Re: Exhaust backpressure

[John Erickson <amigos@elp.rr.com>]

"Brad Sterling"  wrote:

> 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

Don't confuse "creating backpressure" with "increasing velocity."  I believe
the optimum velocity of exhaust gases in most exhaust systems is around 300
ft/sec.  With big pipes and low RPMs, velocity tends to drop and there goes
your torque.  By reducing the diameter of the tube with a valve, the gases
are forced to travel at a faster speed and you regain the benefits of
scavenging and effect the acoustic wave as well.

This topic will make engineers out of us yet.

-- John
'87 16v



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