Flame Summary: Lugging

[Bruce Augenstein <Bruce.Augenstein@xxxxxxxxxxx>]

Well hey! Hardly any temperature at all in this discussion! Only one
content-free post, from a guy who suggested I'd be better suited with a
Corvette -  which may be true if the new Vette hardtop is as good as they
say it is.

In any event, here's how it broke down:

Three people said yup, lugging is OK, and you won't hurt anything.

One person said hey, you'll get better mileage due to reduced pumping
losses.

One person said it was the preferred thing to do.

One person said it would ruin the transmission, and offered an anecdote
about an acquaintance who had the input shaft bearings go bad on his truck,
after incessant and habitual lugging. I personally would lend more credence
to this theory if I could figure how lugging actually affects transmission
bearings, but I can't, since about every street clutch I know of has a
sprung hub.

One person said it'd end up twisting something in the driveline, which I
would personally lend more credence to if the engine were making any
significant torque down near (or at) idle - but it isn't. Even full-throttle
bangs way down in the rpm range lack punch, and by definition, it's a lot
harder to lug in first gear than it is in fifth or sixth, so everything
south of the tail shaft is safe, so to speak.

A couple of people mentioned ping or knock associated with lugging, and of
course I'd agree that any significant pinging or knocking over time can hurt
your engine. That's why I mentioned  "short of prolonged detonation in a
vehicle without knock sensors" in the sentence that said lugging wasn't bad.

Everybody else (including a number of folks who emailed me instead of
posting on the digest) said that lubrication (or lack of it) was the reason
you shouldn't lug an engine. Most of these responses said that you'd have
metal-to-metal contact between bearing surfaces because of a) Loss of the
hydrodynamic effect due to too-slow relative bearing surface speeds,
combined with b) reduced oil pressure (and therefore volume, I presume)
because the engine-driven oil pump won't pump enough oil down near (much
less at) idle.

I reject this idea, primarily because ANY metal-to-metal contact between
plain-bearing surfaces will toast them in a very short time - period. While
it is possible, in theory, to have this problem, I've never heard of an
engine spinning a bearing because of lugging - even when it's driven right
out of the showroom with a shudder, in high gear.

One person, a Mr. Peter Fellingham, responded with what I thought was an
intriguing theory. Mr. Fellingham doesn't subscribe to the digest, but sent
mail after a digest subscriber forwarded our little flame topic. It is his
contention that lugging an engine leads to increased cylinder and ring wear.

More on that shortly, but first, let's try and more closely define lugging.
I described a couple of examples of lugging to Mr. Fellingham that I was
sure would not have lubrication problems - at least not of significance. One
is a '90 Saleen Mustang that I just turned back over to one of my sons after
having had it as a project car for the two plus years that he was overseas.
This vehicle started as a stock 5.0 (302) V8 that was happy to pull away
smoothly from a thousand rpm in fifth gear. Now, it's making ludicrous power
(an estimated 500+ horsepower at the clutch, a measured 458 at the rear
wheels, under SAE Net "standard day" atmospheric rules). It also lugs at
anything under 1500 rpm in high gear, due mainly to cam timing, and intake
and head design. My contention is that this car won't suffer any lubrication
problems at, say, 1200 rpm in high gear, even though it'll lug badly at that
rpm. It has the same short block it had before the modifications, including
rings and bearings, and will therefore be safe from a lubrication
perspective wherever it ran smoothly before the modifications.

The other example is my Neon. This vehicle has the DOHC engine, which will
grumble a bit if you try to pull away from, say, 1400 rpm or below in fifth
gear. My experience has been that the SOHC engine will pull smoothly from
1400 rpm in fifth, even though fifth gear in the SOHC model is significantly
taller than fifth gear in the DOHC model. Since the short block is
essentially identical, I'd have trouble believing that the DOHC engine will
trash its bearings at (low) engine speeds where the SOHC model won't.

Mr. Fellingham conceded that lugging due to less than optimum gas flows will
be "safer" (my phrasing, not his), but went on with the following, which I
will reproduce without permission:

"........In any event, I am considering the wear potential of loading an
engine down well below idle speed, irrespective of when it starts
"lugging"."

After defining the operating envelope, he then went on say:

"You mention rod and main bearings; these may be damaged if the engine's oil
pump does not provide enough pressure at low speed.  I am more concerned
with the fact that the rings almost always make a large degree of metal to
metal contact.  If the engine is loaded hard at low speed (dependent on
breathing), there may not be enough splash lubrication for sustained
running, and as I mentioned oil film formation depends on speed.

As the pistons stop at the top and bottom of every stroke and move slowly
near TDC and BDC, dropping the engine speed increases the distance the rings
travel with questionable lubrication film formation.  The same logic applies
to the side loads on the pistons, except that these peak halfway through the
stroke."

Interesting, no?

I think Mr. Fellingham is on to something; namely, that lugging at extremely
low rpm can lead to significantly increased cylinder and ring wear, due to
the low speed of the moving parts at a time when lubrication may be
questionable.

In any event, it is my personal opinion that lugging is extremely bad for
your teeth and jaw muscles (at least it is to mine), and, in fact, may lead
to at least as much cylinder wear as running to red line does. That is to
say, habitual lugging may cause as much "damage" as habitual flat out
driving does. Occasional lugging is probably just as safe as occasional red
lining.

On the other hand, even though my 3 liter M3 will pull away smoothly  from a
thousand rpm in fifth (if I'm gentle), it's a bunch happier when it's at
3500 or more, and so am I :-).

Bruce

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