Brake booster - Vacuum lines before and after TB

[Panici]

Question about the booster vacuum line setup on my M20.

There appears to be a connection before and after the throttle body, through a Y splitter.

All booster systems I've had experience with only have a single line to vacuum, with a check valve.

Why is the E30 system designed this way?

[Northern]

late model has a single hose with check valve from that lower hole, like:



I don't know why they previously used the setup you show, but in the late model cars, the top hole is not drilled through to the TB

[Tom.]

My late model (11/88 ) had the two lines going to the TB. I converted to the E28 check valve (I think?) so I could run the einstein57 silicone boot.
It's very straight forward to do. The new check valve is like $30, though.

[TobyB]

My THEORY on this is that if the throttle sticks open and you pump the
brakes repeatedly, you get no boost, and then die, because you're not
smart enough to switch off.

So BMW designed this thingy to generate some boost at WOT so that you
will die of other causes and not be able to sue them. Kinda like that thermostat
they stuck on the heater core....

But it's just a theory. I tore mine off before I ever had a chance to test it.

t

[raudonis]

I asked about this on my local BMW board, because I wanted to plug of that Y splitter shut to drop the idle RPMs on my old ass Jetronic m20 with shit thermostat ICV.

The answer was different ICVs - L shape vs T shape:

http://i217.photobucket.com/albums/cc152/lifeiskaos/DSCN1711.jpg

http://i9.photobucket.com/albums/a85/jahnaboi/IMG_3373.jpg

Has nothing to do with brake assistance, just allows some extra air to pass around throttle body to maintain correct idle RPMs with the corresponding ICV.

So I plugged it shut and there was no difference whatsoever except the dropped idle RPMs like I intended.

[Panici]

My THEORY on this is that if the throttle sticks open and you pump the
brakes repeatedly, you get no boost, and then die, because you're not
smart enough to switch off.

So BMW designed this thingy to generate some boost at WOT so that you
will die of other causes and not be able to sue them. Kinda like that thermostat
they stuck on the heater core....

But it's just a theory. I tore mine off before I ever had a chance to test it.

t

But at WOT the intake is at atmospheric pressure anyways. How would having access to air before and after the TB generate any more brake boost?

I asked about this on my local BMW board, because I wanted to plug of that Y splitter shut to drop the idle RPMs on my old ass Jetronic m20 with shit thermostat ICV.

The answer was different ICVs - L shape vs T shape:

http://i217.photobucket.com/albums/cc152/lifeiskaos/DSCN1711.jpg

http://i9.photobucket.com/albums/a85/jahnaboi/IMG_3373.jpg

Has nothing to do with brake assistance, just allows some extra air to pass around throttle body to maintain correct idle RPMs with the corresponding ICV.

So I plugged it shut and there was no difference whatsoever except the dropped idle RPMs like I intended.

Kind of like a bandaid for a poor ICV.
Why would they go to the trouble of integrating it with the brake booster stuff though?

[TobyB]

at WOT the intake is at atmospheric pressure anyways

Mostly. There is some pressure drop, though, enough to generate flow.

As to a controlled leak, there's already a bypass screw to do that...

...

...


t

[jeenyus]

REVIVE! Any actual answers and not conjecture?

[jeffnhiscars]

Conjecture is so much more interesting

[TobyB]

I stand by my conjecture.

The thing is a 'jet-sucking-pump' for the booster.

If I hadn't tossed it, I'd test it.

t
backing his own bs since oh, about 1971...

[varg]

It's essentially a venturi pump. It takes the pressure differential between the intake hose and the intake manifold and uses it to drive what is essentially a small, weak vacuum pump as a secondary source of vacuum. It'll provide some vacuum for the brake booster when the manifold vacuum is not sufficient.

[ForcedFirebird]

But at WOT the intake is at atmospheric pressure anyways. How would having access to air before and after the TB generate any more brake boost?

Venturi/ported vacuum sources are different. The angle provides vacuum as the air passes the opening when at an angle.

It's essentially a venturi pump. It takes the pressure differential between the intake hose and the intake manifold and uses it to drive what is essentially a small, weak vacuum pump as a secondary source of vacuum. It'll provide some vacuum for the brake booster when the manifold vacuum is not sufficient.

This.

[cscracker]

I have no idea why they made it this way either. I tapped threads into my throttle body, plugged one hole with a grub screw, and put a 90 degree fitting in the other one with a straight through check valve to the booster. Switched to the other intake boot without the extra hole. No ill effects. No noticeable change, other than fixing the vacuum leaks from all the old cracked rubber and plastic bits I junked. Mine's an '89 iX, so I'm not sure it's a early/late model thing.

[turbski]

is the check valve between the throttle body and brake booster a must. Im trying to sort my new to me car out before winter and do not see a check valve there at all??

[ForcedFirebird]

is the check valve between the throttle body and brake booster a must. Im trying to sort my new to me car out before winter and do not see a check valve there at all??

Definitely need a check valve, I have never come across any car/make that didn't have one. The fitting that the hose attaches to at the booster might be the check valve in your case.