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RE: Backfire

Just to add to the debate - I've worked on Subaru engines for a few years, 
and they use a relatively simple reed valve to pull fresh air into the 
emission stream -  no pump involved.   Air is drawn from the air cleaner 
assembly and distributed to each pair of horizontally opposed cylinder 
heads near the exhaust ports.  In addition, the exhaust system on these 
cars does not appear to be anything special.  A single exhaust pipe 
services each pair of cylinders, and then merges before the catalytic 

Erik Pullman
73 Squareback
71 Campmobile
(and 87 Subaru)

-----Original Message-----
From:	Jim Adney [SMTP:jadney@vwtype3.org]
Sent:	Tuesday, July 21, 1998 6:43 PM
To:	type3@vwtype3.org
Subject:	Re: Backfire

> >JimA=> the exhaust is under positive pressure and will almost never
> >draw  air INTO itself.
> I disagree.  the exhaust is not always under positive pressure but
> under a series of pressure pulses.  The are some basic air injection
> systems for emissions that are nothing but a simple valve that
> allows fresh air to enter the exhaust during the low pressure period
> between the pulses.

The systems I'm familiar with all have an air pump.  Who gets by
without one?

> (And to push this thread into  a different
> direction) This is also how scavenging works on headers...  The
> pulse from one cylinder will help draw the exhaust from another
> cylinder at the collector improving the overall flow...

I don't think this is quite correct.  I believe equal length headers
allow the pulses from the different cylinders to enter the muffler
with even timing, which allows each cylinder to operate under the
same conditions, thus giving more even running.  There is a resonance
effect with a tuned exhaust, but I really don't know if the pressure
ever goes below atmospheric.  All it has to do to have a beneficial
effect is to go below average (at the right time, of course.)

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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