H. Whelply said:
My '04 Forester XT owner's manual says, as do the manuals for any car I've
had, that for the first 1,000 miles the engine shouldn't be run continuously
at the same speed for long periods of time. Can someone explain why that's
bad for a new engine? And how long is a "long period of time"? I'm assuming
that setting cruise control on 75 and staying that way on the open
Interstate for an hour or two would be a "no-no." But what about 30 minutes?
20? 10? Thanks.
HW
It mostly has to do with wear in of the piston rings to the cylinders.
With freshly machined (i.e., relatively rough) finishes on the mating
parts, initial wear is a lot faster than later after things are worn in
(think of big peaks and valleys rubbing against each other - on a
microscopic scale) to a smoother, and in some senses, a harder surface.
When an engine is running at higher speeds, the parts set up resonances
(vibrations relative to each other on a microscopic scale). At a given
engine speed, the rings will be vibrating (contracting and expanding)
and always hitting or pressing harder on the same places on the cylinder
walls. This can wear grooves in the cylinder walls. Since this is a
relatively rapid wear period (remember microscopic peaks and valleys),
these grooves could become significant during the wear-in period. Now
the normal resonances can get amplified and re-inforced by hitting these
grooves, and the grooves will, in turn, set up new resonances and get
bigger over the life of the engine. That engine will typically have a
shorter life, not be as efficient, and use oil.
By varying the engine speep during the intial wear in period, the points
at which the grooves hit or press hardest against the cylinder walls
during each revolution of the engine get more randomly spread out over
the length of the cylinder wall, so no grooves will form to amplify the
normal resonances.
This is not nearly as critical as it was some years ago, as machining
techniques, tolerances, and surface finishes have improved greatly - so
the "mountains" and "valleys" start out much smaller (effective hardness
is greater and initial wear rates are lower so those grooves are less
likely to form, and the critical wear-in period is much shorter).
Some stop and go and running thru the gears with moderate acceleration
would probably be one of the best things you could do. And when
cruising, just make an effort to continutously vary the speed between
your top cruising speed and maybe 10 or 15 mph below that. Explain to
any passengers what you're doing and why.
Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")