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Thread: Question about the 911E hydropneumatic suspension

  1. #1

    Question about the 911E hydropneumatic suspension

    As many of you know, I'm currently selling a very original 1971 911E over on BaT.

    https://bringatrailer.com/listing/1971-porsche-911e-10/

    As expected, I'm getting a lot of interest in the car and I'm doing everything I can to be transparent about the car's condition and to answer questions as accurately as possible.

    One question sent to me has me stumped. The car was recently in for service at Eagle Racing and Steve verified for me that the car is still running it's 911E suspension. I just had someone ask me "With the hydropneumatic suspension if the car sits a few days does it settle in height? Then pumps itself up to driver height once driven again?"

    I'll be honest in that I've only driven the car 300 miles in five years so I'm not as familiar with this car as some as my others but I'm really confused now. Is that what is supposed to happen? If so, how much does it settle and rise? And how does it "pump itself up"?

    Would appreciate insight from anyone who is familiar with the E suspension setup.

  2. #2
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    The 72E that I had used the struts you speak of ( front only) and they had long since lost their ability to self level.Although I think the feature was only designed to compensate for load carrying in the trunk (a common thing with 911 cars)

  3. #3
    Quote Originally Posted by LiveFromNY View Post
    One question sent to me has me stumped... I just had someone ask me "With the hydropneumatic suspension if the car sits a few days does it settle in height? Then pumps itself up to driver height once driven again?"

    ...Is that what is supposed to happen? If so, how much does it settle and rise? And how does it "pump itself up"?
    No, it doesn't pump itself up.
    It's being confused with the Citroën Hydropneumatic Suspension system, once used on some Mercedes-Benz models, which was pressurized by an engine-driven pump...

    https://en.wikipedia.org/wiki/Hydropneumatic_suspension

    Jon B.
    Vista, CA

  4. #4
    The Boge self levelling strut does have a pumping action but there isn't a pump fitted to the vehicle in the same way as a Citroen.

    The pumping effect is generated by wheel oscillation during driving which will raise the suspension to a predetermined level.

    If the car settles it should return to the preset height within 300 - 1500 metres of being driven - unlike a Citroen which pumps up whilst stationary.

    The strut has an oil filled cylinder with a damping piston and a hollow rod and a hydraulic accumulator. This accumulator has a high pressure and low pressure chamber separated by a flexible diaphragm (These accumulators have an identical function to the 'globes' in a Citroen and are common place in hydraulic circuits.)

    The high pressure chamber is gas filled and is separate to the oil. The low pressure chamber if filled with a gas/oil mixture
    .

    The gas charge in the high pressure section is set to exert a force equivalent to around 90% of the vehicle's kerb weight.

    Bumps in the road cause the hollow piston to move relative a stationary rod and a combination of internal valves transfers oil from the low pressure to the high pressure circuit. This increases the gas cushion pressure in the accumulator and raises the car to is preset level.

    Effectively the ride height adjustment is provided by the cycling of oil between the high and low pressure circuits and the ride height is effectively controlled with a relief action which opens a port when the damper piston reaches a preset position.

    When driven on rough roads the ride height will tend to rise as at the preset position the relief port will only be open for half of the cycle and more oil will be pumped than can flow back into the chamber.

    To prevent the car from gaining too much ride height the end of the piston rod is bevelled so the pumping effect is eliminated when the strut extends more than 10mm from its preset position.

    Adding weight to the car will lower the ride height and the pumping action will raise the suspension to the preset position.

    Once the weight is removed the suspension will initially rebound as in a conventional car but it will then settle to the preset height.

    I think that this design is a subtle attempt to obtain some of the benefits of the Citroen System without the pumps and pipes.


    Last edited by BillJones; 04-18-2019 at 12:53 AM.

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