Re: 5000 brake booster (servo)

[Frank Deutschmann <fhd@interport.net>]

Alan Cordeiro sez:
> Frank  Deutschmann mantioned ( June or so ) that he was considering eliinating the hydraulic pump, hoping to save 60 or so HP.

Yes, I did.  And I'm still working on it.  I will be going to a no-booster brake
system, with a dual master cylinder arrangement (not a tandem), and much larger
brakes.

My current challenge is the steering rack; to eliminate the hydraulic pump, I
am also going to a manual rack, but that is not so easy in this car.  Audi uses
a rather strange arrangement where only one end of the rack is used for output,
with both tie rods connecting at that point.  It is interesting, as the nominal
position puts the tie rod inner pivot point right on the centerline of the vehicle,
but it is also difficult to replace for that reason.

I experience quite a bit of bump steer with the stock configuration, and I would
like to fix that, so I am currently making lots of measurements, etc to perhaps
relocat the steering pickup on the front suspension.  This has become a major
project!

Unfortunately, as it is all tied together (brakes, steering, wheels, etc), none
of the pieces have been independantly implemented.

On another note, I have been contemplating playing with tire circumferences and
final drive ratios to effect an uneven torque ballance (more rear-biased).
Unfortunately, even if I get this worked out from a gearing point of view (we
have to keep the total ratio -- final drive + tire circumference -- between
front and rear the same, to keep the Torsen happy), I will loose ABS, as the
ABS sensors will see uneven tire rotational speeds front and rear.  Reprogramming
the ABS computer is out of my league, sadly... so I'm not sure if I
will attempt this.

(Side note: this is one area where the viscous coupling really shines:
it allows the use of tire circumference to fine-tune torque ballance
front-rear, without the risk of frying the center diff.  Seems like
Porsche has taken advantage of this aspect with the 993, too.)

> Before I rebuilt mine, I considered getting
> an ELECTRICAL 12 VOLT PUMP, with a cutoff switch and a pressure
> accumulator. It would take the constant one or two horsepower
> load off the engine and put it onto the battery. This pump would
> only run when the pressure in the accumulator fell below
> 2000 PSI. I can get manufacturers names if anyone wants.

This is not a bad idea, tho you may find that the pump is hard to
mount, and it may end up running most of the time.  Also, keep in mind
that the accumulator is only used for the brakes, not the steering rack.

> If you replaced the brake booster, how would you replace the
> steering system need for high pressure mineral oil.

Ahh, now this is a problem.  If you simply remove the brake booster,
and connect the pressure supply for the booster to the return, you
will probably throe away gobs of power and fry the pump in the near
future.  OTOH, if you simply plug the pressure supply and return, I'm
not sure what the result will be.  This was one of my motivations for
dumping the whole system...

-frank
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