Re: Exhaust Mod

[Steve Moresi <smoresi@xxxxxxx>]

Ok, after reading these posts referring to the "restrictor" and "resonater" 
in the ALFA exhaust I would like to throw in my 34 Lira.

One of the things we want to do in an exhaust system, is tune the length of 
the pipes.  As each cylinder releases hot gas into the exhaust manifold 
(header, whatever)  it creates a high pressure wave that travels down the 
pipe.  As the exhaust valve closes and the pressure wave travels down the 
tube, it creates a low pressure zone behind it.  This low pressure zone is 
important because if we can time the opening of the exhaust valve so this 
low pressure zone is there at the same time, more exhaust gas will be 
pulled from the cylinder.

In order to create these waves, and time them properly, the length of the 
pipe needs to be right.  Like a musical instrument, we need to tune 
it.  Some of this is done with the lengths of the individual branches of 
the exhaust manifold, and some of it is done with the resonator, or 
collector.  That mysterious device after the second joining of the exhaust 
serves to trick the exhaust gas into thinking it has reached the end of the 
pipe.  Really.  By causing a discontinouity in the diameter in the pipe a 
"stop" is created for the pressure waves.  What this causes is a standing 
wave in the exhaust system at certain RPMs of the engine, and  keeps the 
low pressure zone at the exhaust valve, giving your ALFA more power.

Removing, or changing the volume of, the collector will change the RPM at 
which the exhaust system resonates, and/or just make you lose some hp!

There is a lot more to it than this, naturally, but that is probably long 
enough.

Steve




At 11:47 AM 8/9/2003 -0400, you wrote:

>        I have welded up my own Alfa exhausts for years.  Most of the time, I
> just weld the stress cracks the seem to develop in the Alfa exhausts.
>        The front resonator is not a restrictor.  It adds "resonance" to the
> exhaust note.  My experience is that removing it causes the exhaust to be very
> raspy or tinny sounding - not at all pleasant, in my opinion.
>        The center muffler on the Alfa does all most all the muffling.  If you
> want LOUD, replace the center section with a straight pipe.  At that point,
> you would have the front resonator and a rear muffler section of your choice.
>        If you want medium noise, replace the center muffler with a glass
> pack.  Then run a straight pipe on the rear or a rear section with the muffler
> of
> your choice.
>        I really like the idea of a MAGNAFLOW center section.  If anyone knows
> where to get one, please let us know.
>
>        If you take the car to a muffler shop to have them replace some or all
> the mufflers with straight pipe or glass packs, make sure they reuse the
> existing pipes.  The flange in front of the rear axle is commonly replaced on
> custom made exhaust systems.  That makes the exhaust hard to remove for other
> repairs and you cannot replace only a portion when the system fails in a few
> years.
>        I also like the flange on the front of the center section on the Bosch
> spiders.  The early slip joints are hard to get apart for repairs /
> replacement.
>        So cut and weld to your hearts content.  Or just buy a Stebro.  I have
> never tried the Stebro, but they come highly recommended.
>
> Ciao,
> Russ Neely
> Oklahoma City
>
>
>
> In a message dated 8/8/2003 11:27:37 AM Central Daylight Time,
> owner-alfa-digest@domain.elided writes:
>
> > There was a number given for the MAGNAFLOW center
> >  section in a discussion about a year ago, anyone remember the MAGNAFLOW
> > number?  Joe  Faherty
> > 74GTV
> >
>
> Date: Fri, 8 Aug 2003 12:56:42 -0400
> From: "skippatnode" <skippatnode@domain.elided>
> Subject: Re: Exhaust Mod
>
> Nils,
>     The purests are going to jump on me for this but, Alex and I have both
> taken our Alfas and many others to a local exhaust shop and had them
> fabricate
> an exhaust system using glass packs (or the equivelent thereof) and standard
> pipes.  The result has been a good mellow sound, plenty of power, a neat
> installation and a reasonable price.
>     The glass packs are not much larger than the pipes so you can put as many
> or as few as you need to get the sound you want and have it the way you want
> it before you leave with it.
> Skip Patnode
> 67 Duetto
> Norfolk, Va
>
> In a message dated 8/9/2003 12:14:23 AM Central Daylight Time,
> owner-alfa-digest@domain.elided writes:
>
> > by "restrictor" do you mean "resonator" ? it really doesn't do
> > much restricting.  how much volume and resonance do you
> > like inside your car from the exhaust ? i like my vehicles fast
> > and loud ( you should hear my motorcycle ), but for street
> > driving i put my resonator back on after only a couple thousand
> > miles.  the noise made long drives very taxing.   sounded great
> > in the twisties and at the autocross, though.
> > <snip>
> >
> > dion fields
> > los angeles
> > 73 gtv
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