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Re: Backfire

On 20 Jul 98 at 15:34, type3@vwtype3.org wrote:

> I don't know enough to be commenting personally, but I happened to
> show this to a friend of mine (mechanic of 10 years) and this is his
> 2 cents...
> :: By the way...
>   Cars DO backfire from exhaust leaks, anyone who can tune an engine
> well enough to burn every last bit of fuel before it enters the
> exhaust should be applying for the position of GOD!!  they would
> probably get it LOL    On a car, its absolutely NOT realistic to
> expect that clean and efficient of a burn. Thats teh reason that
> cars have thigns like air injection systems, catalytic converters
> etc...  The EPA wants them clean, but even they understand it cant
> all be done in teh engine, some of its is finished in teh exhaust
> and lots of it is NEVER finished. There is most likely a leak if its
> backfiring in teh exhaust, but a really poorly running engine can
> also do it.. The exhaust is definately MOST likely :)

Your mechanic is as entitled to his opinion as I am to mine, but if 
you look at the ppm of unburned hydrocarbons that the EPA expects, 
even in a 67 or 68 T3, then you will see that there is not enough 
there to burn, let alone explode.  For every combustible gas, there 
is an explosive range; for gasoline it is rather narrow.

The exhaust leak hypothesis assumes that the leak will allow oxygen 
to leak into the exhaust system, mix with the unburned gasoline, and 
explode.  This ignores the fact that the exhaust is under postiive 
pressure and will almost never draw air INTO itself.  Without this 
mechanism, the hypothesis looses strength rapidly.  It also ignores 
the fact that there is as much unburned oxygen in the exhaust as 
there is gasoline, so the addition of extra gas is unnecessary.

I stick with my opinion (labeled as such.)

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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