Re: holes in the muffler

[C2slayman@xxxxxxx]

Edward,
       This sounds like an urban legend to me.  The argument Tom and Ray give 
is that lower backpressure leads to higher exhaust velocity over the valves.  
That may be,  but I doubt it since the exhaust gas is so damn hot and the 
pressure is do damn high there's probably little difference for the bulk of 
the gas.  Where the difference comes in is scavanging out the last bits of 
spent exhaust gas.  That's where low backpressure plays a role.  But with 
lower backpressure, you get more new air/fuel into the engine which in turn 
produces more power.  That's what probably causes the valves to run hotter.  
But then that's equivalent to saying "I burned my valves because my engine 
was putting out more power."  Maybe if you were running NOS or a 
supercharger/turbo.  I doubt it from just improving the engine breathing.  
(Eric - you're the expert.  Care to comment?) 
       The argument that the valves run hotter because the gases are faster 
is also incorrect from a heat transfer standpoint.  There's a certain amount 
of heat in the gas (again assuming fixed power output).  How much it 
transfers to the valves depends on residence time.  The longer the gas sits 
in the chamber, the  more heat it can transfer.  The less time it sits around 
(ie higer velocity), the less heat its going to transfer to the chamber and 
the more heat it's going to dump out the exhaust.
       There was a similar legend when I was an autoshop student - if you ran 
an engine without the exhaust manifold or mufflers, you would damage the 
exhaust valves because cold air would creep back in after shutdown and crack 
the valves from thermal shock (thermal shock from air diffusing into  the 
exhaust?  nahhhhh!). I never believed that one either.
       I'd be happy if there was any real engineering data  to show the 
opposite.  But no annecdotal stories or urban legends masquerading as 
automotive engineering facts, please! ......though I do enjoy urban legends 
for their tongue in cheek factor. ;-)
       But the real point Tom and Ray pointed out (though not the first on 
their list) was the potential for carbon monoxide  getting into the cabin.

Cheers,
Charlie


> Date: Tue, 01 Apr 2003 13:32:14 -0500
> From: "Overly,Edward A." <eoverly@domain.elided>
> Subject: holes in the muffler
> 
> My spider needs a new exhaust system.  I was reading Click and Clack this 
> weekend
> and they were saying holes in the muffler could cause the valves to burn 
> due to not enough
> back pressure:
> 
> http://cartalk.cars.com/Columns/latest.html
> 
> This got me to worrying about free flow exhausts and how extractor exhausts 
> 
> work.
> I suppose as long as the closest muffler to the engine is in good shape you 
> 
> are ok-
> i.e., you will have sufficient back pressure. 
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