RE: bmw-digest V9 #93

[Thi VanAusdal <tvanausdal@xxxxxxxxxxxxx>]

> Date: Sat, 22 Aug 1998 15:34:56 +0000
> From: andrew.j.thomas@domain.elided
> Subject: Rattle-rattle (engine lugging)
> 
> Hi all.
> 
> An engine-rebuilding friend of mine recently opined that engines were
> generally given a harder time lugging at low speeds than occasionally
> being
> revved excessively. The deal was that "lugging"
> (three-quarters-throttle,
> top gear, 22 mph kind of behaviour beloved of minicab drivers) ruined
> bearings and big ends (listen to that knocking!), whereas the
> occasionaly
> foray to the upper reaches was not detrimental, provided that engine
> maintenance was increased to accommodate this.
> 
> I would bet that your European average four-cylinder 1.6, designed to
> operate continuously between maybe 2,500 and 4,500 rpm, is more
> sensitive
> to being "lugged"than a big 6 in-line or V8. Why exactly this is,
> beyond
> the obvious reasons of size and stress, I don't know.
> 
> Any thoughts?
> 
> Andrew T
> '95 318is.
> 
Andrew is correct about 4-cylinders being more sensitive to lugging.
Six- and eight-cylinder motors have their power pulses spread out
in multiple planes.  Fours have a point at TDC where the power
pulse from the last fired cylinder is complete, the compressing cylinder
is not
yet fired (well, at speed, it has, due to ignition advance, but bear
with
me), one piston is just done with intake stroke, and the last is done
with its
exhaust stroke.  A six cylinder has power strokes every 120 degrees of
crank
throw, an eight every 90, and a twelve every 60 degrees.

If you extrapolate away from a single-cylinder, 4-stroke motor, which
would
operate very poorly at low revs without a massive flywheel (*), you can
see
the six is better than the four at leveling out the power output, and
the twelve
is really wonderful.

 - Thi VanAusdal

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