Underdrive Pulley?

[Jenevive]

Will an underdrive pulley really make much of a difference for my power?

I'm not on the market to purchase one right away, I need to figure out my current car situation a little more. I've got a 1994 Toyota MR2 Turbo, but I'm on the market for a circa 1992 Maxima or Talon TSi. Just not sure what I'm going to go with yet, still weighing out all of my options. But I'd like a little more info on what exactly type of difference the purchasing of an underdrive pulley will make.

Thanks!

[04Mazda6s]

Underdrive Pulleys add horsepower by reducing the drag placed on the engine to drive the accessories such as the air conditioning, power steering and water pump. In an effort to increase horsepower the pulleys slow down these accessories to allow the engine to turn with less effort. The pulleys have no adverse effect on the operation of A/C, power steering, charging or cooling systems and installation is simple and straight forward.

[ljasonl]

If you're talking about a crank pulley, I'd really recommend sticking with the stock pulley. Dyno tests on Subarus have shown, among stock sized lightened pulleys, 1 hp at the wheels was the biggest gain for any brand. I'd expect similar results for most other imports.

If you're one of the people who just want the answer and don't care about the why, skip this next part and go straight to the end. It's long and uninteresting unless you're a nerd like me. ;)

The stock crank pulley is designed to be a harmonic balancer, AKA it cancels out vibrations. I'm not gonna pretend to have more than a basic understanding of harmonics, cuz that's and entire physics class in itself. But here's a basic explanation. You have 1st order, 2nd order, 3rd order, so on harmonics. If you drop a rock into a bathtub right next to the wall. You'll see the waves go across to the other wall, and bounce back. When they get back to the 1st wall, they'll bounce back, you drop another rock in the same spot. Now you have big, fast waves from the rock you just dropped, and small slow waves from the earlier rock. The big waves are 1st order, and the small ones are 2nd order. Every so often, the waves will catch up and occupy the same spot, since they're moving at different speeds. This makes a harmonic spike to a much bigger wave. Your crank has all these little vibrations going through it too, from all kinds of different sources, but mainly the pistons and connecting rods. At one end of the crank is the flywheel, which if stock probably weighs like 40 pounds, at the other end you have the crank pulley, which only weights 5 pounds or so. The flywheel, having so much weight rotating around it, smooths out the crank on that end and makes it move at a constant rate. On the pulley end, the vibrations can become really bad, and the bearings can become worn, or the crank can literally twist in half. Manufacturers spend hundreds of thousands of dollars trying to find the exact right combination of weight for the crank pulley and how it should be placed within the pulley to cancel out the vibrations and twisting. The idea is to move the spikes (remember, where the waves converge) into an area of the RPM range where they won't damage anything, either very low in the RPM range, or way up beyond redline. Pulley manufacturers don't test their pulleys for this, not even AEM, and they're known for pulleys. The equipment is just too costly.

Short version - a couple horsepower is not worth the risk, especially when you can get more power from the same money in other parts. Change all the other pulleys if you like, but leave the stock crank pulley.

[Triepsyn]

Good observation, Jason. Most people go with an aftermarket, although the stock setup is sometimes the way to go. In this case, you should probably go with the stock setup. Thanks for the long-winded reply Jas, sometimes people need a thorough explanation.

Now, if you want to set free some serious hp (Not like 100 or anything), I'd say check out an aluminum flywheel (Maybe by Fidanza). They boast power gain equivilency of taking out your two front seats.

[04Mazda6s]

Originally posted by ljasonl@Oct 27 2003, 08:27 AM
The stock crank pulley is designed to be a harmonic balancer
I beg to differ. "Harmonic Balancer" is a term that is used loosely. Technically, this type of device does not exist. The "balancer" part comes from engines that are externally balanced and have a counterweight cast into the damper, hence the merging of the two terms. None of the applications that we offer utilize a counterweight as part of the pulley as these engines are internally balanced. The pulleys on most of the new import and smaller domestic engines have an elastomer (rubber ring) incorporated into the pulley that looks similar to a harmonic damper. The elastomer in the OEM pulley serves as an isolator, which is there to suppress natural vibration and noise from the engine itself, the A/C compressor, P/S pump, and alternator. This is what the manufacturers call NVH (Noise Vibration & Harshness) when referring to noticeable noise and vibration in the passenger compartment. It is important to note that in these applications, this elastomer is somewhat inadequate in size, as well as life span, to act as an effective torsional damper. If you look at the pulleys on some of the imports there is no rubber to be found at all. Acura/Honda, the Nissan Altima, 1.8L Eclipse, 2.3L Fords, Chrysler 2.2L's, and 1.8L VW's, to mention a few. This is not to say that with the pulleys you will hear a ton of noise or feel more vibration from your engine compartment. Most who have installed and driven a vehicle with pulleys will notice the engine actually feels smoother. This is a natural result of replacing the heavy steel crank pulley with a CNC-machined aluminum pulley. NVH is variable and unique to every car. NVH will increase with the installation of an aftermarket intake and/or exhaust, for example. Think of OEM intake systems in newer cars, they use baffles and resonators in the intake to quiet all the intake noise. Aftermarket intakes eliminate these resonators and create dramatic increases in engine noise from the throttle opening and closing.

