Undersized drive pulley issues

[SVXRide]

Originally Posted by benebob

snip* Just think, if all us car nuts had physics questions like this we'd all be rocket scientists like Bill.

-Bill

[dromano]

Originally Posted by dcarrb

As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

I have an Unorthodox Racing billit aluminum under drive crank pulley, the marketing pitch is: the billit aluminum pulley weighs 5 lbs less than the oem pulley, the gain in power to the wheels is in the weight savings of the pulley.
Plus mine is a cool anodized red

[SpoonChucker]

The gain is in efficiency, which allows more power to be transferred to the wheels. The simple equation is that K=1/2*I*w^2, where K is kinetic energy, I is the polar moment of inertia, and w is the angular velocity. Furthermore, I=1/2*m*r^2 for a cylinder rotated about its axis, where m is the mass, and r is the radius. So the diameter and mass removed from the outside of the pulley (cross-drilled pulleys) have a larger effect on the energy than just the weight reduction. This is however is energy and not power, power is energy per time, so its effect will only be noticed during acceleration. The effect on the power steering and alternator is function of the angular speed of the crank pulley (ratio of the diameters) so if the diameter is held constant the performance of the power steering and alternator remains unchanged.

[b3lha]

Originally Posted by SpoonChucker

The gain is in efficiency, which allows more power to be transferred to the wheels. The simple equation is that K=1/2*I*w^2, where K is kinetic energy, I is the polar moment of inertia, and w is the angular velocity. Furthermore, I=1/2*m*r^2 for a cylinder rotated about its axis, where m is the mass, and r is the radius. So the diameter and mass removed from the outside of the pulley (cross-drilled pulleys) have a larger effect on the energy than just the weight reduction. This is however is energy and not power, power is energy per time, so its effect will only be noticed during acceleration. The effect on the power steering and alternator is function of the angular speed of the crank pulley (ratio of the diameters) so if the diameter is held constant the performance of the power steering and alternator remains unchanged.

So you are saying that it is better to keep the same radius of pulley but make it lighter. Particularly towards the outside. What about the slave pulleys. Surely the same should be applied to them also?

How much gain in power to the wheels are we talking about here, percentage wise? We're splitting the power from the engine between the alternator/steering and the driveshaft. But what is the initial ratio? Surely the force needed to accelerate the pulley is insignificant compared to the force needed to accelerate a 1.5 ton car. I'm confident that I could spin the alternator and power steering pump with one hand. But there's no way I could turn the driveshaft and make the car move by hand.

Suppose the force needed to turn the driveshaft was 50 times the force needed to turn the accessories (just a random figure). If you could reduce the force needed to turn the accessories by half (quite a lot), then would you gain 1% in power? Or am I over-simplifying the situation? Obviously I'm just making these numbers up, but it seems to me that the gain in actual performance of the car must be very very small.

[svxhunter]

Sure you're oversimplifying it! Spoonchucker is not!

The slave pulleys don't have exactly the same effect- they are not generating any power, they are only being driven. They are not contributing to power gains, they are detracting.

Rotational mass is a fairly significant consideration in engine design. That's why high performance engine internals are both light and strong.

Lightened flywheels and wheels are the 2 easy after market changes that we all can do and feel the difference.

No one has ever had huge gains from a lightened pulley, or we'd all know about it and be running out to buy them. But it is rotational mass, and it does make a difference, even if it is a small one.

I run a lightened underdrive pulley on my SVX racer. We think it makes a difference, but we have no dyno tests, etc. to prove it. I run a lightened flywheel on my 5 spd. I KNOW it makes a very large difference.

[McTaff]

I think that there also needs to be a clear distinction on what you are trying to do.

As I pointed out in my original post in this thread, if you change the size of the crank pulley, you are changing the ratios for ALL of your pulleys, and thus you get a greater gain across the board. However, this lessens the amount of revolutions transferred to the Power Steering, Air Con (if any) and Alternator.

If you change the size of one of the driven pulleys, then you only change the ratio to that device (but for a lesser gain).

Ideal setup would probably be: lightened Drive/Crank pulley, smaller and lighter Air Con, lighter Alternator and (slightly) smaller and lighter Power Steering.

As stated before, the lightening would probably do little, and the change in size with the pulleys would help a little. To be honest, although it is fairly cheap, there are probably better things to spend your money on elsewhere in the engine bay? I'm not sure how much custom pulleys would cost you in the US of A, but here they aren't cheap.

[benebob]

Originally Posted by svxhunter

I run a lightened underdrive pulley on my SVX racer. We think it makes a difference, but we have no dyno tests, etc. to prove it. I run a lightened flywheel on my 5 spd. I KNOW it makes a very large difference.

Yeah I agree it makes some difference, even if it is just "feel". I think ours though is making the difference the other way with issues with the PS but if we can solve that we'll keep it. Every pound helps.

[SpoonChucker]

For mine I kept the radius the same, I used one from Cobb Tuning (weighed in at 1.25 lbs.). It is difficult to quantify any gains, a typical dyno plot of hp versus rpm would look identical to one with a stock pulley. In theory, a plot of hp or rpm versus time would be shifted to the left (quicker acceleration). I do not notice much improvement from the pulley alone.

Companies such as AEM have made lightened power steering pulleys for the WRX. It’s not a bad idea, but like it was mentioned earlier the improvement isn’t much and might not be worth the cost. When I get some more time, I’ll design and mill out a set (alt/ps).

[UberRoo]

I wish my power steering pump was electric like the XT6s. If I crank the wheel too quickly in a parking lot, I run out of pressure. I can actually feel the pump playing catch up. This only happens when my foot is off the throttle. If the engine is over about 1500 RPM the pump seems to be able to keep up with me. Very annoying though - especially when you think you're going to easily make a turn and avoid colliding with something very expensive, but suddenly learn that you're going to have to forgo the luxury of a phone booth to change into your spandex and cape, and instead summon all of your Herculean strength on the spot to avoid missing said unspecified expensive object with only inches to spare.

The problem: The 1500 RPM 'sufficient speed' is almost twice the idle speed. It would require a significantly undersized power steering pulley or oversized crank pulley to fix my problem. The power steering pulley would probably not be adequate to handle the power, or the crank pulley would cause the alternator to spin so fast you could build a housing around the alternator cooling fan and use it as a supercharger. That, or the centrifugal force would make the alternator windings explode and fill the engine compartment with copper spaghetti.

[benebob]

Originally Posted by UberRoo

I wish my power steering pump was electric like the XT6s.

More incentive to get on the drawing board . I too love the 6 steering! Just not the cost of the liquid gold fluid it requires.

[Beav]

Ubie:

It sounds as though you might first try flushing (or having it flushed) the p/s system. Old fluid and/or gunk can cause that feeling. Also, a restricted line or a failing pump could do the same. Flushing isn't just changing the fluid in the reservoir, it includes using a cleaner designed for the job followed by several exchanges of clean, new fluid. It takes about 20-30 minutes with the right equipment or an hour or so (and probably a bit of a mess) to do at home.

[UberRoo]

Originally Posted by Beav

It sounds as though you might first try flushing (or having it flushed) the p/s system.

There's probably a lot of truth to that. I've had the same problem with quite a few vehicles, so I suspect the primary cause is that I simply have a tendency to flick (or attempt to flick) the wheel from lock to lock more than the average driver. ...but any improvement at all would be worth something. With your suggestion, the current problem is now a matter of apathy.