The Truth About The Underdrive
Crank Pulley
INSTALLING AN UNDERDRIVE CRANK
PULLEY WILL DECREASE YOUR ALTERNATOR OUTPUT.
The underdrive crank pulley dates back many years
when hot-rodder's were squeezing every last
horsepower out of their rides. The underdrive crank
pulley is any form of pulley smaller in diameter
than the one originally supplied by the factory.
Back in the day cars were carbureted, had mechanical
fuel pumps and definitely did not have an electric
cooling fan. Installing an underdrive crank pulley
in one of these older vehicles probably saved a few
horsepower by slowing down the accessories like the
water pump, air conditioning compressor, power
steering and alternator, which was more than
adequate to begin with. Older vehicles sometimes
came with an alternator that only produced 30amps!
Slowing one of those units down with an underdrive
crank pulley really didn't matter as the vehicle
contained few electronics that would notice the drop
in alternator output.
Today's vehicles are packed with essential
equipment that can draw large amounts of power.
Most OEM crank pulleys are 6" in diameter while most
OEM alternator pulleys are 2". This gives us a 3:1
ratio between the crank speed and the alternator
speed. At an 800rpm engine idle speed the
alternator is actually spinning to the tune of
2400rpm. It's very important to maintain the 3:1
ratio or else the alternator can't do it's job,
especially a high amp unit that needs every last rpm
to operate effectively.
We've searched the web and have seen wild claims of
10%-15% power gains, better fuel economy and some
underdrive pulleys will even make your hair grow
back (ok, just kidding about that last part). What
is even more amazing than these claims are the
prices of one of these pulleys. There are plenty of
people eager to spend up to $400 on these devices
that only hurt your vehicles performance.
How can an underdrive pulley actually hurt
horsepower?
It's pretty simple. For today's vehicles to run at
optimum performance a lot of things need to happen,
correctly. There are two very important parts of the
rpm range when talking about alternator output. Idle
and WOT (wide open throttle). If you didn't notice
there is no longer a carburetor or distributor on
almost any vehicle even 10 years old. What controls
your vehicles fuel delivery and timing then? It's
called an ECM (or ECU for the import guys!) and it's
voltage dependant. You see the cars today are so
smart that if your electrical system isn't healthy
the computer will simply retard ignition timing or
alter fuel delivery at WOT to compensate for low
voltage, saving a costly engine disaster. The
computer knows that if your vehicles voltage is
lower than expected the fuel pump will be delivering
less fuel, the injectors will open and close slower
and some adjustments will be made to guard
against engine damage. Install an underdrive pulley
and you may end up underpowered.
If you couldn't tell by now, we don't like
underdrive crank pulleys. Since we are in the
business of increasing alternator output these have
been a thorn in our side for quite sometime. Any
time a customer has reported low alternator output
on one of our units it was because of an underdrive
crank pulley, insufficient battery cable size or cheap
battery. More articles on the other two culprits
coming soon.
We can't compete with the hundreds of thousands of
dollars put into marketing products that you just
don't need for your car, but we hope you've at least
heard the other side of the story.
