My understanding of running an engine with insufficient backpressure is
that it can cause the exhuast gas to exit the combustion chamber too
fast, which can burn the exhaust valves. The velocity of hot gases
flowing past the exhaust valves and through the exhaust port too fast
can actually cut grooves into the exhaust valve.

But there could be other reasons, as well.

Dan

Dave Nadeau wrote:

> Well, I started my trip on Friday from Phoenix to San Diego
> with high hopes, some time by the ocean, all of that stuff.
> About 45 minutes out, my soft exhaust note became noticeable
> different.  The Connecting Pipe on my exhaust system ('84
> 1.9L) had broken in half.
>
> I called a mechanic here in Phoenix and asked about running
> the engine without the muffler and cat.  He said that it
> wasn't the greatest thing to do, and I shouldn't run it this
> way for long periods.  Based on that information, I returned
> home.
>
> My question is this:  what is so bad about running the
> engine without back pressure?  Is it bad on the valves?  The
> noise, of course, is much higher, and certainly illegal.
> But what kind of damage might have been done, or might be
> done to the engine?
>
> Of course, my solution is to get the Connecting Pipe and
> install it immediately, which I will do.  Thanks in advance
> for any explanations!
>
> Dave Nadeau
> Phoenix, AZ
> '84 Westy