Re: Lugging

[Bruce Augenstein <Bruce.Augenstein@xxxxxxxxxxx>]

<ChrisBourk@domain.elided> takes issue with my summary post on lugging, and it
therefore appears to me that I need to explain a bit more........

I thought Mr. Fellingham's comments were pretty straightforward, but
evidently not, so let me flesh it out a bit more. His comments about
cylinder and ring wear under low engine speed and high load conditions were
largely concerned with hydrodynamics; that is to say, higher speed
differentials between adjacent parts promotes hydroplaning on the oil film.
Mr. Fellingham used a water skier analogy in his note to me. I usually use
the high-speed-hit-a-puddle-with-MXX3s analogy, but no matter the analogy,
the effect is the same. Speed promotes lift, as it were.

When lugging, you will need more throttle to accelerate the vehicle, because
the engine is operating inefficiently (cylinder filling is compromised,
generally because cam timing/lift and port sizes are optimized for another
part of the rpm band), and perhaps more importantly because you are not
getting many bangs per second to accelerate the vehicle. Therefore, you need
to create bigger bangs by using more throttle. This, in turn, creates more
forceful side loading against cylinder walls, at a time when piston and ring
speed (relative to cylinder walls) is low. 

When ring speed is low, the tendency to hydroplane is also low. If we can
assume (and we can) that there will be some relative speed where the rings
will stop hydroplaning, then we can assume that the rings will be below this
critical speed for a greater percentage of the piston's travel (on the power
stroke), and therefore will spend more "time" (expressed in degrees of
crankshaft rotation) in this state, where side loads will be significant,
because of rod angle and combustion pressures in the cylinder.

Since we've postulated that side loads will be relatively high at this time
(because of copious throttle use), then the rings will likely stop
hydroplaning for an even greater amount of crankshaft degree time - just as
a heavy car won't hydroplane as easily as a light car, everything else being
the same.

When you compound the problem with a potential loss of lubrication (from
reduced "splash"), then you're looking at the potential for significantly
increased wear rates.

As an anecdote that partially supports this idea, I'll mention an
engineering/business study that Chevrolet did back in the late '60s. They
found that repair records on Corvettes varied a bit based on final drive
ratios. Vehicles with aggressive gearing (i.e. - 4.11 or lower) tended to
have more problems associated with valve train wear, and *fewer* problems
associated with cylinder and piston/ring wear. Those vehicles equipped with
less aggressive gearing (i.e. - 3.08, 3.36, etc.) had fewer problems with
valve train wear, and *more* problems associated with cylinder wear.
Overall, there wasn't any significant difference on overall repair needs -
just that a "typical" repair varied.

Interesting, no? :-)

Re <ChrisBourk@domain.elided>'s objection to my paragraph to the effect that
occasional lugging may lead to as much cylinder wear as occasional red
lining does, I reject it. He took two words ("flat out"), and chose to
ignore everything else in that paragraph in order to construct a mini
diatribe about racing conditions. This, at best, is a non sequitur.

On the other hand, it would be interesting to see what would happen if
everybody lugged their engines as much as possible during a race :-).

Re the criticism that describing an engine as "happy" in a certain rpm band
was not scientific: I agree that it wasn't scientific. On the other hand, it
was an effective use of words, and I dare say that everybody except perhaps
<ChrisBourk@domain.elided> got the concept.

Bruce

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End of bmw-digest V9 #105
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