I have been patient not to wage endless argument on baseless grounds, but
now I shall venture one message to the list on this subject. Watching this
OD theory be thrashed about vigorously for about 10 days now, I thought
someone would get it right, eventually. Yes there are those benefits of
slightly reduced wear and significantly reduced noise, but the greater
pursuit is cash savings! The correct catch phrase finally popped up twice,
but wasn't clearly explained. Bully to Bill Eastman for mentioning it,
"The real thief at high rpm is pumping losses". But why, and what exactly
is it?
An internal combustion engine acts like an air pump, pulling air in one
port and pushing it out another other port. As fuel is mixed with that air
and the mixture is burned, what actually comes out is exhaust gas (which
has a considerably larger volume than the intake air). To the extent that
the combustion gases are at higher pressure on the power stroke than the
pressure of the air charge was on the intake stroke, the engine can produce
useable power output. When the exhaust valve opens, there is still
pressure left in the cylinder. This remaining pressure, to the extent that
it is above one atmosphere of pressure, represents some lost energy (but
that's not the pumping loss).
Now consider the other component of the exhaust gas, the amount of gas that
is required to exactly fill the cylinders at one atmosphere of pressure.
This amount of gas stores and returns energy like the spring in an
accumulater, with the intake charge being compressed, and then the exhaust
charge being expanded to recover the energy. This net energy flow is zero.
There is a certain amount of fuel being burned to create this amount of
exhaust gas, and that amount of expended fuel going out the exhaust port is
the pumping loss.
The pumping loss varies directly with engine displacement and engine speed
(and atmospheric air pressure and fuel mixture ratio being minor
contributors), and is totally independent of all other operating parameters
(as best I understand it). So first, if you cut the engine displacement in
half for a given engine speed and load, you cut the pumping loss in half.
Secondly, if you cut the engine speed in half with a given engine
displacement and load, you also cut the pumping loss in half.
Most of our LBC's are generally good at the first example, because they
have relatively small engines, so they get pretty good fuel economy inspite
of running rather fast. Still, if you can slow the engine down some with
an overdrive gear you can reduce the pumping loss even more. This works as
long as the engine is not lugging under load at speeds far lower that the
peak of its efficiency curve.
At fairly high engine speed and moderate travel speed, the pumping loss can
be about equal to the energy used to overcome mechanical friction and wind
resistance. In this case, when you reduce the engine speed by 30% with an
overdrive gear you may reduce the fuel consumption by 15%. This is the key
reason that so many new cars are equipped with such gear ratios to make the
engines run slower, as the manufacturers need to meet the mandated CAFE
numbers in the US, and the economy demands of (some) customers in all parts
of the world. As someone stated, the results do depend on the engine speed
being fairly well matched to the peak of the efficiency curve, but there's
a wide window of opportunity there.
Now aside from helping to meet the mandated requirements, in the real world
OD may well not make good economic sense. When you figure the added cost
of manufacture of the OD unit (especially an OD unit separate from the main
gearbox), and the added maintainance cost of the OD unit in the long term,
these may well out weigh the value of the fuel savings. These costs are
quite obvious to those of us dealing with these older cars now in need of
repairing or replacing the OD units, or wanting to install OD in a car that
did not previously have it.
And there are many new car buyers who have no idea what OD is or what it
does and who would be very happy to pay a little less for a car without OD.
And there are many who do know the value of OD, but are on a tight short
term budget and would rather pay less for a new car regardless of the fuel
economy numbers.
So in summary, if it wasn't for our government mandates, there would be far
fewer OD cars on the road, and many more satisfied car owners. You see,
not everyone wants one (including me), in spite of the obvious benefits.
For our beloved old LBCs, I believe OD is a loosing economic proposition,
so if you like and/or want OD it should be founded on other reasons, like
reduced noise. I for one happen to get off on the neat noises my MG makes,
and couldn't bear to neuter it with OD.
$.02,
Barney Gaylord
1958 MGA with an attitude (and a happy 4400 RPM @ 75 MPH)
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