I'm interested in underdriving my power steeering pump. I've looked around and it doesn't seem like anyone makes a larger diameter power steering pulley, instead they underdrive the crank pulley. The PS pulley has a splined hub and is retained by a nut. I was looking at some OEM pulleys on line and noticed that there might be some larger pulleys out there from stock vehicles that could be used as long as they have the same type of hub. Has anyone tried this? If so, what vehicle was the pulley from? Tanks in advance!

 

FWIW, Unorthodox, who makes underdrive pulleys, attributes 85% of any hp gain to weight loss, not underdrive. Apparently that's why they underdrive at the crank to maximize weight-loss there. If that's accurate, and since the PS pulley appears to be some type of plastic, I'm not sure there is there much to gain.

 

The rotational inertia (or rotational inertia change possible) of something that small is very small. Plus, in dyno work, acceleration is usually quite low, and readings are usually taken under pretty-much steady-state conditions, so weight or rotational-inertia effects are essentially negligible.

So, I do not understand their claim. I think it is pure marketing hype.

OTOH, power (and rotational-inertia) gains from not driving larger and heavier power consuming attachments (WP, alternator, etc.) too fast are VERY measurable under most conditions.

So, the bottom line, IMO, is that a given % of underdrive, however accomplished, will achieve essentially the same results.

 

I have no background in this area and certainly not going to argue. I was just passing it on and qualified it as a "FWIW".
From Unorthodox.....
These gains are obtained in two ways. First, and most important, is the weight loss. 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 our gains are from weight loss.....The rest of the gains are from underdriving, which accounts for about 15% of the total gains. We do not push underdriving for two main reasons. Our main reason is to keep charging systems, air conditioning, power steering, and water pumps moving fast enough for every day driving. Second, not much horsepower is gained from extreme underdriving.
All of our underdriving is done with the crank pulley, not the accessory pulleys (most applications). This is done so maximum weight loss can be achieved at the crank.....

 

Well, to clarify my reasoning...

2.7 HP gain per # lost really is a mis-statement. On the crank, in steady state, there will be no HP gain on the dyno just changing pulley weight. However, especially in 1st gear, there would probably be an acceleration gain due to the reduced rotational inertia and reduced mass. That acceleration gain under certain conditions (high HP setup, so very rapid acceleration in 1st), might possibly be equivalent to the increased acceleration noted with a 2.7 HP gain, so maybe they are stating it that way to impress the masses.

Having said all that, ANY reduced rotational inertia and weight will improve performance, but the difference may not be worth the expense, unless it is on an all-out race car.

 

I'm interested in a PS underdrive too. Not so much for the HP gain, but to change the feel of the PS a bit. It's too light for my liking, but I don't want to remove it or loop the lines.

 

That is my intention as well Aaron. I'm looking to slow down the ps pump to tame the steering, not gain hp.

 

This commentary is based on my knowledge of other PS systems, not specifically on knowledge of the FD PS system. However, here is what I think:

I would be worried that slowing down the pump will not have your desired effect. Most, if not all, power steering units get their "feel" or "assist %" by the calibration of a torsion bar or similar in the unit. A certain amount of torque on the steering column starts to open a hydraulic valve which then feeds pressurized oil to the rack, assisting steering. More torque opens it further. The pump usually bypasses the excess oil back to the reservoir, or otherwise limits the oil's peak pressure. All slowing the pump down, IMO, would do would be to limit the SPEED at which the unit can respond, but will not reduce its assist level.

So, be sure you really understand the system before you proceed.

EDIT: In addition, IIRC, the FD system REDUCES assist as RPM rises, so slowing the pump may actually have the reverse of the effect you want.

 

hmm, ok. I don't know a lot about PS so I assumed it would help. What would you suggest to do to tighten up the feel without removing or looping the lines?

I would go full manual, but the car is a daily driver so I would rather not...

 

To be honest, my last post pretty much exhausted my knowledge on the subject. Maybe someone else has done something to do what you want. If I were to guess, you might have to make a new torsion bar (make it stiffer) for less assist. But, since I don't really know the FD system, I could be completely off base. I even did several brief searches to get smarter on the FD system while I was writing my previous post, but I found nothing useful.

Dave

 

I've seen it demonstrated on the dyno that putting a big brake kit on a front wheel drive car will reduce horsepower due to the increased rotational mass.

It was an Acura, and it cost about 5hp at the wheels IIRC.

So how is this different than weight change on the crank?

 

It's not conceptually different. But once stuff is moving, and not accelerating, the rotational inertia (rotating mass) does not, in itself, consume HP. Unless this dyno was measuring HP during rapid acceleration, something else was going on, like increased brake pad drag, air-pumping HP losses due to increased fin area on the brake disks, etc.

The key to whether rotating mass is important is how far it is from the axis of rotation (i.e., what is it's rotational inertia), and how quick does it have to accelerate. While heavy brakes will slow the car due to having to accelerate and decelerate them, due to their mass alone they will not cost HP at a steady speed, and very little HP in higher gears due to the low acceleration rate.

It's similar in concept to a flywheel. The heavier and larger in diameter the flywheel, the harder it is to accelerate, and the longer it takes to slow down. But, once spinning, it takes very little energy to keep it spinning, as long as there is little aero or bearing drag.

 

I did find some descriptions of the FD PS system which stated that there is a "quill" or "reed" that is essentially a torsion bar like I described. And, also, the "speed sensing" is based entirely on the RPM of the PS pulley. So what I said before was correct. Here is a thread that offers a bit more insight:
https://www.rx7club.com/showthread.p...ng+rack+column

Dave

 

Gordon,

Thanks for adding some less vague suggestions to this thread. Very interesting.

I don't understand what you mean by "flywheel." Could you explain that more, plus state what effect you would like it to cause?

Dave

 

Just remove the power steering all together, and install a manual rack. Problem solved.

 

aaron_bc said he didn't want to go full manual, he liked PS for daily driving - that's why all this discussion.

Dave