Engine Sucking from tailpipe

["Martin Stanczak" <martin@xxxxxxxxxxxxxxxx>]

After reading through all the posts on engine sucking through the exhaust I
feel left out by not pitching in my 2c worth.  Here is my VERY strong
opinion:
An engine is VERY LIKELY to suck through the exhaust under some conditions!
Just think of what is happening at relatively high rpm when you take your foot
off the gas pedal:  The throttle plate closes very nearly air tight, that
creates very high vaccum in the intake manifold.  At the same time, fuel is
cut off completely (on FI vehicles the fuel will be shut down in this
situation until engine rpm drops to about 1400 or less, at which time enough
fuel to create an idle mixture will start flowing).  What doeas this mean?
The cylinders are sucking in almost nothing from the intake manifold, there is
no combustion taking place, the exhaust valve opens close to the piston at
Bottom Dead Centre and.... WHOOSHHHHH, lots of air from the exhaust rushes to
fill the cylinder momentarily, before the piston comes up to Top Dead Centre
and in the process pushes the air back out again into the exhaust!  You can
actually feel this in action if you have someone rev the engine up and take
their foot off the gas at high rpm and you cover the tailpipe with your hand -
you will definitely feel the pulsing of the air in and out of the exhaust as
the engine rpm drops back down to idle.
That's what my idea is behind how the engine works.  I strongly believe that
things loose in the exhaust pipe, especially from a cat which is close to the
manifold, can find their way into the engine.
Regards to all,
Martin
'95 318ti
'87 MR2

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