Any difference between the stock aluminum pulleys vs. stock sized billet pulleys? Are the stock serpentine set even aluminum? Is there even a weight difference....why is billet more desireable?
The stock set just doesn't look good on the new engine...so my options are as follows:
1. buy new billet (stock size) pulleys
2. buy new cog pulley set but $400 is a little more than what I wanna spend
3. get the stock set powder coated

Thanks,
- Joel

 

First, the stock sheaves (pulleys) are steel, not aluminum.. That may answer a few of your other questions.

Since they are steel, there is a difference in mass. The steel sheaves are definitely going to have more mass. That isn't necessarily a problem, however. Lower mass sheaves are only an advantage on acceleration. Once the mass is spinning, there is no advantage to a lighter sheave set. Very rudimentary physics.

To me, the white metal sheaves are NOT desireable. A far superior set would be lightweight steel, not aluminum. Normal belt friction will wear the aluminum surfaces much faster than steel. You'll be replacing the sheaves (or living with the slippage) eventually.

As for the underdrive sheave systems, a car with even a 140amp alternator putting out 13.8-14.1 volts, which equates to a maximum output of 1974 watts, or 2.6471 horsepower to run the alternator. So, accounting for the inefficiencies, your alternator is only pulling 3HP at peak output (all accessories running). More often, it's more like 1.5HP at a 70A output, where a 1/3 underdrive sheave would save you only about 0.5HP. At best, you'll save 1.0 HP. If you only have a 70A alternator, you can cut those "gain" figures in half.

Actually, this is the best case for the sheaves. In fact, the alternator will attempt to output current to match the system demand at any RPM's above idle. This means that the only affect of the sheaves will be to reduce idle output from 75-100 amps to around 50 at idle. You would no longer be able to run all accessories at once without discharging the battery. Additionally, there is more drain on the alternator once you start to run the RPM back up because it has to recharge the drained battery. And the under-driven alternator will operate at a higher temperature since it is attempting to generate the same current at a lower RPM, all while the alternator cooling fan is running more slowly. Beyond that, the output frequency of the alternator will be lower, subjecting your electrical system to larger voltage peaks and less pure DC voltage at this lower RPM. Electronic control systems don't really appreciate that kind of power, and can have problems because of it. I'm sure your ignition boxes and ECM/PCM won't like the "dirty" power, either.

The power steering pump merely circulates oil under zero pressure when the steering wheel is at rest. The valving in the steering gearbox simply reverts all pumped oil back to the reservoir, so aside from nearly immeasurably small hose losses and pump inefficiencies, there is a zero net gain in running the pump more slowly. As a matter of fact, input power requirements will be higher with a slower turning pump once there is some steering input, since each pump cavity discharge will be at a higher pressure to accomplish the same amount of work.

The belt tensioner is basically an idler, and its only inefficiency is in the frictional losses from the roller bearing in the hub. If you have measurably higher frictional losses at higher RPMs, it's time to start shopping for a new tensioner, not underdrives.

The air conditioning compressor is also basically an idler until the air conditioning is turned on, so losses are minimal to nonexistent with a higher drive RPM. And, like the power steering pump, running a loaded AC compressor at lower RPM actually introduces more load, belt loss, and will cost you power. Then again, in the hunt for peak power, most of us turn off the air conditioning. And when you're sitting at idle in traffic and want the AC to work, you want it to work well.

The A.I.R. injection pumps on emissions-controlled vehicles (or those that still have them) are also basically no significant load. If your's is no longer connected or even on the engine, you have no issue with it. If yours is still operating, rest assured that in a worst case, the small volume of air being moved at the ridiculously low pressure differential will consume about 1 HP at peak engine RPM. Underdriving that by 1/3 will easily free up 1/3 HP at peak RPM on a bad day. If the diverter valve are working correctly, it should be even less than that. If you want a clue as to the power requirement, look at some of the older cars with V-belt drives, and study the size of the FHP belt used to drive them. It doesn't take a rocket scientist to realize that the input power is ridiculously low.

