RS2 turbocharger

[Bernard Littau]

> Okay, now you've struck upon something I was wondering before.
>
> What is it that makes lugging an engine bad?  Is it related to
> boost pressure at all?  Or is is just that it's bad for the motor
> because it is spinning too slow to make proper use of the
> air/fuel being put in?  I know it sounds like knocking, so maybe
> that's what it really is?

Hi Ken,

I'm not technical car guy, so this is my own definition :-)

Lugging occurs when the explosion of the mixture in the cylinder occurs with
more force than the piston can absorb by moving to make use of the explosion
energy.

A knock is when the main force of the explosion occurs before the piston is
moving downward, that is, while the piston is moving upwards.

Both knocking and lugging result in higher cylinder pressures than normal.
The sound you hear is the pressure spike.  My sense is that knocking is far
worse than lugging, but either of them can damage the head gasket, or worse.

The more mixture (potential energy) you put into the cylinder, the more
likely you are to create an explosion that exceeds the piston's ability to
absorb the energy.  But, the faster the piston is moving, the less likely
you are to have too much energy.  There comes a point where the piston is
moving at the same speed as the explosion front, so it absorbs no energy and
can't make the crank spin any faster.  This is the theoretical redline of
the system.  At low rpm, the explosion always occurs "too fast", but the
main force of the explosion is delayed until the piston is moving downward
and there is more volume so the pressure spikes are tolerable.

So yes, it is somewhat boost related, but boost is not the only thing that
affects how much mixture you put into the cylinder -- the throttle is really
the final arbiter of how much mixture goes into a cylinder.  At the rpms
lugging can occur, I doubt you would want to start cracking your WG; better
to throttle back.  The newer drive by wire systems that control the throttle
plate with a servo do this for you.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq