RE: Back pressure

[LORI ROSE <lrose@marineone.com>]

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-----Original Message-----
From: Matt Zajac [mailto:MTZajac@beloit.com]
Sent: Monday, January 03, 2000 3:25 PM
To: jettaglx@igtc.com
Subject: Re: Back pressure


Hi Bill
Thank you for explanation. The only thing that I don't really understand is
why backpressure is impoortant for low end torque. From what I've heard so
far is that too low backpressure will hurt low end torque. Perhaps you can
help me with this.
Thank you very much.

Sincerely 
Matt Z.

>>> Bill Hussey <vfrboy@home.net> 12/28 4:38 PM >>>
    Backpressure is when your exhaust system doesn't release as much air
as the engine is putting out.  Like dripping oil into a funnel ... the
funnel doesn't flow as much as the larger-necked quart of oil bottle so
the oil builds up in the reservoir.  Same thing as air .. if the exhuast
can can't release enough, the gases build up and cause a pressure.  As
pressure builds, it pushes back up the exhaust system ... thus the term
"back pressure".

    As pressure increases inside the exhuast system, it moves up towards
the engine and stops air from coming in (not all of it of course, but
some) ... this is why a restrictive exhaust system cuts power, because
air flow, and thusly fuel flow once the EFI compensates, is cut.  Most
engines only push enough air to cause back-pressure at higher RPM
ranges, which is why you only get an upper-rpm power increase when
throwing in a new exhaust system.

    Diameter of the exhuast and header tubes is not the only factor
either .. bend quality and smoothness is very very key.  If the inside
surface of the pipes are rough or the bends are angled sharply, the air
flow is distrupted and will slow down which will cause a "traffic jam of
air" if you will, and again build up pressure.  Straight pipes with
gentle bends and polished insides produce the best power, but sound
louder than hell.  I'm going into a huge tangent now .. so I'll stop.
Hope this helped.

Bill
'98 GLX

Matt Zajac wrote:

> Hi
> Can someone please tell me what backpressure is?
> All I know it refers to exhoust.
> Thanks in advance
>
> Matt Z.
>

 
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