Vacuum Brake Tuning: Improve Your Brakes

Hello everyone,

I've been reading a lot of old posts lately about poor braking performance in VAG vehicles, or high pedal forces.
ULF had opened a corresponding thread in January 2006.
However, all of these contributions get lost in fundamental discussions, larger issues, and things that are outside the scope of what is permitted.

My problem is: My wife really doesn't like driving my A4 because she can't seem to get the brakes to work properly; she never reaches the ABS threshold. I'm also honestly worried when she brakes with 'half force.' (I'm used to much stronger braking forces!)

My brake rotors are all relatively new, with ATE Powerdiscs on the front. Otherwise, the car brakes perfectly and consistently. The vacuum system is airtight, even after days of inactivity.


Question: What is the desired vacuum pressure in the braking system of an Audi A4 manufactured in December 1998? (0.6 bar? 0.8 bar?)
2. Has anyone tried to achieve something by increasing the vacuum pressure? If so, how? Did you change the pump or use a different/additional pump?
3. How is the vacuum in the system regulated, or does it depend on what the pump can achieve?

Please don't suggest things like larger rotors or racing brake pads. I'm too concerned about the risk of an accident caused by improperly modified brakes.

Well, the only thing I can contribute is that a week ago, I was driving around with insufficient vacuum pressure, at least according to my log files where the load data was recorded.

Now I've replaced a few hoses and the lambda value is now following the target line exactly, as is the air mass. Before, it was significantly lower.

What I mean is, I never felt anything different with the brakes before, and I don't feel any difference now. I think you might be experiencing a low-pressure regulation problem.

I also believe that increasing the vacuum pressure wouldn't change the braking force. I had a porous connection to the vacuum pump before. The problems only occurred when the vacuum collapsed too quickly due to a leak. Perhaps another vacuum reservoir would work more reliably than reducing the pressure, because lowering the pressure could lead to other problems with the vacuum system.

Can you rule out the possibility that something else is causing the problem with the BKV? Rainer had issues with his A3 before, so it's possible that the problem lies with the BKV rather than the vacuum system.

Forgot to ask: Have you ever compared your car to a similar one? My Passat and my wife's Golf (VR6, though) both brake in a way that allows her to easily engage the ABS.

Since the tires are quite good, it naturally becomes more difficult to achieve proper pressure buildup, even for me (but that leads to the sizing, which I consider appropriate in both cases, and probably applies to your situation as well). However, with a panic braking maneuver, you'll easily stay within the ABS limits. Think of it like a motorcycle brake; if you squeeze too hard and too quickly, you're more likely to end up going over the handlebars than locking the front wheel.

Hello everyone,

Thank you for the answers.

I believe that my A4 from the year 1998 has more power, but my wife is comparing it to other cars, including an A4 from 2004. And the difference is quite significant.

Whenever I'm driving a rental car, I sometimes make the person following me nervous with sudden, unexpected hard braking.

The mention of the influence of turbocharger boost pressure control is good; I had forgotten about that. It would be worth trying.

What surprises me, but is also a good point to consider, is that the vacuum should not affect the braking force pressure. I don't quite understand that yet.

Has anyone online found a detailed description or pictures of the brake booster?

Brake force amplifier.htm

Even with the link, I don't understand why it is like that. Since I assume that the vacuum pump creates more vacuum at higher speeds (or in gasoline engines, there is more vacuum), it shouldn't have a significant influence. Imagine how little force you would need at high speeds; you would have to adjust your power delivery based on engine speed .

@Schraubi - Thank you.
'@ Antjie: I would say that the braking force during emergency braking is definitely pressure-dependent.'


Specifically in throttle-controlled gasoline engines, vacuum is only generated during idle or at its highest level during rapid acceleration.
The fluctuating pressure is released through the pressure accumulator; in most cases, the volume of the buffer tank is sufficient.

In a throttle-less engine with a pump, the pump generates the vacuum, which I believe is not very speed-dependent but rather depends on the flow rate.

Regarding partial braking, which is the area I'm interested in, I'm not yet sure and might just need to test it out. Currently, I would say that this area is constructively independent of pressure because the brake control valve acts like a regulator up to the point where atmospheric pressure prevails on the pedal side.
(There's still the thought experiment: What happens if I ventilate with compressed air...? I could generate and store compressed air using the turbo.)

One thing has become clear to me now: the only way forward is to first measure the pressure conditions.

look again: http://www.osz-teltow.net/~ahlgrimm/bkv.pdf

or: http://www.continental.euromediahouse.de/bild.las?kat=bosPtXhbAISEryXa7fb6&listskip=31

garth.brooks wrote:

1) @ Antjie: I would say that the braking force during full braking is definitely pressure-dependent. Specifically in throttle-controlled gasoline engines, vacuum is only generated during idle or at its highest level during rapid acceleration. The fluctuating pressure is released through the pressure accumulator; in most cases, the volume of the buffer tank is sufficient. 2) Another thought experiment: What happens if I ventilate with compressed air...? I could generate and store the compressed air using the turbo. 3) Regarding partial braking, and that is the area that interests me

.
1) Theoretically, I would agree with you, but my practical experience is different for reasons I can't explain. I imagine that the difference might be measurable, but not noticeable, because, according to the link, a "vacuum relief valve" (which allows excess vacuum to escape) is not installed.

