🚀 go-pugleaf

vanderby@mprgate.mpr.ca (David Vanderbyl) writes:
> jmorriso@ee.ubc.ca (John Paul Morrison) writes (and draws a fancy picture):
> > vanderby@mprgate.mpr.ca (David Vanderbyl) writes:
> > >(This, incidentally, is why a hole in your muffler cannot help
> > > you pass emissions testing.)
> > 
> > Depending on the shape of the hole, and its position, the exhaust gases
> > could easily, and conceivably create a vacuum, and suck in more air into
> > the hole, thus diluting the exhaust.
> 
> As to this vacuum theory, yes it is conceivable that fresh air may enter
> the system.  However, it is extremely unlikely since the muffle/tailpipe
> create a back pressure in the system.  This is why air pumps are necessary
> for air injection anti-pollution systems, and the engineers who designed
> the car cannot simply strategically place a well shaped hole to achieve
> the desired effect.

I was assured by three different muffler technicians (all from reputable
muffler institutes) that exhaust system holes *do* suck air into the
exhaust.  This might be due to a vortex effect - I don't know.  Further,
the air injection anti-pollution systems you describe normally inject
right near (or at) the exhaust manifold do they not?  At that point the
pressures are much higher (due to higher temperatures and narrower
channels) so air pumps are necessary.  The air must be injected at 
this point because the idea is to promote more complete oxidation
(ie.: CO -> CO2) - something that is facilitated by higher temperatures.

> kenb@ee.ubc.ca (Ken Buckland) writes:
> While it is true that the test probe would pick up the extra air, this
> would result in the tester thinking you car has low emissions, not that
> you have holes in your exhaust.

In fact the testers asserted that I had a hole *before* we actually found
one.  It may have been that my emissions were just too low to be believed,
but they did mumble something about impossible gas mixtures (for an intact
exhaust system).

Ken Buckland
kenb@ee.ubc.ca