RE: too much free-flow bad?!?

["Aaron Ness" <aaron@makuta.com>]

<x-charset iso-8859-1>I've also heard that if you don't have enough backpressure you'll burn your
valves.  I don't doubt that it could be true, but I haven't heard of any
technical reasoning behind it.

I do know, however, that by properly designing your exhaust system (and
intake, for that matter), you can increase your peak torque  There are
reasonably simple formulae to calculate what the effective length needs to
be for optimum performance.  When running the numbers, you take into account
the fact that a bend in the system has the same effect as a specific length
of straight pipe, hence the term "effective length" rather than "length."

If you pay close enough attention to the newer engines on the market, most
carmakers are going with 2- and 3-stage intake manifolds.  Since the length
is only optimized for a specific engine speed, you can actually give
yourself 2 or 3 peaks in your torque curve by using flaps to open different
sections of intake runners, etc.  It gets pretty complicated, but that's
what you have to do if you truly want to optimize performance as much as
possible.

I don't know of anyone using 2-stage exhausts to accomplish the same thing,
but it would be interesting.

Like I said, I haven't seen any kind of proof that you will burn valves with
too little backpressure, but based on this information, a person may not be
getting as nice a torque curve as they could if they had more restriction.

Aaron
'84 Scirocco - Woo-hoo!
'70 Bug - hibernating
'87 Nissan truck - Soon to be for sale

> -----Original Message-----
> From: owner-scirocco-l@scirocco.org
> [mailto:owner-scirocco-l@scirocco.org]On Behalf Of Cory Fonger
> Sent: Friday, November 19, 1999 2:28 PM
> To: scirocco list
> Subject: to much free-flow bad?!?
>
>
> i was talking to a friend, and he always complains about how loud my
> car is and accuses me of poking holes in my muffler. My system is
> airtight, but by no means sound-tight. Although i do have a
> cat and the
> restrictive 4-1 type manifold, its 2" from after the cat and only uses
> 2 glass pack resonators for sound deadening. it was custom built by a
> local shop and is mandrel bent and welded on there. driving it up the
> street at full throttle reveals that its a pretty free
> flowing exhaust.
>
> Back to my friend. When i keep reminding him that i have no mufflers,
> he tells me that im gonna burn out my valves. something about needing
> backpressure and the heat builds up causing valve failure. ive been
> thinking about it and wondering how could leaving hot gas around make
> things any cooler. then it hurt and i decided to ask the list. anyone
> got any answers?
>
> TIA,
> ~cory
>
> BTW, some kid from school who is into VWs asked to buy my car today so
> he could make it into a rallye car. i might be able to get a better
> rocco, hehe!! :) if youve been to my website then youll have seen my
> car, and thats just the good side.
>
HTTP://elf_9.tripod.com

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