The one place that can save power is the water pump. If you have an LT1/4 engine, that is not an issue, since the pump is camshaft driven and cannot be altered. With any other belt-driven pump, however, pump input power is nearly directly proportional to the input RPM up to the point of pump cavitation. So an argument can be made that the best place to install a larger sheave is on the water pump. Physical space limitations can only allow a slight increase in this size, so the gain you might realize would be relatively small. And given that a water pump at full flow will require about 3.0 HP, the 1 HP maximum gain realized will most likely be outweighed by the problems created.

It would make far more economic and performance sense to install an electric water pump drive. The pump can them be operated at a constant speed, or the clever enthusiast can install a variable speed drive based on coolant temperature, not engine RPM. The real beauty of this system is that even at idle RPM, the engine can be cooled quickly by increasing pump speed. Conversely, at higher RPM, the power saving would be far greater than the underdrive sheaves could ever allow. For those who cannot bring themselves to abandoning the belt driven water pump concept, you could always use the old trick of cutting down the impeller vanes on a stock water pump. The unfortunate part of that is that at low RPMs, overheating is very likely with any kind of load whatsoever.

Consider the fact that thousands of qualified engineers at many large auto manufacturing corporations, with limitless education and resources, test facilities, test vehicles, and time to experiment have been trying to increase fuel efficiency of their fleets for over ten years to improve the CAFÉ results and save money for their employers (not to mention sell more vehicles and make themselves look good). Belt accessory drives are relatively easy to change and redesign. Obviously, we can go the aftermarket and already buy a variety of drives. These people specify and MAKE the drives by the millions every year. They change them as necessary to accommodate all manner of layout and design changes, additions, new accessories, and the like. Doesn't it stand to reason that they would experiment with the drives to provide the most output power for the least energy input? We want the same thing, but for different outcomes - more peak power. The concept is the same. They have carefully chosen the drive sizes to accomplish the task without sacrificing anything. Re-engineering that system makes little sense, especially when the exercise would result in operational problems and a 2.0HP gain for all that time and effort (and cash). Instead or reinventing the wheel, we should be inventing a replacement (like an electric drive). These companies have gone so far as to reduce the oil pump volume to the point of barely holding pressure at idle for a few pennies in fuel saved. To me, risking engine protection for that minimal saving is ridiculous, but when a manufacturer delivers millions of cars, it all adds up, and the warranty risk is theirs to take. It's easy to reason that they would have done the same thing with the accessory drives if there were ANY benefits. ANY.

If you really cannot justify the expense of an electric pump drive for a few horsepower gain, then there is even less of an excuse to install an underdrive sheave. The attraction is that the companies that market these parts exaggerate the power gain claims to a best-case scenario, and make the conversion so easy that even the most mechanically challenged can accomplish the exchange. Then again, those same challenged people probably haven't read this far, not being able to get past the "long division" of calculating a 33% gain.

For those who have made it this far, I'll make this simple. If you have the mechanical ability to change your drive sheaves in an attempt to make more peak power, and the wherewithal to spend £100 or more on the parts, you can easily handle changing your own oil and oil filter. Fair statement? Instead of a best-case potential of 2-3 HP at peak RPM, save your money and buy five quarts of synthetic engine oil and a decent filter for $20.00. You'll get a verifiable gain of 6HP, less engine wear, and better fuel efficiency at all RPMs. For the cost of one set of underdrives that will wear out in 30,000 miles, you can buy at least 20,000 miles worth of synthetic oil changes and enjoy a better power advantage all along the way. At the end of that time, your engine will have worn less, and the fuel savings will allow you to purchase another couple of synthetic changes. Best of all, you won't have to fight a slipping belt from a worn sheave for the next 10,000 miles before you spend another $100+ to repair the problem, again temporarily.

If you want the best of both worlds, lighten a good, unworn stock set of sheaves, then powdercoat them in your favorite color.

That's just my opinion, though.