"The Drokla gasoline engine, like my VR6, should lose its vacuum during low-speed cruising/idle after repeated pumping (it can't "recharge" quickly enough, the "reservoir" eventually runs out), while at higher speeds, the vacuum in the brake control unit (BCU) should increase. It works even after being emptied with the engine off, as long as it hasn't been run at low speed. Be careful here, it's not just the fuel quantity, but also the vacuum – there's no build-up in the BCU reservoir. The first scenario isn't happening, but the second one does (as is the case with a TDI), although this doesn't show any effect on my vehicles... Also, I now definitely have more vacuum (a little air was drawn directly at the pump), and I don't notice any change in the required braking force."

In your case, it all depends on trying; I can only share my experience with you.

2) Due to air pressure. You don't want to install a larger brake system that can be registered, but instead use compressed air, or multiple/different vacuum pumps

Quote:

. I'm too concerned about an accident involving a braking system that has been modified in an unauthorized way

.
I guess you're just leaving it as a thought experiment, right?

3) The system needs to be able to produce an ABS effect, but in my opinion, this happens during a full stop, not during partial braking. However, the force applied in critical situations is generally much higher, and the feeling of "safety" is usually provided within the partial braking range.

Does your vehicle have a larger turbocharger available (perhaps in conjunction with a different intercooler)? If so, you could keep the pressure as it is, and no one would notice.

What surprises me about this discussion is that your wife isn't able to trigger the sudden stop .

Are there any physical reasons or other factors that could explain this? Normally, she should be able to do it without any problems. And I haven't encountered any A4 models where the brake pedal was "extremely" difficult to press.

Perhaps it's only the BKV (battery control unit) that is defective. Did you perhaps have a lot of water in the reservoir before (which might be noticeable by water leaking into the passenger footwell)?

It wouldn't be the first BKV to sink.

Okay, not all throttle-body gasoline engines use the vacuum created by the closed throttle to generate a vacuum. The beloved 35i 9a had a vacuum supply that was separate from the engine. I know this, because it was often towed... However, I never questioned it, because the car wasn't worthy...

matthiasTDI96 wrote:

Wait, not all throttle body gasoline engines use the vacuum created by the closed throttle to generate a vacuum.

"That's right, the VR6 doesn't always have it like mine either (in that case, the intake manifold is sealed with a plug), but then you move towards external vacuum generation, similar to what's used in TDI engines. The external supply is much more consistent than, for example, the one in my VR. I want to use this example (and the one from my Passat) to argue that changing the vacuum system might not achieve its intended purpose."

Hi,

"I agree... even with the absolutely porous and leaky hose connecting to the vacuum pump (BKV), through which the vacuum escaped so quickly at the end that it wasn't even enough for the shut-off valve to function properly, there was no noticeable difference in pedal force."

Let's do a quick calculation: A BKV has quite a large membrane... let's assume, for simplicity's sake, that it's around 200 cm² (based on the housing size, it could actually be even more).

In a perfect vacuum, a pressure of 1 kg would act on every square centimeter, meaning at least 200 kg would be exerted on the entire object.

For the desired purpose, a fraction of that force should actually be sufficient, even though the human user benefits from the leverage provided by the pedal – and therefore requires correspondingly less pressure.

Best regards,

January.

Hello Jan,

Your calculation regarding area/braking force is interesting and sounds like a lot at first, but:

With a fully flexed foot, an adult man can easily exert a pressure of 200 kg! (for example, to move furniture).
However, in a car without ABS, it's not enough to prevent full braking, which means I believe there are other forces at play.

@ Schraubi: No, the problem isn't anatomical but psychological; any driving safety instructor can confirm this. Women typically drive in a way that requires less braking force than men. However, when things get critical, they are not accustomed to applying such strong brakes, and this usually happens in situations where there is no time for reflection.
Another observation: Cars typically marketed towards women often have very low braking force.

If the pedal force required for ABS activation were lower, it would result in a relatively stronger braking effect.

I now understand that the braking force below full braking is constructively defined within the brake control system (BKV) and cannot be changed by pressure conditions. These pressure conditions only become relevant during a full braking maneuver.

With that:
Applying full brakes with insufficient vacuum pressure will not work.
Applying full braking with sufficient vacuum pressure is effective; however, increasing the vacuum pressure further has no effect because it then places the system back within the brake force-regulated partial braking range from the perspective of the ABS control unit.

@ Antjie
It makes sense that, as long as the vacuum is sufficient for full braking, you wouldn't notice a difference.

For me, it's either: install a larger battery or leave it as is...

Quote:

I think that my A4, manufactured in 1998, has more power, but my wife compares it to other cars, including an A4 from 2004. And the difference is quite significant.

The difference shouldn't be that significant. Something is fishy.

If I understand correctly, this is an A4 with an automatic transmission. According to the vehicle's documentation, it has an electric vacuum pump and an external pressure sensor installed before the brake control unit (BCU). This system would also be capable of diagnostics.

Possible errors may be stored in the engine control unit (ECU).

That's where I would start first.

My uncle has an A4 with 110 horsepower and automatic transmission, and the brakes work normally. At least, that's my opinion, and neither he nor his wife complain about them. So, something must be wrong.