UNDER DRIVE "PERFORMANCE" PULLEY KITS
Metroplex Alternator & Starter does not sell or install Underdrive Pulley kits and we do not endorse or condemn their use, but we do have considerable experience with them and how they effect charging system performance in real world-day to day driving conditions. We average about a call a week from technicians and individuals who are trying to understand the effect of these kits on charging system performance. Typical calls include questions about undercharged batteries, volt meters in the instrument cluster showing low voltage, and the most the common is low alternator output at engine idle, causing dim headlights and stereo system problems.
Due to ever the increasing list of electrical accessories found on modern vehicles the general trend from the automotive manufacturers is to decrease alternator pulley diameters in order to increase alternator output at low engine speeds such as at idle or in stop and go traffic situations. On some cars the pulley is made as small as it can be while still being able to use a socket to tighten the nut that holds the pulley in place.
Here is some information about the effects of underdrive pulleys on charging system performance.
This information will be useful for people using their vehicle in a daily driver, commuter type situation.
If your car is for racing only your needs will be different and this information may not be of use.
The typical underdrive kit comes with three pulleys. A crank pulley, an alternator pulley and a water pump pulley. The underdrive crank pulley in all of the kits is of a smaller diameter than your engines stock crank pulley. The alternator and water pump pulleys vary in diameter from one brand of kit to the next and from kit to kit within brands.
Under drive or performance pulley systems work on the principle that underdriving (slowing down when compared to the stock setup) the speed at which belt driven components such as the alternator, water pump, a/c compressor and power steering pump are turned the mechanical load (drag) on the belt drive system is decreased. Decreasing drive belt system load (drag) means less horse power is required to run the belt system which means this horse power is freed up and available elsewhere (drive wheels).
To put it another way, the belt driven components are all doing work and work requires power and that power comes from the engine by means of the crank pulley and belt. By slowing down the speed at which you spin these components you reduce the amount of work they are doing thereby reducing the amount of power required from the engine. This power is then available elsewhere.
When you install an underdrive pulley kit your engine is not suddenly producing more horsepower. You are simply using less in one area so that it is available for use elsewhere. The typical claim is that these pulleys slow the belt driven components down anywhere from 20% - 30% and 10 to 15 horsepower can be freed up by installing an underdrive pulley kit as well as improved fuel economy and improved torque......all with no adverse effects.
Common Sense Check: If vehicle manufactures could increase horsepower, increase torque, and increase fuel economy by simply installing a smaller diameter stock crank pulley , which would cost them nothing extra, then these pulleys would be standard equipment on every model of every vehicle made by every car manufacturer on the planet. Just saying.
The claims sound great but as we all know nothing comes for free....
and there in lies the rub. While it takes less horsepower to spin a thing slowly than it does to spin that thing fast it is also true that the slowly spinning thing can do less work than it could if it were spinning fast. A water pump moves less water when it spins slower and an alternator will put out less power if it spins slower and an a/c compressor will produce less pressure if it spins slower and a power steering pump will provide less power assist if it spins slower.
Now depending on what vehicle you drive and how you drive it these changes might not cause any noticeable problem at all and if that's the case then that's great. Happy Driving ! If you do notice any ill effects it will usually at times when engine rpm is low such as sitting at stop lights or driving in stop and go slow moving traffic. So if you do encounter problems the information below may shed some light on why you're having charging system issues.
NOTE:
We're going to focus on the alternator and charging system from here because it's what we know.
So the word "Alternator" can stand in for any of the other belt driven components.
THE CRANK PULLEY DRIVES.
All other pulleys are DRIVEN.
Therefore if you put your foot on the gas pedal and increase engine rpm the rpm of all the belt driven components will also increase at the same time. Conversely if you take your foot off the gas pedal and engine rpm decreases the rpm of all the belt driven components also decreases at the same time . To understand the relationship between the crank pulley and the alternator pulley you must understand the ratio of the diameters of the crank pulley and the alternator pulley.
To calculate this ratio you divide the crank pulley diameter by the alternator pulley diameter.
Simply use a tape measure or ruler to get the diameter of your crank pulley and alternator pulley.
