Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Turbo and vacuum assist

Here’s a general question about which I have long wondered. Some things work off of engine vacuum, like power assist brakes. When an engine is turbo charged, there is not vacuum, but positive pressure, in the intake manifold. How do you still have power brakes?

Because on a turbocharged car you will have normal engine vacuum at idle/low RPMs.
The boost pressure only comes on when the accelerator pedal is nailed down.

You could see this by either fitting a vacuum/boost gauge to a turbo car, driving it, and noting the vacuum/boost readings or by test driving a car with a factory boost gauge. (SAAB for example)

Supercharged cars (SC T-Birds, SC Buicks or Pontiacs, etc.) almost work the same way except in these cases there is a bypass built into the Supercharger system which allows normal engine vacuum at idle/low RPMs. Once accelerating, a vacuum pod closes a door and boost pressure builds until the throttle is released.
Hope that answers your question.

Isn’t the turbo downstream of the throttle? There’d be vacuum available between the two, if that’s correct.

No, turbos are upstream of the throttle body and even if one had a turbo downstream the impeller simply won’t be turning fast enough at idle/low RPMs to generate any boost.
The Ford/GM Roots superchargers are downstream but those all have bypasses built in.
Here’s an example of a Ford Super Coupe SC.

Note on the top right there is a small vacuum pod (to the top left of the letter “S” in the word supercharger) and a few hoses. Normal manifold vacuum allows the pod to operate a flap in the unit. This permits normal engine breathing. At some point when the RPMs climb enough the spring in the pod will override the vacuum diaphragm, close the door, and boost pressure will begin.

No. There would be no point in having the turbo then.

I don’t get your point. A throttle body wouldn’t negate the turbo, it has to be somewhere. GM’s factory superchargers are downstream of the throttle body, for example.

The point of a turbo is to get more air into the engine. If you put a choke point somewhere in the line, so that you have generate a vacuum, then you are eliminating that whole point of adding a turbo in the first place.

“If you put a choke point somewhere…you are eliminating that whole point of adding a turbo in the first place.”

There has to be a throttle somewhere. At least for driving on the street.

Yes, there does. And the right place is where it is at now. If you want to try it on the other side of the turbo, then go ahead.

There were plenty of turbocharged engines with a carburetor upstream from the turbo.

"There were plenty of turbocharged engines with a carburetor upstream from the turbo."
Keep up with the thread, that’s where I am saying it is.

No, you’re saying the carb/throttle body is downstream of the turbo, between the turbo and the intake manifold, right?

Okay, I have worded this badly and got caught up in the wrong argument. My point is this. When the turbo is supplying boost, the throttle is wide open and there are no restrictions to cause a vacuum to form. After all, why would you work really hard to get extra air into the engine and then do something to restrict that airflow?
It’s okay, you don’t need vacuum under heavy acceleration because why would you brake and try to accelerate at the same time? If the climate control uses vacuum, then all cars have a check valve to keep from losing the stored vacuum back to the engine, and many cars allow have a vacuum reservoir for the AC control system (usually a big metal can or plastic globe).

And there are turbocharged cars with carburetors located downstream of the turbocharger. (referred to as blow-throughs)

Still, there is vacuum behind the butterfly most of the time. Auto engines spend most of their time making less than 20 horsepower and that means vacuum in the intake manifold most of the time.

Diesel engines are a whole different game. There is no throttle in a diesel engine so there is never a vacuum in the intake manifold. Diesels need a separate vacuum pump to boost the brakes.

I guess designing PCV in turbos and diesels is a serious challenge.

Blowby is pretty filthy and most abundant when running with boost.