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For the most part, accessory loads can have a dramatic effect on engine
performance. Based on some test and analysis that GM has done, the simple
removal of mechanical driven power steering pumps can affect the fuel
economy by as much as 1 mpg. Keep in mind that this is completely removing
it and using an electric power assist servo. The energy required to drive a
slightly larger alternator is significantly less than the mechanical
hydraulic pump. Fuel economy can be affected by accessory load, but often it
is a big trade-off.
After talking with a driving instructor that has a totally built VR6
Corrado, I do not recommend slowing the water pump at all. If you live in
hot climates, or drive under heavy loads (extended performance driving on a
track, or favorite canyon road/backroad romping) you are putting serious
strain on the cooling system. This guy, who had cams, worked intake
manifold, chip etc. had seen oil temps pushing 310 degrees. It was enough
for him to roast two engines. His oil was not keeping up. The primary reason
stems from the fact that the front 3 exhaust runners go through the head to
the manifold, adding considerable heat to the aluminum head. He did see
lower temps after he had the runners ceramic coated to keep the heat in the
runners, but he stuill saw elevated temps.
By slowing the water pump, you are reducing the amount of heat that the
cooling system can draw away from the engine, and put even greater strain on
the already taxed oil system. Keep in mind that oil is a coolant AND a
lubricant. It just doesn't lubricate to well the hotter it gets. Synthetic
oil helps, but is far from a complete cure.
Underdrive pulleys always have these trade offs. When you start adding power
to the car, you are also putting a higher load on the cooling system. In
fact, radiators are sized based on the the horspower rating of the engine.
Therefore, if you have increased the output of the engine significantly, say
25 HP, you are looking at nearly a 15% increase in output. Slowing the water
pump by installing underdrive pulleys may reduce the accessories by
typically 10%. Now it is hard to say that a 10% reduction in water pump
rpm=10% loss in cooling capacity, but there is a marked decrease in the
cooling systems capacity. Put that together with an engine that has had its
output increased, on a design that already has a small margin for excess
cooling capacity, and you may be looking at potential trouble.
As far as increases in rotational masses, it is a very weak theory. Compared
to the rotational inertia of the long block, the inertia of a couple of
pulleys is relatively insignificant. The lighter pulleys will not affect the
power output, though it may have a small, very small affect on throttle
response. It is nothing compared to a lightened flywheel.
When you consider balancing/harmonic damping, lighter pulleys would not
affect it greatly. However if the stock pulleys had some counterweighting,
and the flywheel was not properly counterweighted after lightening or the
pullys were spun eccentric, then you may have a problem with damaging the
crank. This was a debate on the 90-92 Miatas. Many owners encountered
cracked cranks after their first timing belt change. Almost all of them were
traced back to improper crank sprocket installation/a design that did not
make improper installation obvious. For an I-6 engine as the BMWs use,
external balancing is not necessary, as the crank configuration leads the
engine to have no first or second order vibrations other than those of
production variances. I would be surprised if there was any counterweight on
the flywheel to balance the engine. If any, they would be very small. I-6's
are the smoothest engine to have as far as harmonics.
Another example of the effect of water pump speed reduction is on my street
rod. Since I dropped in a complete 5.0 motor, including the serpentine belt
drive system, I installed a set of UD pulleys. I saw a marked reduction in
cooling capacity, even with a 3" - 4 row radiator and no hood (long story).
The radiator was the same one as what a firend had in his 650HP super
modified roundy round car, and with the UD pulleys, a humid 90 degree day in
Detroit would have me watching the gauges. I had one 225 degree spurt, but
that was after running down the highway at 90, and having to slow for
traffic. A couple of minutes of light load driving, coupled with the
electric cooling fan, and I had kept from really boiling over......that and
a 22# rad cap ;-)
I will probably put the stock pulleys back on, actually I installed them in
the middle of summer after the retaining nut on the alternator backed off,
roaching the alt. drive pulley. That and since I had no hood the red
anodized pulleys did look good in in front of the polished aluminum
alternator, p/s pump and brackets..... next time I have to be sure I torque
that alternator nut back down after I polish a housing....
Anyway, it is just my $0.02 ($2.00 Canadian :). Flame away if you think you
need to.....
Michael Kaczmar
Analysis Engineer- Energy Management Group
Vehicle Synthesis Analysis and Simulation - Passenger Car Division
General Motors Corporation NAO.
How's that for a title huh!!??
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Thu May 7 14:06 CDT 1998
>, <gti-vr6@cobra.ccsi