If your crank pulley diameter is 6 inches and your alternator pulley diameter is 3 inches then the ratio is 2 to 1(6 ÷ 3 = 2) Your alternator pulley rotates 2 times for every 1 time the crank pulley rotates.
(SAFETY TIP.....TAKE ALL MEASUREMENTS WITH THE ENGINE OFF)
If your crank pulley diameter is 4.75 inches and your alternator pulley diameter is 3.25 inches then the ratio is 1.46 to 1(4.75 ÷ 3.25 = 1.46) Your alternator pulley rotates 1.46 times for every 1 time the crank pulley rotates.
The formula for this is: A ÷ B = C ( to 1)
A (crank pulley diameter)
B (alternator pulley diameter)
C (ratio) to 1
Due to ever the increasing list of electrical accessories found on modern vehicles the general trend from the automotive manufacturers is to decrease alternator pulley diameters in order to increase alternator output at low engine speeds such as at idle or in stop and go traffic situations. On some cars the pulley is made as small as it can be while still being able to use a socket to tighten the nut that holds the pulley in place.
Here is some information about the effects of underdrive pulleys on charging system performance.
This information will be useful for people using their vehicle in a daily driver, commuter type situation.
If your car is for racing only your needs will be different and this information may not be of use.
The typical underdrive kit comes with three pulleys. A crank pulley, an alternator pulley and a water pump pulley. The underdrive crank pulley in all of the kits is of a smaller diameter than your engines stock crank pulley. The alternator and water pump pulleys vary in diameter from one brand of kit to the next and from kit to kit within brands.
Under drive or performance pulley systems work on the principle that underdriving (slowing down when compared to the stock setup) the speed at which belt driven components such as the alternator, water pump, a/c compressor and power steering pump are turned the mechanical load (drag) on the belt drive system is decreased. Decreasing drive belt system load (drag) means less horse power is required to run the belt system which means this horse power is freed up and available elsewhere (drive wheels).
To put it another way, the belt driven components are all doing work and work requires power and that power comes from the engine by means of the crank pulley and belt. By slowing down the speed at which you spin these components you reduce the amount of work they are doing thereby reducing the amount of power required from the engine. This power is then available elsewhere.
When you install an underdrive pulley kit your engine is not suddenly producing more horsepower. You are simply using less in one area so that it is available for use elsewhere. The typical claim is that these pulleys slow the belt driven components down anywhere from 20% - 30% and 10 to 15 horsepower can be freed up by installing an underdrive pulley kit as well as improved fuel economy and improved torque......all with no adverse effects.
Common Sense Check: If vehicle manufactures could increase horsepower, increase torque, and increase fuel economy by simply installing a smaller diameter stock crank pulley , which would cost them nothing extra, then these pulleys would be standard equipment on every model of every vehicle made by every car manufacturer on the planet. Just saying.
The claims sound great but as we all know nothing comes for free....
and there in lies the rub. While it takes less horsepower to spin a thing slowly than it does to spin that thing fast it is also true that the slowly spinning thing can do less work than it could if it were spinning fast. A water pump moves less water when it spins slower and an alternator will put out less power if it spins slower and an a/c compressor will produce less pressure if it spins slower and a power steering pump will provide less power assist if it spins slower.
Now depending on what vehicle you drive and how you drive it these changes might not cause any noticeable problem at all and if that's the case then that's great. Happy Driving ! If you do notice any ill effects it will usually at times when engine rpm is low such as sitting at stop lights or driving in stop and go slow moving traffic. So if you do encounter problems the information below may shed some light on why you're having charging system issues.
NOTE:
We're going to focus on the alternator and charging system from here because it's what we know.
So the word "Alternator" can stand in for any of the other belt driven components.
THE CRANK PULLEY DRIVES.
All other pulleys are DRIVEN.
Therefore if you put your foot on the gas pedal and increase engine rpm the rpm of all the belt driven components will also increase at the same time. Conversely if you take your foot off the gas pedal and engine rpm decreases the rpm of all the belt driven components also decreases at the same time . To understand the relationship between the crank pulley and the alternator pulley you must understand the ratio of the diameters of the crank pulley and the alternator pulley.
To calculate this ratio you divide the crank pulley diameter by the alternator pulley diameter.
