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Yes and no. Perhaps I didn't explain properly (I tend to do this <G>).
Most wear takes place during start up. For example, start car and let it
idle with the choke for a couple of minutes to improve smoothness when
driving off. The low temp. causes corrosive wear, the over rich mixture
causes oil to be washed off cylinder walls and cold oil does not reach all
the parts it needs to lube. As you drive off, the engine becomes warmer (as
indicated on the water temp. gauge - which tends to show an average) but the
temp. still varies during stop-start driving, especially at the top of the
piston. This variation causes corrosive wear. The engine wear rate has now
dropped significantly, however. As you get out onto the freeway, overdrive
in or not, wear drops to almost nothing - constant temp., throttle opening
etc.
As piston speed increases, the lubrication film becomes thick enough to
eliminate metal to metal contact. Most wear takes place where the top ring
comes to rest i.e. TDC, as we all know, and the stopping of the piston
leaves a thin oil film. The top ring causes most wear because of increased
tension on the cylinder wall cause by combustion pressure. The greater the
throttle opening, the greater the MEP, the greater the wear. This also
shows why speed is independent of wear. This type of wear is not as
significant as corrosive wear. Corrosive wear is what happens during start
up and during temp. variation. The thin layer of oil at the top of the
cylinder can not protect the wall from acids formed during the combustion
process. When the engine is hot, these acids are expelled as gasses. In a
cold engine they condense onto the walls, attacking where the oil layer is
thin. Research shows that the temperature for these acids to be formed into
gasses should be roughly 120 degrees Celsius which translates to a water
temp of 60. I don't have conversion factors at hand for degrees F. This is
the most important type of wear. The other type of wear is abrasive caused
by carbon/poor filtration.
So, maybe I misunderstood the question but the point I was trying to make
was that an overdrive will not necessarily reduce wear on its own. I always
use my overdrive on the highway but not hoping to reduce wear, better fuel
consumption (maybe but not much), definitely better comfort.
The best way to reduce wear IMHO is to allow the quickest warm up with out
stressing the engine. Drive off immediately after starting, small throttle
opening and dispense with the choke ASAP.
Cheers
Mike
-----Original Message-----
From: ccrobins [SMTP:ccrobins@ktc.com]
<mailto:[SMTP:ccrobins@ktc.com]>
Sent: 24 February 1999 06:53
To: Mike Venables
Subject: Re: MGB Overdrive?? - engine wear
OK, so if the temperature of operation is the big deal, then it
follows that, when in OD and burning less gas at lower RPM, the
engine
is experiencing less wear.
Simple, eh?
CR
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