Nickshu

I bought an underdrive pulley from Tarett awhile back and have not installed it. I was staring at the box yesterday wondering if I should put it on. My 2003 996 C2 is used mostly for track use and that use is 25-30 minute sessions at PCA and NASA HPDE and the occasional open track day in the winter when the weather permits. I do drive my car to and from the track and I carry my Hoosiers on my RhinoRack roof rack and swap out upon arrival.

Keeping in mind that the reason to have an underdrive pulley is to keep the power steering from overheating can anyone tell me if this is something I should even be worried about running 25-30 minute sessions in the hot summer weather, or would it really only be a concern if I were running longer sessions such as enduros, etc?

Thanks!
Nick.

Bill Lehman

Short answer is No, you don't need it. If you notice P/S issues, add a cooler.

mark kibort

in fact, you need a lot of steering force in some of the tighter, high g turns, and if the PS pump turning too slow, (too tall of a gear selected for a turn) it can effect how much assistance you get by using a larger diameter pulley to make it spin marginally slower. i put one on the alternator, as that makes a little more sense to help with wear of the alternator.

wgn

"Worth it" is hard to say. But for a car that is mostly a dedicated track car, the accessory drive spins faster than you would like. It is designed to drive the accessories properly in a street car that typically spins low RPM (idle, 2500-4000 rpm, say). In a track car that is almost always above that, all of the accessories spin faster than they need to. Power steering just tends to be the place that causes issues, since pumping and recirculating PS fluid heats it up, and on some systems this can cause issues.

The underdrive pulley I'm used to is a smaller pulley on the drive (crankshaft), not a larger pulley on the power steering. This way, it reduces the speed of all accessories driven from the belt. Benefit: a slight reduction in power required to drive the accessories (think small increase in wheel HP), and less stress on systems that would rather not be driven at high speed for sustained periods (like power steering). Down side? I suppose if the car were used a lot at low rpm, accessories might be compromised. But unless someone drives at idle speed a lot of the time, I doubt this is an issue since the car as designed needs to function at relatively low rpm.

Difficulty of install depends on the car. In my Cayman, it was a pain in the a$$ since it's not easy to get to the drive belt side of the motor. In a 911, I expect it's really easy.

cheers,
WN

mark kibort

The gain of even a small hp benefit could absolutely be debatable. just because you are spinning an alternator or PS pump slower doesnt mean its requiring less power to spin it. this is only true with the water pump, that isnt a part of the drive belt system. for example, the PS pump and alternator both will require forces to get the job done... this changes for the load , not the speed of the accessory. the alternator is the beneficiary of a larger diameter pulley, as it spins much faster than the PS pump, so in racing applications, where the engine is at 5-7000rpm range, that reduces speeds to save bearing/ brush wear. on a PS pump that is spinning much slower, a smaller crank pulley or smaller PS pulley is much less effectve in a change.

Slakker

Definitely! On a 911, you can install it while standing outside of the car instead of removing a seat and kneeling inside of the car while installing. Lol. I'm just finishing my first Cayman project and just can't get used to working on the engine from inside of the car.

I've run underdrive pulleys on both of my 996s on the track and street and never had any issues.

wgn

[OK, I realize that to some people EVERYTHING is debatable.] But I am pretty sure that, for equal load conditions, things spinning faster require more power than things spinning slower...even if just due to bearing/belt friction. Notice I indicated "slight" reduction in power required.

Never mind that power steering pumps are essentially positive displacement units, so pump flow increases with RPM (and....uh....with discharge pressure controlled, more flow = more power). Luckily the excess pump flow is recirculated back to the reservoir so the car doesn't steer REALLY fast at high RPM, but the pump still sees the higher flow.

mark kibort

Sometimes trusting your intuition isnt the best sourse of knowledge. the reality is, the power required to drive the alternator bearings is pretty close to nothing. the power required is due to the magnetic forces and brush friction. when an alternator puts out 14volts and 20amps, thats 280watts ... about a 1/3 of a HP, that might require 1/2hp after all the efficiency losses. this is almost regardless of rotational speeds. so what you are doing with the alternator by slowing it down is just saving bearing life.
As far as the power steering pump, yes, it is a positive displacement , but if there is no load on it, the fluid is just circulating, which doesnt take much HP. plus the power steering pump is spinning much slower through less of a reduction . so, just because you have more flow, doesnt mean that reducing the flow gives you any measurable HP savings. most of the power drives the pump when under load.
the point is power is torque x RPM....in most cases, unless the pumps or alternator is turning too slow, the force goes DOWN as the RPM goes UP (aka- trade off ) giving a near constant power requirement. The net net of this is that you shouldn't expect any measurable HP gains by slowing your PS pump or Alternator down by 10-20%