need to find it. It was hugely useful for
building motorcycle engines. There are a host of fundamental issues that
can be optimized, and this program takes all the critical parameters into
account. The way sound works in a column (and that's basically what we're
dealing with) is that a open end reflects back a pressure wave. When you
have a trumpet you stretch out the set of frequencies that reflect and
extend the tuned length of the tract. Any sudden change reflects much more
strongly than the rest of the trumpet, which makes for harsh and
disruptive changes in the pressure throughout the intake. You also get
turbulence from a sharp edge that disrupts flow in the mouth of the stack.
The position of the carb in the tract is important, because if it gets
stuck in the wrong place the resonant high or low pressure points can be
located right over the jet. That makes sudden and very RPM-specific
changes in the mixture. The overlap is also a critical factor since that's
the source of the blast of air coming back out of the carb at low RPM. To
optimize, you have to take into account the desired RPM range, the overall
length of the tract, from intake valve seat to the end of the stack, the
location of the carb, the beginning and ending points of the trumpet
section, the choke size of the carb, and the diameter of the stack. The
calculation is one of the few I've found that connects precisely from
computer to reality. When I was building a motorcycle drag motor back in
the early seventies, the mule engine lived on the dyno. The software was
written in FORTRAN and lived in three boxes of punch cards in my locker at
Long Beach Community College. I developed the software from formulas
cribbed from Ricardo's _High Speed Internal Combustion Engines_ and a few
other books. We'd make a change to the engine, I hop on Little Jeeter, my
incredibly crappy but reliable Yamaha Twin Jet 100 parts runner bike, and
go run the program with new data. Then we'd do what the program told us to
do. Most of the tuning consisted of moving the carb back and forth to
clean up the jetting. We almost never changed jets. At the end we were
getting 70+ horsepower out of a Yamaha RD350 motor (which was good for
about five passes before it needed a new set of crank and big end
bearings). Of course we were dealing with intake port timing and a reed
valve, and pumping the mix into the crankcase instead of cam timing and a
compressed reversion charge, but it works the same way.
I rewrote the program in BASIC, then Turbo Pascal, then C. It's still
around somewhere.
-----Original Message-----
From: Brad Eells [mailto:bradlnss@lightspeed.net]
Sent: Thursday, October 16, 2003 5:41 PM
To: Friends Of Triumph
Subject: Velocity Stacks (air horns)...
I recall a post a while back suggesting the below link was the ultimate
velocity stack/air horn to use...
http://www.twminduction.com/AirHorn/AirHorn-FR.html
Anyone had experience with them??
Brad
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