Hi Carl,
This is a question about the rate of wear in the engine that's
acceptable to the owner. The rate of wear increases as engine speed
increases, so one supposes that for maximum longevity one would drive on
flat roads at 2500 rpm or less and live boringly ever after. But the
horsepower peak is 5000 or so, and in order to enjoy the car more one
drives it faster, albeit harder. The old rule-of-thumb is that 2500
ft/min piston speed is the max for long happy life. That's approx 4200
rpm for the TD engine. At the net gearing of the cars with original
5.125 gearing and today's radial tires, that's about 61 mph.
Carl Cedarstrand wrote that 4000 rpm for hours of cross-country
driving leads to early bearing cracking. 4000 rpm for hours of
cross-country driving probably leads to the driver cracking too. That
train of thought leads to discussions of alternate rear-axle gearing.
A consideration for the longevity question is that we probably rebuild
engines today to a standard higher than MG used, and we have balancing
and fuels and lubricants which are so much better that wear should be
lower than it was in the 1950s.
If you assume that a rebuild would ordinarily be called for at 60,000
miles of normal driving (whatever normal driving is) and you push it hard
and get only 40,000 miles before a rebuild is called for, at 3500 miles
per year, that's still eleven years of use.
For driving around with the gearing you have in the car you have, you
won't go wrong at 3500 rpm cruise, a speed you can maintain forever and a
day; 3750 isn't flogging it either, and MG did set the red line at 5500,
so it's not going to self destruct immediately.
Perhaps the answer to your question is that you can drive it as fast
as you want (or as you dare) on secondary roads, but once on the 70 mph
highways you should hold the revs to 4200 max and let the modern
machinery roll by.
Bob
On Mon, 19 Apr 2004 18:00:01 -0700 (PDT) Carl French
<leylandauto@yahoo.com> writes:
> Kimd of a newbie question. What is the max I should stay away from
> and what is the highest comfortable (for the engine)rpms for a Mk1
> TD?
> I am not sure the speedo works correctly, what is the rpm to speed
> formula as the tach seems to be ok.>
> Carl French
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