What's the source of p/s overheating?

[69427]

What's the prime cause of overheating the power steering fluid? Is it the pump vanes, the fluid squeezing through the bypass valve, the steering rack valve/orifice, something else, or all of the above? I'm just trying to get a better understanding of the physics behind this. If it's the pump speed and the internal pump flow (and not the rack) then I would expect that land speed racers would also be boiling fluid (assuming no cooler). On a road course a lot of us have boiled the fluid, so is it the repeated action of the fluid being squeezed through the rack control valving, or again, the pump running at high RPM? What's the main source/cause of the heat?

Thanks.

 

[phantomjock]

Fluid under pressure = Heat That is a fact!

Work Generates Heat, so maybe:
Rapid movement of the steering wheel, on the track/autocross?
Rack and pinions more likely a problem?
With/without Hydroboost?

Just a few ideas - sorry no definitive "I know the answer for you."

Cheers - Jim

 

[BBShark]

That's a good question. Wish I had an answer. If I were to guess, I think it must have something to do with the bypass valving. I say this because some C4's had PS pump coolers built into the line. It wasn't much more than an extra long line with some fins on it.

 

[JIML82]

The combination of high engine/pump rpms along with a full lock turn will quickly raise the power steering fluid temperature. A full lock turn basically blocks any fluid flow through the system and causes the pump to develop relief pressures as 100% of the high pressure fluid is recirculated inside the pump. The pump relief pressure (roughly 950/1000 psi in a Corvette P-pump) along with just a small amount of fluid being recirculated can cause the fluid temperature to quickly exceed 300F.

What you want is a power steering pump cam ring that has limited rise and has lower fluid displacement. Most people think that the Corvette power steering pumps were basically the same (except for the reservoir) through the years. This is not true. The displacement (output per revolution of the pump shaft) varied through the years. You want a low displacement pump!

Corvette Model Year - Displacement
1967 through1969 - 1.75 gallons/min
1970 through 1973 - 1.60 gallons/min
1974 through 1981 - 2.90 gallons/min
1982 - 1.90 gallons/min

What you want is a low displacement pump not the 1974-81 type. I doubt whether any of the remanufacturers actually build replacement Corvette pumps with different displacements for different years.

Here is a rule of thumb for original Corvette pumps:
All Corvette pumps with nut retained pulleys (male thread on the driveshaft) had between 1.6 and 1.75 gpm displacements (except 1974). All pumps with pressed on pulleys (smooth pump driveshafts) had 2.9 gpm pumps (except 1982.)

I did find this information: Any Chrysler minivan built before 2000 will have a Saginaw P-pump with a low displacement 60 series pump ring. That ring will be a direct replacement for the Corvette ring.

You could also contact steering system experts like Tom Lee who may have direct information as to reducing steering system temperatures under competitive conditions.

A low displacement ring should help to reduce steering fluid temperatures. You can also consider a return line cooler for your system. Also, it might be good to use genuine GM power steering fluid in your system.

Jim

 

[69427]

Thanks for the information, Jim. It's greatly appreciated.

What prompted my curiosity was that I recently put a smaller OD crank pulley on my '69, and while thinking about all the side effects on the accessories, I was hoping that this would reduce the p/s fluid temp a bit on track days. I've got a C5 fluid cooler sitting on the shelf but I haven't gotten around to installing it yet. I'm running a C4 suspension with a (Unisteer) Corvette style rack and an '89 Grand Am pump. I have no technical details on the pump. I chose it because it fit my requirements: Light weight, rack compatible, light car compatible, and V-belt pulley. So far it works like a champ, although it did belch a small amount of fluid on a 99* track day last July. (I do suspect I overfilled the reservoir a touch when I first plumbed the system. I've since siphoned a bit out.)

I still need to cut and modify the p/s cooler bracket so I can swap the direction that the hoses connect to it.

 

[TimAT]

In my world (big airplanes), high hydraulic temperature is usually traced back to bad actuator. It leaks thru the valving under high pressure. I've seen some of the actuators actually turn blue from the heat. But we cheat too- the hydraulic systems all have temperature gauges and coolers. SOme of the planes have a minimum fuel requirement just to run the hydraulics- the cooler is in the fuel tank and it has to be covered with fuel to work.

 

[Stroker-427]

Phisics....

The efficiency of the pump is of course less tahn 100%..... so the oil will be very hot!

Even if you don't steer at all!

 

[JIML82]

The valving in a power rack and pinion steering gear should not cause any higher system temperatures than the bigger size valve in the Saginaw 800 recirculating ball gear. Note that the new Saginaw 600 recirculating ball gear is advertised with state of the art "rack and pinion valving."

If you overfill the pump reservoir, the fluid expands with higher temperatures and the fluid has to go somewhere. The Saginaw production reservoirs are designed with adequate air volume on top of the fluid take accommedate the expansion.
Jim