Underdrive Pulleys

This month I will discuss what under-drive pulleys are and how they can benefit you. I will also discuss installation of a crank under-drive pulley on my 1998 Dodge Dakota 5.2L 318 V-8.

First, let’s talk about parasitic horsepower. Parasitic horsepower is where equipment is driven off the engine to provide power for the vehicle. This includes the alternator, power steering pump, air conditioner compressor, water pump, and anything else powered by the engine. It takes some horsepower to rotate these items and that is why it is called parasitic horsepower.

By reducing the rotational speed of these items, we can free up more horsepower. That is where under-drive pulleys come into play. Under-drive pulleys reduce the rotational speed of some items. Under-drive pulleys start with a new smaller crank pulley. A complete set can include a new crank pulley, alternator pulley, and water pump pulley.

Now the question arises about how efficient will the alternator and water pump be if they are under-driven. The truth is that if you slow down the alternator, it will take a greater RPM to bring it to full charge. That is why most people just change out the crank pulley and leave the alternator and water pump alone. The manufacturer of the under-drive pulleys should be able to tell you what RPM the alternator goes to full charge. On my 1998 Dakota it is 900 RPM for just the crank pulley and 1100 RPM for the full set. So you see, under-driving the alternator too much will make more work on your battery and charging system if you idle too long. That is why I decided to use just the crank pulley on the Dakota.

Installation of under-drive pulleys is pretty simple. Just remove the old pulleys and install the new ones. Depending on what pulleys you install, you may need a smaller belt. The under-drive pulley manufacturer should be able to provide you with a part number for the new belt.

Installation on my 1998 Dakota was pretty simple. I installed a new crank pulley from ASP which required a new belt. The first step was to remove the fan and fan shroud. Once that was done, I removed the old belt and unbolted the pulley from the engine. I then installed the new pulley in place of the old pulley and installed the new belt. That’s it, nothing else to it. One note to remember when installing new belts, check the belt tensioner (if it is a serpentine belt) and replace if needed. The belt tensioner should be replaced every 60,000 miles.

After the new pulley was installed, I noticed a drop in voltage on the alternator at idle. This was to be expected. The drop is not too bad, but would have been worse had I replaced the alternator pulley. The engine revs faster and I can tell a difference in off idle response.

I hope this helps explain a little better on what under-drive pulleys are and how they can benefit you. The horsepower gain should be about 5-9 horsepower depending on the pulley set you install. Just remember that if you do the full set, you will need to not let your vehicle idle too long or you will stress your battery.

CYA next month!
Vincent Wright
sales@kmwperformance.com
http://www.kmwperformance.com