Pipe Physics
I was looking through some old emails and ran across this gem
from Chris BeHanna (behanna@syl.nj.nec.com):
Subject: Pipes and Performance (Was: Re: SuperTrapp Staggered
Duals)
Jane Caban writes:
>Can someone explain what performance means in terms of pipes? I have always
>had the Supertrapps on my bike so I don't know how performance would be
>affected if I had another kind.
Couple of things:
1) Flow: you want to be able to flow as much exhaust gas as is
practical. This means low-restriction.
2) Scavenging: you want the flow to have as high a velocity as
possible so that it properly scavenges the exhaust gases out of
the head when the exhaust valve opens. This places a practical
limit on the inner diameter of the pipe that can be used with a
given engine.
3) Reversion: when the pressure wave from the exhaust pulse
reaches the end of the pipe, it will be reflected. At certain
engine speeds at which the whole system (exhaust AND induction)
are in the proper resonance, this pressure wave (reflected as a
pulse of rarefied air), will reach the head at the precise moment
of overlap in which both the exhaust valve and the intake valve
are open, producing a partial vacuum that helps to draw air into
the cylinder. In general, this will only happen over a narrow rpm
range. For a Harley, there will only be one such range on the
safe side of redline. At all other engine speeds, a compression
pulse blows through to the carburetor and is carbureted once on
the way out, and again on the way in, producing a rich stumble
and a resultant loss of power at that rpm range. In general, the
"sweet spot" where the reversion pulse helps will be
well over 5000rpm, so pure straight pipes do NO GOOD AT ALL on
street machines.
Generally, you will want some mechanical device (usually
baffles of some sort) to break up the reversion pulse so that the
rich stumble goes away. You trade off the high end power to gain
low end and midrange since you do not want multiple carburetion
of the exhaust pulse when it blows through the carb and gets
sucked back in. The way to prevent this is by using baffles of
some kind. This "breaking up of the reversion pulse" is
often confused with "backpressure."
Repeat after me, folks: BACKPRESSURE HAS NOTHING TO DO WITH
ANYTHING. We are dealing with an acoustic phenomenon here, as
well as pure wave mechanics. The purpose of the baffles, besides
keeping the bike quiet, is to break up the reversion pulse so
that it doesn't screw up your carburetion. Period.
In general, flow and scavenging should be matched to the
capabilities of the induction end of the system, as you have wave
mechanical difficulties if one end of the system can flow more
than the other. Generally, you should pick the pipe AFTER you've
picked the carb, cam, and headwork. Just slapping on a pipe
because you like the sound will hurt overall performance rather
than help it.
Ideally, you'd have laminar flow through the system, except
for the turbulence you need in the combustion cycle to get a
complete burn. By matching the capabilities of induction, head,
cam, and exhaust, you will most closely approximate laminar flow
(laminar meaning "ordered". As an example, when people
line up, they can get through a doorway much faster and more
easily than when a crowd tries to push through the door. The
lined up crowd is laminar flow; the pushy crowd is turbulent
flow).
My $.02,
Chris BeHanna DoD# 114 1975 CB360T - Baby Bike
behanna@syl.nj.nec.com 1991 ZX-11 - needs a name
Disclaimer: Now why would NEC
agree with any of this anyway? I was raised by a pack of wild corn dogs.
Some performance data:
From Seth Zirin:
Here is some pipe data I got from Dennis at Bub Enterprises on
6/2/92. I forget the year bike but think it was a 1990 FXR. It
had a stock carb, stock ignition with rev limiter disabled
(Screaming Eagle), and an Andrews EV-3 cam.
They set the bike up on a DynoJet dyno and tested the
following sets of pipes under the exact same conditions. The
pipes tested were:
HD Stock The stock HD exhaust system with the
crossover as it left the factory.
HD Off Road HD "slit fit" off-road mufflers on
the stock header pipes with the
crossover.
Python I The original Python Pipes.
Super Trapp 2->1 (18 disks) A SuperTrapp 2 into 1 system with
18 disks installed. He didn't mention
the type of end cap.
Rich 2 into 1 The Rich Products ThunderHeader that
has been the topic of recent discussion.
Bub Bad Dog Bub Enterprises Bad Dog pipes. This is
their product. I have these on my bike.
Rear Wheel HP at RPM 2500 3000 4000 5000 6000
------------------------------------ HD Stock 17 26 40 42 41 HD
Off Road 19 33 49 57 54 Python I 20 32 46 55 38 Super Trapp
2->1 (18 disks) 19 34 46 52 56 Rich 2 into 1 18 30 50 58 60
Bub Bad Dog 21 35 50 60 71
During their testing, they found that the engine produced 2 HP
more at 6000 RPM with a full tank than with a half tank. They
compared results with S&S and Jerry Branch and they also have
seen this. A fuller tank probably keeps the float bowl at the
full line a larger percentage of the time.
I don't remember how the following noise measurements were
taken. The actual method isn't really important since all
measurements were taken under the exact same conditions and are
meant to be compared, not judged as absolute values.
Noise (dB) at RPM
2500 3000 4000 5000 6000
------------------------------------
HD Stock 80 80 81 82 84
HD Off Road 86 88 92 94 96
Python I 87 89 91 94 96
Super Trapp 2->1
(18 disks) 82 82 87 96 93
Rich 2 into 1 88 90 92 96 99
Bub Bad Dog 83 84 90 93 93
Thanks to Seth Zirin (shz@mare.att.com)
for this info.
Response to the above from Hoyt McKagen as posted on
the yam650@micapeak.com
discussion group.
There's no doubt that the pulses line up the wrong way and
kill portions of the powerband but it's as much or more from
stifling the flow as it is double carbing. There's not much
backflow in modern motors and that's because it's a problem.
Hence it's designed around. The usual cam timingshave small valve
area exposed on overlap. And ex systems can kill backflow; all
muffler types do it by construction, for example. Racing systems
have used reverse meggas for decades for same reason. I've seen
this site before BTW and think he's a bit extreme. The reverse ex
flow problem may get more obvious with longer duration cams. But
double carbing isn't a big issue with four-strokes.
Best wishes,
Hoyt McKagen
Belfab CNC - http://www.freeyellow.com/members/belfab/belfab.html
Best MC Repair - http://www.freeyellow.com/members/batwings/best.html
Camping/Caving - http://www.freeyellow.com/members/batwings/caving.html
When wise man argues with fool, people cannot tell
the difference!
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