Underdrive pulleys
Encyclopedia
An Underdrive pulley refers to an aftermarket
crankshaft
or accessory pulley
(such as an alternator pulley
) that is designed to drive a vehicle's accessories at a slower rate than stock. Underdrive in general means the input rate of rotation in a system is greater than the output rate of rotation. However, an underdrive pulley is considered to be an accessory pulley that is larger than the stock pulley, or a crankshaft pulley that is smaller than the stock crankshaft pulley, even though the accessory may not necessarily be underdriven with respect to the crankshaft pulley.
and horsepower
output of an engine by reducing parasitic drag
caused by belt-driven accessories, but more importantly by reducing the moment of inertia
. Horsepower
gains from underdrive pulleys alone are possible. Independent dyno tests have shown up to 15hp increase at the wheels after installing an underdrive pulley (see references).
effectiveness that may be noticeable, especially at idle. The headlights may dim when coming to a stop, or the stereo may lower in volume, for instance. Too much underdrive for a race car is not much of a concern, but for a daily driven vehicle it can be a major annoyance.
Changing the original crankshaft pulley can have negative effects if the replacement pulley is not manufactured properly. A crankshaft or accessory pulley that is not machined or balanced properly can cause severe damage. For most American V8s a harmonic damper
is necessary to absorb crankshaft vibrations. Earlier V8's kept the pulley and damper as separate components. Most late model V8s now integrate the damper and the pulley into one part. In general, for small displacement engines (4.0L and under) the original crankshaft pulley only serves to quiet noise vibration and harshness (NVH) heard in the occupant compartment.
Aftermarket (automotive)
The automotive aftermarket is the secondary market of the automotive industry, concerned with the manufacturing, remanufacturing, distribution, retailing, and installation of all vehicle parts, chemicals, tools, equipment and accessories for light and heavy vehicles, after the sale of the...
crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
or accessory pulley
Pulley
A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...
(such as an alternator pulley
Overrunning alternator pulley
During normal operation alternator pulleys can rotate at up to 18,000rpm and coupled with rapid acceleration and deceleration can lead to premature alternator, pulley and drive belt failure...
) that is designed to drive a vehicle's accessories at a slower rate than stock. Underdrive in general means the input rate of rotation in a system is greater than the output rate of rotation. However, an underdrive pulley is considered to be an accessory pulley that is larger than the stock pulley, or a crankshaft pulley that is smaller than the stock crankshaft pulley, even though the accessory may not necessarily be underdriven with respect to the crankshaft pulley.
Typical Usage Scenario
Underdrive pulleys usually are marketed as a performance enhancing item that increases the torqueTorque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
and horsepower
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
output of an engine by reducing parasitic drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
caused by belt-driven accessories, but more importantly by reducing the moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
. Horsepower
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
gains from underdrive pulleys alone are possible. Independent dyno tests have shown up to 15hp increase at the wheels after installing an underdrive pulley (see references).
Usage Concerns
Poorly designed cheap pulleys or copies and knock-offs of major brands can have severe side effects because of too much underdrive of the alternator, power steering, and/or air conditioning units, which are not spinning fast enough. These lead to dips in alternator voltage, power assist, and air conditioningAir conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
effectiveness that may be noticeable, especially at idle. The headlights may dim when coming to a stop, or the stereo may lower in volume, for instance. Too much underdrive for a race car is not much of a concern, but for a daily driven vehicle it can be a major annoyance.
Changing the original crankshaft pulley can have negative effects if the replacement pulley is not manufactured properly. A crankshaft or accessory pulley that is not machined or balanced properly can cause severe damage. For most American V8s a harmonic damper
Harmonic balancer
A harmonic balancer is a device connected to the crankshaft of an engine to reduce torsional vibration and serves as a pulley for drive belts....
is necessary to absorb crankshaft vibrations. Earlier V8's kept the pulley and damper as separate components. Most late model V8s now integrate the damper and the pulley into one part. In general, for small displacement engines (4.0L and under) the original crankshaft pulley only serves to quiet noise vibration and harshness (NVH) heard in the occupant compartment.