Tube blocked off on top of manifold?
 

Hi All,

I noticed that my automatic 2015 BRZ does not have a tube connected to the manifold next to the tube that connects to the PCV valve.

What is this tube (drivers side) for? Mishimoto catch can install says that's the brake booster tube but I don't think so. Radium video says that the crankcase vent. I think this is correct.

If there's no tube because mine is an automatic, why don't I have a crankcase vent? The port is just blocked off straight from the factory. Last time I checked, I do have a crankcase. :)

Vic

PS. Google search was no help. I couldn't find a single photo of this area with the port blocked off like it is on my car. 1st owner so I know it came like this from the factory.

There's already an EXACT post about this.

I've been trying to find this info using search here for the past hour.

"blocked port, tube next to pvc port, plugged port, mystery port, plugged connector on top of manifold etc..... NOTHING"

Kindly point me in right direction.

I believe that's plugged from the factory. Probably another vacuum port for diagnostics

Manual uses that for brake booster. Auto uses something else so it does not need/use it.

Because auto doesn't go fast enough for boosted brakes?

Is that sarcasm? Hard to tell just from reading.

That is sarcasm.

This is not sarcasm.

Auto uses a separate pump.

Why? Is there a reason for this? I've been looking for days and can't find a definitive answer. The AT and MT version both have the same Brake Booster part number, but one gets vacuum from the intake Manifold (MT), and the other gets vacuum from the Cam-driven Vacuum Pump on the rear, passenger side of the engine (Where the leaky cam plate is on the MT version). But why?

What's stopping me from just using the plugged port on the intake manifold for brake booster vacuum? If nothing, then why have the pump at all?

No idea. If the engineers that designed the engine felt that was a good reason for the two different systems I have to believe them. Each little difference adds cost and they don't add cost unless they have to.

Might be for reading vacuum?

I've never looked at the PCV diagram for a CVT, but I would bet $1 it's because an auto does not pull nearly as much manifold vacuum when you let off the throttle. So an external vacuum source is required.

If you transition from throttle to brake in a manual, you get a lot of manifold vacuum. In an auto, you take your foot of the throttle it upshifts (and some CVTs go to idle) and you have little-to-no vacuum.

Not a CVT. If I had to guess I would say the pressure difference is why though. though.

Well then, go to the brake booster and follow the vacuum line? Does it go to an external vacuum source like the 2022 models?

The auto goes to a separate pump. The manual does not. That is the whole point of this question.

What do the boosted guys do for vacuum?
If you have a positive manifold pressure you won't have any vacuum for the brake booster.

I know my GTI has a vacuum generator to boost the brakes to counter act the turbo.

What gives?

Thanks for the replay, that does make some sense, but the FRS does not have a CVT. The gear ratios are a little different, but being in 6th gear in the auto and 6th gear in the manual should pull almost the same vacuum. Even in 6th gear, an angine with an AT will pull heavy vacuum during engine braking, same as an MT.

To further the point, and Edelbrock Supercharger on these cars is the same for AT and MT, and it needs vacuum for the bypass valve. If the vacuum was different on MT and AT, then the Bypass vacle should act differently or incorrectly if you have an AT, which it does not.

The brake booster effectively stores vacuum since there is a one way check valve inline.

As long as the booster and check valve aren't leaking then you will regenerate booster vac on decel. It is possible to get into a case where you get on and off the brakes without lifting throttle under boost and that ends in a hard pedal, but it is a series of steps that doesn't normally happen, especially in a RWD car. I experienced it in a turbo Miata and it was an interesting discovery.

Not sure. If the vacuum lines are plumbed after the TB, then even on a boosted application, the manifold sees high vacuum since the TB is closed and the engine is still trying to suck in air. But that's just a guess. Assuming that the setup is standard: turbo -> throttle body -> intake manifold.

My mistake, but it's an auto, which is essentially the same.


They do the same thing as before being boosted. None of the older boosted cars have vacuum pumps. No STi does. You have to do some pretty odd things (even beyond 'normal' track use) in order for it to cause an issue.

If you really want a vacuum source, you might be able to retrofit the vac pump from a 2022. They all have a vacuum pump. Same with the newer FA20/24 WRX.

Not really sure why you are so concerned with this. Send an email to the Subaru engineers and ask them why. They obviously have a reason but everybody making up guesses is just pissing into the wind.

A M/T sees multiple vacuum spikes on acceleration through the gears, an A/T doesn`t see any.

I only flat-foot shift. Besides, brakes are for losers.