Here is the deal behind the Underdrive
Pulley. The good and its negative effects on your car's belt driven accessories.
Note this mod is said to be good for
almost 10HP. You can pick one up for around $75 for a 1994-98 and $175 for a
99-2004.
Keep in mind when purchasing there is a difference between 2000 and 2001- so
make sure you get the right one for your stang!
Basically the argument is that with
reduction in pulley size it is going to have a negative affect on your belt
driven accessories: i.e. power steering, A/C, engine cooling (water pump), and
the big one.....your electrical system (alternator).
Now unless you're running a high-end
audio system or something else demanding alot of electrical power you should
be okay- but lets find out why.
To find out how the pulley will affect
your accessories, first you must find out what the "norm" for them
is in the first place. Now the issue is, with the smaller pulley, all of the
acc. pulleys will be at lower RPM's then normal (25% less to be exact, hence
the 25% reduction pulley). lets do a little math shall we...
Now to find out what the pullies are
doing under the stock setup...you can find the RPM of the acc. by the following
equation:
RPM of the driven acc. pulley = RPM
of driver pulley X the diameter of driver pulley / diameter of the driven pulley.
The stock pulley diameters are as follows,
Crank= 7 inches
water pump= 6 in.
A/C= 5 in.
Power steering= 6.5 in.
Alternator= 2.5 in.
So for example if you wanted to know
the RPM of the water pump when the engine is at 1000 RPMS:
1000 RPM's x 7" (dia. of crank)
divided by 6" (dia. of water pump)= comes out to around 1166 RPM's
Now, when you install an UD pulley,
you are making the crank smaller.
The 25% version that is most common is about 5" in diameter. With that
in mind, obviously the acc. will be underdriven by about 25%.
Now if you are running a high end audio
system this undercharging could be very bad for your battery.
The alternator is the only one of the accessories that has been proven to be
negatively affected by the smaller crank pulley. I've had my pulley for a little
more than 10,000 miles and the A/C runs like a dream in the summer, and I have
had no problems with overheating even in the scorching Kansas summer heat. Let's
go back to our equation and see if we can't get that charging issue fixed.
With the stock crank pulley at 1000
RPM's, (1000 RPM crank X 7 in. divided by 2.5 in.) the alternator is spinning
at about 2800 RPM's. Now with the smaller crank (5 in) the equation spits out
(1000 RPM crank X 5 in divided by 2.5 in.) comes out to 2000 RPM. Now you can
obviously see the difference and the nagative charging affects this will have
on your electrical system.
With this is mind, 25% reduction pulley,
a 25% smaller alt. pulley would seem to be the answer. I have yet to see a 25%
smaller alt pulley but I am sure that you could have one custom made. The only
one i have seen is a 10% smaller pulley, (or about 2 in. as opposed to the stock
2.5 in.) but see this will only yield 2500 RPM as opposed to the stock 2800
RPM's (oh at 1000 RPM crank speed).
High-end audio systems needs to have at minimum the stock alt. output. So everyone
keep their heads up for a 25% pulley for the alt.
Hope this helps clear stuff up for people...
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