Simply use a tape measure or ruler to get the diameter of your crank pulley and alternator pulley.
If your crank pulley diameter is 6 inches and your alternator pulley diameter is 3 inches then the ratio is 2 to 1(6 ÷ 3 = 2) Your alternator pulley rotates 2 times for every 1 time the crank pulley rotates.
(SAFETY TIP.....TAKE ALL MEASUREMENTS WITH THE ENGINE OFF)
If your crank pulley diameter is 4.75 inches and your alternator pulley diameter is 3.25 inches then the ratio is 1.46 to 1(4.75 ÷ 3.25 = 1.46) Your alternator pulley rotates 1.46 times for every 1 time the crank pulley rotates.
The formula for this is: A ÷ B = C ( to 1)
A (crank pulley diameter)
B (alternator pulley diameter)
C (ratio) to 1
These statements below are true regardless of the actual diameter measurements.
- If you decrease the crank pulley diameter ONLY and leave all the other pulleys unchanged - you have decreased the speed at which ALL the other belt driven components spin.
- If the crank and alternator pulleys are equal in diameter then 1 crank pulley revolution drives the alternator pulley 1 revolution. The ratio is 1 to 1
- If you decrease the crank pulley diameter and the alternator pulley remains the same the alternator will spin slower.
- If you increase the crank pulley diameter and the alternator pulley remains the same the alternator will spin faster.
- If you decrease the alternator pulley diameter and the crank pulley remains the same the alternator will spin faster.
- If you increase the alternator pulley diameter and the crank pulley remains the same the alternator will spin slower.
- If you decrease the crank pulley diameter AND increase the alternator pulley diameter you have effectively slowed the alternator down TWICE.
- IF YOU SPIN THE ALTERNATOR SLOWER YOU DECREASE ALTERNATOR OUTPUT.
- IF YOU SPIN THE ALTERNATOR SLOWER YOU DECREASE ALTERNATOR COOLING.
Let's look at examples assuming an engine idle speed of 700 rpm.
#1. If the crank and alternator pulleys are equal in diameter then 1 crank pulley revolution drives the alternator pulley 1 revolution. The ratio is 1 to 1
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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#2. If you decrease the crank pulley diameter and the alternator pulley remains the same the alternator will spin slower.
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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# 3. If you increase the crank pulley diameter and the alternator pulley remains the same the alternator will spin faster.
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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#4. If you decrease the alternator pulley diameter and the crank pulley remains the same the alternator will spin faster.
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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#5. If you increase the alternator pulley diameter and the crank pulley remains the same the alternator will spin slower.
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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#6. If you decrease the crank pulley diameter AND increase the alternator pulley diameter you have effectively slowed the alternator down TWICE.
Engine RPM Crank Pulley Alternator Pulley Ratio Alternator RPM
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The Bottom Line
If you believe an underdrive pulley kit really makes a noticeable performance improvement on your vehicle then do the installation and happy driving....just be aware of the possible adverse effects.
High Flow Electric Fuel Pumps require More Alternator Output.
High Volume Electric Cooling Fans require More Alternator Output.
Extreme Stereo Systems require More Alternator Output.
Super High Voltage Ignition Systems require More Alternator Output.
Auxiliary Driving Light Kits require More Alternator Output.
Underdrive Pulley Kits usually provide Less Alternator Output.
High Flow Electric Fuel Pumps require More Alternator Output.
High Volume Electric Cooling Fans require More Alternator Output.
Extreme Stereo Systems require More Alternator Output.
Super High Voltage Ignition Systems require More Alternator Output.
Auxiliary Driving Light Kits require More Alternator Output.
Underdrive Pulley Kits usually provide Less Alternator Output.
ALTERNATOR
LAMP BRUSH ARMATURE BRUSH HOLDER |
STARTER
FUSE RELAY SWITCH SOLENOID |
WINDOW REGULATOR
DIODE BEARING CLUTCH PULLEY BATTERY ISOLATOR |
WINDOW MOTOR
DRIVE BUSHING BATTERY CABLE VOLTAGE REGULATOR |
GENERATOR
BULB ROTOR FIELD COIL WIRE SUPPLY |