Throaty engine

Sorry that this message is not riddled with sex, jokes or politics.

I was pondering why my engine sounds so throaty (ie. the exhaust has a
low frequency rumble to it) when it is cold (and on cold mornings it is
more pronounced) - and when it warms up that tone goes away.

Heating the metal of the exhaust system wouldn't change the dimensions
that much to re-tune it.

The exhaust gas should be plenty hot when coming out of the engine so
that shouldn't do it. (although, the speed of sound scales with
temperature and as the speed of sound changes the frequency for a given
wavelength changes - but would a cold pipe make that much difference to
the exhaust gas temperature?)

Anyone have any inklings on this (idol) question?


JGB

ps. overly physics ladened answers are welcome I studied a bit o'
physics once

 

Maybe it's not the pipe. Thicker oil means more throttle, etc.


-tc

 

Good point - that and the gas not being completely bunrt in the cylinder
could explain it.

BTW: I conducted a simple test - after riding around on a cold day, I
let the bike sit for an hour until the pipes were cool to the touch.
When I started it up, the engine was slightly warm (I didn't need any
choke... or whatever they call it on BMW's) and no throaty sound. The
engine temp was on the 'bottom bar' [Usually when it sounds throaty
there are no bars showing].

This seems to fit with your hypothesis.