Iighter steering/higher pressure pump
08-02-2014, 10:32 AM
#16
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Short answer: flow. Pressure comes with flow. I'll try to explain:
I speak without experience on 928. But I have experience tweaking and finding the limits of power steering, mainly in racing.
The rack is a valve that opens more the more muscular force you make on the wheel. In fact, it is related to a little "twist" angle that you can feel if you turn all the way to one end, with the wheel in the air. When the wheel hits the stop, you can still turn some 5 or 10 deg the steering, while you listen the max pressure valve open and the idle drop a little.
The system is open center, meaning no force in the wheel, all flow to tank. When the valve "opens", the return to the tank is closed progressively as the feed to the rack opens progressively in the correct side.
1. If you change the rack, you change the ratio between rack force and hand force in every situation. This is what we need.
2. If you increase the flow, (smaller pulley, bigger pump, higher revs...) given the same amount of "opening" of the valve, the pressure drop in the semi-closed return to tank would be greater, so the rack force would be greater too. But the pressure loss in the restrictor would be higher, too (this is its function, to keep assistance more or less stable with rpm); so this mod alone is not very promising. You will be heating the oil more, and using more HP trough the steering pump belt, what could decrease its life.
3. If you decrease the restriction in the feed line, the rack force will also be greater, because the pressure loss would be less, but it will be more noticeable as flow (rpm) increase. So you will have a little bit more force at idle and a lot more at 3000rpm.
2. + 3. If you increase the flow and reduce the restriction, it will have noticeable more force at idle and a lot more force at speed. Good if you like it like this, maybe too little feedback at speed.
I hope it is more clear now what I mean when I say GTS rack is the way to go, keeping all other hardware undisturbed: That being said, pump and/or pipe would help if you don't mind hotter oil or light steering at speed. (personally I think light steering at speed is not a problem unless you are looking for the grip limit in the front tire, which in open road it takes place long after you are making a fool of yourself).
I speak without experience on 928. But I have experience tweaking and finding the limits of power steering, mainly in racing.
The rack is a valve that opens more the more muscular force you make on the wheel. In fact, it is related to a little "twist" angle that you can feel if you turn all the way to one end, with the wheel in the air. When the wheel hits the stop, you can still turn some 5 or 10 deg the steering, while you listen the max pressure valve open and the idle drop a little.
The system is open center, meaning no force in the wheel, all flow to tank. When the valve "opens", the return to the tank is closed progressively as the feed to the rack opens progressively in the correct side.
1. If you change the rack, you change the ratio between rack force and hand force in every situation. This is what we need.
2. If you increase the flow, (smaller pulley, bigger pump, higher revs...) given the same amount of "opening" of the valve, the pressure drop in the semi-closed return to tank would be greater, so the rack force would be greater too. But the pressure loss in the restrictor would be higher, too (this is its function, to keep assistance more or less stable with rpm); so this mod alone is not very promising. You will be heating the oil more, and using more HP trough the steering pump belt, what could decrease its life.
3. If you decrease the restriction in the feed line, the rack force will also be greater, because the pressure loss would be less, but it will be more noticeable as flow (rpm) increase. So you will have a little bit more force at idle and a lot more at 3000rpm.
2. + 3. If you increase the flow and reduce the restriction, it will have noticeable more force at idle and a lot more force at speed. Good if you like it like this, maybe too little feedback at speed.
I hope it is more clear now what I mean when I say GTS rack is the way to go, keeping all other hardware undisturbed: That being said, pump and/or pipe would help if you don't mind hotter oil or light steering at speed. (personally I think light steering at speed is not a problem unless you are looking for the grip limit in the front tire, which in open road it takes place long after you are making a fool of yourself).
Last edited by littleball_s4; 08-02-2014 at 10:59 AM.
08-02-2014, 10:49 AM
#17
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Do you know what could be awesome?
A pressure switch in the pump pressure line that sent 12v to the ISV to open it fully if we are working hard on the steering wheel. 99% of my complaints of heavy steering is when the car is idling and stationary (parking). Rising the idle to, say, 1400rpm would help in that situation as much as doubling the size of the pump.
Maybe doable for those of you programming custom brains.
A pressure switch in the pump pressure line that sent 12v to the ISV to open it fully if we are working hard on the steering wheel. 99% of my complaints of heavy steering is when the car is idling and stationary (parking). Rising the idle to, say, 1400rpm would help in that situation as much as doubling the size of the pump.
Maybe doable for those of you programming custom brains.
08-02-2014, 09:08 PM
#18
Nordschleife Master
I think it should be pretty easy to make the original pump put out more pressure, as I understand it is just a spring/ball operated pressure relief valve.
But these ZF pumps look very similar so I will check if another pump fits, so I do not risk damaging my original pump.
Will keep you guys posted :-)
But these ZF pumps look very similar so I will check if another pump fits, so I do not risk damaging my original pump.
Will keep you guys posted :-)
08-03-2014, 01:19 PM
#20
Chronic Tool Dropper
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As Littleball says, the secret to different wheel effort is ultimately the little torsion spring inside the spool valve in the rack. This spring determines the wheel effort needed for moving the two sections of that valve, with more displacement meaning higher assist. I haven't had the pleasure of taking a 928 rack apart (yet). Perhaps one of the steering rack gurus could comment on whether it's possible to get to that spring and do a little trimming on it.
FWIW, GM spool valves used to be available as a separate item. You could buy different valves from different cars that offered different steering feel. These mounted in the "steering gear", a worm-and-sector box to which tie rods attached. In their wisdom, GM used just a few different-size steering gear boxes across the whole fleet, with each car fittied with a spool valve appropriate to the "class" of car. A little creative fitting could make a big car feel like a GT, or a Chevy-II steer like a Coupe DeVille.
FWIW, GM spool valves used to be available as a separate item. You could buy different valves from different cars that offered different steering feel. These mounted in the "steering gear", a worm-and-sector box to which tie rods attached. In their wisdom, GM used just a few different-size steering gear boxes across the whole fleet, with each car fittied with a spool valve appropriate to the "class" of car. A little creative fitting could make a big car feel like a GT, or a Chevy-II steer like a Coupe DeVille.
08-03-2014, 02:46 PM
#21
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