The purpose of a traditional harmonic damper is to protect against crank failure from torsional movement. This is not necessary in most modern engines because of the many advances in engine design and materials. Factors such as stroke, displacement, inline, V configurations, power output, etc., do determine when and how these harmonics and torsional movements occur.


"Pulley manufacturers don't test their pulleys for this"
I do know for a fact that Jet Performance (owner of the ever so popular Jet Chip) does test their pulleys in all aspects.


"On the pulley end, the vibrations can become really bad, and the bearings can become worn"

This is a fear many prospective owners have and is a valid concern since we are dealing directly with the rotating assembly. Fortunately it is another urban myth with no basis in fact. The fact is that pulleys have the opposite effect on engine bearings. The combination of tight tolerances, quality control, perfect balance, and dramatic weight loss versus the stock pulleys reduces stress loads on your engine, extending the total service life you can expect from your engine. Engine bearing problems are purely associated to poor engine maintenance, use of heavier than factory recommended oils, improper engine building practices (which includes poor balancing), excessively revving engines when they are cold, and owners expecting their factory oil pumps to handle engine power outputs beyond 3, 4, even 5+ times the stock power levels.

I have underdrive pulleys on my crank and alternator from JET (on the camaro) and have had them for over a year and I have had nothing bad to say about them. Fuel economy increased a little, and throttle responce is a little more sensative. All in all I bought them for $55 off ebay and am very happy with them.

[ljasonl]

In test results on the Subaru 2.5, the stock crank pulley shows harmonic noise over the entire rev range, but it's fairly low. Testing with an afterkarket pulley actually showed it to be smoother than the stock, way less noise everywhere... except for one spot just before redline, where there was a huge spike. Going by the "feel" of the smoothness, you would probably never notice this. Anyways, the spike was quite a bit larger than anything from the stock pulley, but still within the factory specs, although pretty close to the limit. Further modding of the car to make more power could easily have pushed it over the limit. And the Subaru engine has one of the shortest cranks out there, being a flat 4 it's only 2 "cylinders" long. On a longer crank, the vibration would be worse.

[04Mazda6s]

i suppose i can buy that but thats with the extreme smallest. As far as this 'spike' you speak of, till today I've never heard of that

[Jenevive]

Underdrive Pulleys add horsepower by reducing the drag placed on the engine to drive the accessories such as the air conditioning, power steering and water pump.

Thats what my buddy told me.

I'll check into it more later, thanks guys for all of your help. :D

[7thGenVic]

Pulleys are part of the accessory drive system on you engine. They provide power to the power steering pump, water pump, air conditioning compressor, and alternator. These are driven via the serpentine belt from the crank pulley.

Underdrive pulleys free up power in two ways:
1) Changing the rate accessories turn
2) Reducing rotating mass

The biggest gain we will see comes from number 2, reducing the rotating mass. There is a finite amount of power that an engine produces. This power has to do a number of things; turn the accessories (above), drive the car, turn itself far enough for the next combustion cycle to take place. By reducing the amount of power that goes into turning the accessories, you make more available to drive the car.

[Triepsyn]

There is an average of 2.7 HP gained from every pound lost off the crank shaft. With the lighter accessory pulleys, as much as 15% to 30% more power can be found. 85% of gains are from weight loss. Rest of the gains are from underdriving, which accounts for about 15% of the total gains. A small amount of underdrive is used to keep the charging systems, air conditioning, power steering, and water pumps moving fast enough for every day driving.

:thumbup:

[Maliwho97]

You've got to watch for some of them though, if they adjust your ratio too much, you will lose RPM's on your alternator pulley, and it won't charge as well.