hydro vs vac

[dspl1236]

ok so what are there performance differences between the hydro assist or vac assist?


I can go both ways...what would be better? looking for pros and cons

[A80Avant]

i am biased, because i have been crashed and burned due to the hydraulic system, and i am changing to vaccum assist next week.

Main pro for the hydraulic is that it allows to have assist on the brakes even ON boost, for left foot braking.
Cons: very expensive to rebuild when it goes wrong, bomb accumulators cost half the amount of a vaccum assist servo!!!! (not from Audi,as from audi they are more expensive than that!!!), if the servo goes the price is about 600-700euros to replace.

Vaccum assist: Pros: very cheap to run-maintain, only has 1 check valve and that's it.
Cons: you dont have assist during boost,and if you want that, you need to install an electric pump inline to produce that assist.(many new audi cars have this), you may have to modify areas of the engine bay to fit properly.

I will take photos of the system once installed next week. I have had such a big problem with the hydraulic,although it is very reliable, been running perfect for 15 years, then all of a sudden, just wont work no matter what. Changed servo, PAS pump, lines, master cylinder, nothing.

[Pirvu Eugen]

As A80Avant.....I tried hydraulic assisted brakes and i installed at the end...after soo many problems...the vacuum booster.
My advantage was that i have the 85 coupe wich had the original vacuum assisted brakes.
For the S2, RS2...will need some modifications at fuel lines, some hoses..etc.
I wait the end result from A80Avant..... i hope he will be happy.
My car has run without issues for 50k km after conversion...but never drive on track.
For daily driving and even a little sportier..no problems for me.
In the future i want to install an electric vacuum pump inline with the connection from the manifold and an vacuum switch to start the pump when the vacuum is less then -0.4 bar. But with a good check valve and a big vacuum accumulator...i am sure will be perfect without the pump.
All the best!
Eugen

[A80Avant]

i have found that i can connect the electric pump through my aftermarket ECU (VEMS) easily, but i think ill try it as it is. Im thinking that even for fast driving, solong as you dont need left foot braking, it should work perfectly.

But even then if you do left foot braking, all racing cars dont have assist, so you need brutal force to work the pedal, and with more pressure on your left foot they can still be applied!

[martinws]

I'm putting vacuum assist with my typ85 20vt swap. I don't see any problems apart from left foot braking on boost or if the check valve breaks. I dunno if this is daft but could the central locking vacuum pump be used to assist or is it not up to the job?

[Pirvu Eugen]

i have used the car as it is....for more than 50k km...no issues...
In the event that the check valve brakes.....then...is a problem...you need ALOT off power to stop the car...but usualy this is not going bad instant.
As i said, no issues for me in many km.
The vacuum pump from the central door lock is way to small for this job.

[BRY]

There are alternators with vac pumps built in from Transit / Land rover / kia / Vauxhall.
Could one of these be utilised?

[BRY]

The pump on the transit seems to be a separate entity:

[A80Avant]

what i dont understand about the electric vaccum pumps, is how do they know when to switch off so as not to work continuesly when the servo is already fully loaded?

if i have an electric pump, id rather it only worked only when the amount of vaccum stored in the servo is less than what needed

[WabTheKing]

what i dont understand about the electric vaccum pumps, is how do they know when to switch off so as not to work continuesly when the servo is already fully loaded?

if i have an electric pump, id rather it only worked only when the amount of vaccum stored in the servo is less than what needed

bit like how a pressure switch works but in reverse,

with vac tho its not like a compressor building and building pressure, once it gets to its limit, thats it, it doesnt implode,

bmws have vac pumps driven off the cam shaft which looks interesting.

pembo has an electric vac pump for his brakes on his rally car, works great, cuts in when servo drops below a certain vac

[dspl1236]

all good info, i have been sitting on a vac setup and have a few hydro setups lying around with the stock setup installed.

all i have ever known is the hydro setup and never had any issues on any of my past audi's. my 90cs is the 2nd vac assist car and all i have done is put on a 25mm mc and seems great so far.

with the v8 in place on my 80q, i had to ditch the vac and move to hydro to clear the dizzy. well now i have dropped the dizzy's in my cq and have lots of room. i know i can swap the hydro base out to (200 20v i think) shorten the assembly to have even more room...i guess i need to see what works best for me. i plan on single turbo at some point and may be best to stick with the hydro setup just for the room.

80q short base vac with 12v vr6


80q nothing with v8 - vac booster was in the way of the dizzy. never got a photo with the hydro installed


cq with v8 w/- real close with dizzy





a_cq w/abz with vac and shorter base on motorgeek



now, i have no dizzy's....with a shorter 200 hydro base......and i am not even sure if this car had the stock hydro base in it.

ill just swap everything around just to see what i like best

[Pirvu Eugen]

There are alternators with vac pumps built in from Transit / Land rover / kia / Vauxhall.
Could one of these be utilised?

I thought at this also...but these pumps are oiled.. and i didint like the idea to use my preciousssss engine oil for this pump.
The electric pump...is running only when the ecu tells to run..or iff the vacuum switch says that is not enough vacuum in the system. Driving normally will never start. The only problem i see, is the air fuel ratio...wich will be messed up..because the booster is using some air from the intake. But i didint had problems with my 3B.

[A80Avant]

under vaccum it may become a bit richer as some air is lost for the vaccum assist, but lamda sensor would perfectly capable to compensate.

if vaccum can work like that, i was thinking of an electric pump wired to my ECU. Through the ECU i can specify at what rpm i want it to work, at what kpa, but i cannot specify at which point i need it to stop. That can only be done with a secondary switch, such as the one that sometimes these systems use, which works independently (i think) and allows for the pump to stop once the required vaccum level is reached.

My car idles when hot at around 40-44kpa. When cold at 50ish. I presume once the servo is fitted, it may drop a bit as the servo gets pressurised.
With the electric pump in place i can have it run from completely positive pressure(101kpa and above) but i will have no way of knowning when the pump will stop working.I presume that it will work continuesly regardless of the vaccum level. I presume that wouldnt do any harm to the servo as the engine works similarly and the servo is continuesly under vaccum,right?

[Pirvu Eugen]

The newer cars have an vacuum sensor installed after the check valve before the booster.
That sensor is controlling the start and stop of the pump. Is an sensor..not a switch..my bad Is ecu controled...this way it controls the starting and stoping of the pump.
Without this sensor.....you need 2 switches...calibrated. 1 controls the on...the other stop. Is a little work to do...but can be done.
Dont know iff Vems can use the signal from an external pressure sensor mounted at the booster and control the pump. I am sure it can.
This will be much more nice.

[Greg_S]

I was looking at fuel pumps yesterday and came across the vacuum pump used on the 2.0TDI golf engines too, that has a combined fuel and vacuum pump driven off a cam