RE: Exhaust backpressure

["Brad Sterling" <bgsterling@earthlink.net>]

<snip Dan>
So, my summary is that backpressure is not beneficial in any way, but
the presence of pressure pulses, and how they are tuned and interact
with other cylinders, can make a huge difference in engine performance
or the shape of its power curve.
<end snip>

Okay, I've got to ask.  Up front, I'll say I've been enjoying this thread
and learning a lot.  For that matter, I'm still not sure what side of the
argument I'm on.  One of the things that keeps bugging me is the stock
exhaust on my (ex) Yamaha FZR600.  I know we're supposed to be talking cars
here, but this may be relevant.  The California model of this bike (mine)
came with an EXUP valve.  The EXUP was a servo-controlled valve that was
located in the collector beyond the headers (4-into-1).  In essence, the
valve was a half of a cylinder.  At lower RPMs, the solid part of the half
cylinder would be turned into the path of the exhaust.  I assume this is
limiting flow and creating backpressure.  As RPMs would increase, the servo
motor would turn the half cylinder so the solid part would rotate out of the
way.  This valve was put in place for smog reasons initially, but Yamaha
found that the same motors coupled with this exhaust produced more power in
the low and mid rev range compared to the same engines with a more
traditional 4-into-1.  So, if my assumption is correct that the valve is
creating backpressure, doesn't this add credence to the some backpressure is
good theory?

- Brad
'86.5 Volkswagen Scirocco 16v
'76 International Scout II



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