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Exhaust: Do we really need back pressure?

Good advice here from both Brad and Peter.
Considering how restricted your engine is by the rules it just doesn't need a big pipe.
I'd definitely minimize back pressure, no muffler if the rules will let you, but I'd stick with a 2" system.
And of course as Brad points out, after any change like this you should reset your idle AFR.

Another thought relative to your engine. Do the rules require you to run stock injection?
Cause if not you can definitely pick up some power by going to electronic injection and removing the intake restriction of the CIS metering plate.
I'd suggest Megasquirt, but then you already know that.
OTOH, I have a Haltech E6K that I could sell you cheap if you just can stand to say Megasquirt!!
Dan

From: "Peter" <peter@thescirocco.com>
To: "brad boston" <boston813roc@gmail.com>
Cc: "Scirocco list" <scirocco-l@scirocco.org>
Sent: Thursday, February 15, 2007 9:23 PM
Subject: Re: Exhaust: Do we really need back pressure?


2" is ideal if it's always going to have an 8V, and no turbo.

A 2.5" exhaust will kill your bottom end power, even if it was a 16V, and the 2 L 16V makes more power with a 2 1/4" exhaust.

On Mars I installed a TT tailpipe & 2" Borla muffler, onto the 2" non-resonated Autotech exhaust, and it pulled strong up to 7200RPM, with the 286 cam.

Peter - 302.832.8989
http://thescirocco.com
  ----- Original Message ----- 
  From: brad boston 
  To: Peter 
  Cc: Cory Langford ; Scirocco list 
  Sent: Thursday, February 15, 2007 8:42 PM
  Subject: Re: Exhaust: Do we really need back pressure?


  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 
    >>
    >> _______________________________________________
    >> Scirocco-l mailing list
    >> Scirocco-l@scirocco.org
    >> http://neubayern.net/mailman/listinfo/scirocco-l
    >>
    >>
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