Exhaust: Do we really need back pressure?

[boston813roc at gmail.com (brad boston)]

I agree, the back pressure from a straight though muffler is minimal.  I was
still at work at that point and in no frame of mind to be quantitative, only
qualitative :)  If I remember correctly this is for your '87 1.7 8v race
car? if thats correct you may want to go with something a bit smaller than
2.5 inches.  With something under 2L making ~100 hp you should be looking in
the 1.75 inch range if you want to run it all the way to the back of the
car.  This will keep the gases hot (wrapping the pipe also helps) and will
keep the flow as constant as possible.  Without getting into tuning the
length of the pipe (Math) and diameter (More Math). i would say that
anything between 1.5 and 2 inches is fine and you will see more gains from
tuning the header.

Brad

On 2/15/07, Peter <peter@thescirocco.com> wrote:
>
> ...That back-pressure is probably close to the resistance the exhaust flow
> deals with going through the bends in the exhaust system.
>
> Peter - 302.832.8989
> http://thescirocco.com
> ----- Original Message -----
> From: "brad boston" <boston813roc@gmail.com>
> To: "Cory Langford" <Cory_Langford@bcit.ca>
> Cc: "Scirocco list" <scirocco-l@scirocco.org>
> Sent: Thursday, February 15, 2007 6:36 PM
> Subject: Re: Exhaust: Do we really need back pressure?
>
>
> > If you are running a 2.5 inch pipe from the headers back with no turbo
> you
> > will need to richen up the air fuel mixture a bit from stock.
> >
> > Also, a straight through muffler/resonator (if correctly tuned) will
> cause
> > back pressure due to the resonating pressure waves bouncing around
> inside
> > the resonator
> >
> > Brad
> >
> > On 2/15/07, Cory Langford <Cory_Langford@bcit.ca> wrote:
> >>
> >>
> >>
> >> Just finalizing the exhaust set-up and I was wondering what the opinion
> >> was
> >> on running a completely free flow exhaust.
> >>
> >> One of the tracks has a noise restriction, and one does not.  So I
> >> figured
> >> the simplest design was from the header to 2.5 inch pipe straight back
> >> (no
> >> cat, res. etc).  I figured I would have a removable muffler that can
> >> slide
> >> in when needed, and a straight pipe when not.
> >>
> >> But I have seen some talk on the vw engines needing some back pressure
> to
> >> keep the exhaust gasses hot enough to travel quickly through to the
> end.
> >> So I was wondering if a simple straight pipe would have any down sides
> to
> >> it (beside noise)?
> >>
> >> Cory
> >>
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