Hydraulic to vacuum servo conversion

[Henka]

So, sick n' tired of the bleeding pump and bomb. Just realized that the A4/S4/RS4 has a vaccum assisted brake servo system that relys on a electric pump. Hooking it up should be....Easy? There is some kind of control unit hooked up to this pump. What does this ECU do? What signals does it read? Is the pup running all the time?

Parts list:

-Electric vacuum pump A4/S4/?? (salvage yard?)
-Brake servo 80 2.8 (?) (salvage yard?)
-Control unit A4/S4 (salvage yard?)
-Sensors?
-Tubing
-Pump for steering, A4 100bar
-Master cylinder Audi 80 2.8

Should be cheaper than the bomb+pump (I'm hoping) which I need to buy to fix my system.

Only problem as I see it - hooking up the ECU and fitting sensors or what not.

Or should I just leave it alone? How boring is that?

[porkkis]

No need to change the master cylinder.

Send me the old parts

[Henka]

Yeah, you're right on that one... I change the hydraulic servo-thingy to a vacuum one.

[spiralsmurf]

i was thinking the same thing. let me know how this works out for you. i have a 92 S4 and i hate the old style hydraulic system. vaccum assist is much cheaper and more reliable.

[Henka]

Yeah well as I said the only real issue is the ECU for the vacuum pump. I'm going to check it out today and we'll see. The pump and control unit I believe cost less together than the bomb.

[porkkis]

Actually you don't need the vacuum pump IMHO. Braking is done quite rarely on boost and off boost you have vacuum in the IM. Just add a check valve so that the servo will never see boost.

[Henka]

You know that doesn't feel all that good. My girlfriend is supposed to be able to handle this car at all times --> Safety first. If I convert it'll be with pump and all.

I was thinking of different ways of hooking it up. If I skip the ECU I can connect it like this. Keyfeed via a vacuum-sensor. This means - key on, power to pump and pump ON when sensor says there is not enough vacuum. My guess is that the ECU relys on vehicle speed, vacuum and "engine running" and the decides wether to turn pump on or not. Show be pretty straight forward.

[spiralsmurf]

I know the Impreza/WRX does not use a vaccuum pump and works just the way porkkis described.

[Henka]

What? Do they utilize some kind of vacuum tank? I mean, can they really rely on that?

On a sceptical note I have to say that it sounds way to easy. I mean, if this would work just fine - wy bother putting in a pump in the first place like Audi did?

How much vacuum is needed? Feels like a wierd question, I mean, calculating size of a accumulator is a breeze but a vacuum container?

[S2central.net]

Actually you don't need the vacuum pump IMHO. Braking is done quite rarely on boost and off boost you have vacuum in the IM. Just add a check valve so that the servo will never see boost.

That will work fine in my opinion BUT you get no servo assist if you are partial to a bit of on-boost left-foot braking. I'm no rally driver, but I actually do this quite a lot on roundabouts to help balance out the understeer.

Granted, a better balance suspension and traction system is the better way to solve this, but until I get the time and funds for Torsen/ATB diffs at the front & rear (maybe with a lockable centre), RS2 FARB, w/line RARB, new suspenders and better angles - then a bit of left foot braking will do for me thanks.

Paul

[Henka]

Is it worth giving it a shot? I found a guy willing to sell me ALL of the mentioned stuff for €200 not including the hydraulic pump. I'd really like to have a go at this. I mean, if it's good enough for a RS4 then hey, it's gotta work on a S2. Just the bomb itself is twice the money...

[ottawa coupe]

just doing a little mental math right now and for that price it's definitly worth it. what i wonder is just how easy it is....it wont just BOLT UP just like that will it? woulda thought some fabrication is involved

[porkkis]

Sure it will

[Henka]

He he... No, it probably won't. The servo unit bolts right up. The pump comes with a bracket and a nice plastic hat that would need to be mounted. Wiring. Plumbing. Figuring out how to hok up ECU or how to hook it all up NOT using the ECU. I'm calling him tonight.

[spiralsmurf]

henka, i got a response from some subaru people and they confirmed there is no pump, only a check valve. the impreza works PERFECTLY with this system. i say, do it and let me know because i want to do it too. here dude check it out: http://forums.nasioc.com/forums/showthread.php?t=747329

like the guy said, some cars run a pump when there isn't a good spot for vaccum, but otherwise all you get is a firm pedal occasionally when transitioning from boost.