PROPWASH
4
April 2012
Tuned Pipe Design Factors
By Lohring Miller
NAMBA Safety Chairman
The one component responsible for at least half the power of our engines is the
tuned exhaust system. The way the system does this is by creating high and low
pressure pulses that are timed to the engines porting. Dropping the pressure in the
cylinder while the transfer ports are open encourages flow into the cylinder while
increasing the cylinder pressure just before the exhaust port closes supercharges the
cylinder. Computer engine simulation programs simulate these pressure waves and use
this information to estimate a particular engine design’s power.
See http://www.vannik.co.za/EngMod2T.htm for the simulator used in these
examples. Below is a graph showing an ideal result of a tuned pipe’s pressure waves
arriving at the exhaust port near an engine’s power peak.
Note the prolonged pressure drop to 3/4 atmosphere or around 4 psi below normal
from shortly after the transfers open almost until they close. Remember that port flow
is tiny until the port opens a significant amount. That will be before and after the port
times shown on the pressure plots. As the rpm increases this fixed duration wave lasts
for more of the intake cycle. The second, positive pressure wave arrives as the
transfers start to close. It raises the cylinder pressure 2 ½ atmospheres to almost 37 psi
absolute for a 22 psi boost. This peak arrives later as the rpm increases.
So what parts of the pipe create this result? The slowly increasing section of the
pipe, called the diffuser section, creates the low pressure wave. This area of the pipe
starting at the exhaust flange and running to the biggest diameter expands the exhaust
gas to a lower pressure that is reflected back to the exhaust port. Its length and
maximum diameter determines the strength and timing of the low pressure wave.
Ideally, the diffuser section would be shaped like a horn. The term, horn coefficient,
describes the shape of this horn. A high horn coefficient describes a section with a
smaller “waist, while a small horn coefficient describes a fatter diffuser. Pipes use horn
coefficients ranging from 1.2 to around 1.8. A horn coefficient of 1.2 usually gives the
best power, but larger horn coefficients can help, especially in more complex diffusers
where a stronger suction pulse is needed. The G3s pipe (page 5) has a horn coefficient
of 1.2.
The end of the pipe that decreases in diameter from the largest section down to the
stinger is called the baffle cone. This area acts to reflect the high pressure wave back to
the exhaust port. The stinger, the small diameter pipe at the end, sets the average
exhaust pressure in the pipe by creating a restriction to exhaust flow.
To show how these sections work, I simulated a modified Zenoah with several
different pipe designs. While the results won’t match real engines, the trends should be
News From District One
By Dennis Probst Jr.
District One Director
I would like to first start out saying
thank you to everyone in District One that
voted for me. I would also like to thank
everyone who pledged their support to
help me make my job a little easier and
our district stronger. District One is
growing with new members and new
faces which I saw last year by watching
some of our races.
At the time of this being written the
first district meeting with me on the
opposite side of the table is in the books.
The meeting was well attended by our
members, even though it was a bit far
south of North Jersey and New York. I
would mention our members from
Connecticut but they could not make it -
something about traveling past Giants
stadium in New Jersey after their Super
Bowl win. There was one positive about
our meeting being in close proximity to
Philadelphia International Airport – our
past district director Dave Rychalsky was
able to fly in on his Lear Jet to attend the
meeting.
In closing, I would like to wish every
member in NAMBA a safe and fun
season. I hope the rising fuel prices don’t
keep anyone from attending some great
races along with meeting some of the new
members in our great hobby. I look
forward to seeing everyone throughout
the season and at this year’s Nationals.
Good luck to all and God bless.