How does vacuum and boost mix?

[Guest tony78_280z]

What happens to manifold vacuum under boost? When the manifold is boosted do vacuum based components (ie. brake booster, timing advance, 4 barrel carb 2nds) work differently or not at all? My mind tells me that the boost pressure could nul these systems.

 

If anyone has any good links on how a turbo-charging affects the rest of the engine components I'd be gratefull.

 

Thanks

[mobythevan]

Good deduction. That is why vacuum operated accesories should have check valves to prevent the pressure from going up the line to the device. When vacuum is present in the manifold, air is evacuated from all vacuum pods/actuators, then when higher pressure is present in the manifold no air is allowed to go back and fill the pods/actuators. Think of all it as pressure. Not vaccum or boost, just no pressure or varying levels of pressure. That may help you to how it works a little easier. Atmo pressure is 14.7psi. An n/a engine still has pressure in the intake manifold when the throttle is open, just that it is only 14.7psi of pressure at sea level. A boosted engine running 10psi then has 24.7psi of pressure in the manifold. Just different levels of pressure. Otherwise n/a and boosted vacuum devices have to deal with the same issue (checkvalve).

 

Something that relies on pressure to move the actuator like a wastegate or vacuum advance pod may be possible to move the actuator farther under higher boost until it bottoms out. That is why people like to use a threaded wastegate rod so you can change the loading on the actuator diaphram for different boost levels.

[Guest tony78_280z]

When vacuum is present in the manifold, air is evacuated from all vacuum pods/actuators, then when higher pressure is present in the manifold no air is allowed to go back and fill the pods/actuators.

But when there is no vacuum then these additional devices (ie. brake boosters, timing advance, carb vac2nds) would stop functioning. So you are cruising along and the boost kicks in causing the manifold's vacuum (or psi level) to equal or exceeed the atmospheric pressure and your devices quit working. I know that this is not the case. I'm just wondering why this is not the case.

 

There is like some little piece of info that I'm not understanding or geting on how charged systems work. I'm sure I'll have a breakthrough once I understand this.

[mobythevan]

True, if you are under boost for a long enough time, or you drive an n/a car full throttle long enough you will loose vacuum that is stored(assuming you use the device with stored vacuum, like a cruise control pod). But as soon as you lift your foot vacuum is quickly restored to all devices. So under "normal" driving you will always have vacuum. My Ford dually truck will use up all of its vacuum for cruise control climbing long gradual grades because the throttle is applied constantly. The only answer to fix that type of problem is a larger vacuum storage canister or a vacuum pump.

 

Your thinking is correct, it is just a matter of how long does the vacuum last and how long can you keep you foot in it and need vacuum accesories to work. The standard answer is you won't be flooring it and hitting the brakes at the same time(only answering the brake question).

[Guest bastaad525]

I've always wondered this as well... I know there is a check valve inline with my brake booster, but I always wondered how it would function under boost.

[akeizm]

So do you guys run with your foot on your brake when you are @ full boost

[Guest 73TPIZ]

I believe the Grand National and the supercharged Thunderbirds along with some other factory boosted cars run an electric or belt driven vacuum pump for this very reason.

 

Wouldn't be that hard to fab up if you want to have vacuum to your brakes at all times.

[Tony D]

The 82/83 ZXT's have an integrated electrically driven vacuum pump and reservoir complete with a vacuum actuated switch to pump down it's little vacuum reservoir to allow the HVAC controls to continue to function (as well as the Idle Air Regulator) at all times. If you ever noticed some Fords, the HVAC controls will change state when climbing hills due to the vacuum loss.

The stock Dstsun system will turn on the electric pump to keep the reservoir at a set vacuum level for proper opreation of vacuum operated accssories no matter what---the reservoir is also ducted to the intake manifold with a check valve inline, so under normal conditions, the engine supplies vacuum, but under the conditions talked about above, the electric pump steps in at a predetermined value to keep everything functioning flawlessly.

On the ZXT, vacuum is what moved the heater controls, as well as AC dampers. On a 110 degree day, the last thing you want when you pass that vette is for the AC to start pumping "full heat" onto your feet!

 

For those of us who remember vacuum operated windshield wipers, the first thing that comes to mind after seeing the Datsun Pump Assembly is (that would fit under the dash easily enough, and I'd never have slow wipers again!

PssshTA, PssssshTA, Psssssssssssssss(WHACK!)TA! Some of you guys will never realize how nice electric wipers really are! Even the "slow" 240 wipers! LOL

[drzed]

"So do you guys run with your foot on your brake when you are @ full boost :P"

 

Why yes I do in fact every time I launch at the track !!! Seriously though the brake booster holds enough vacuum to operate the brakes while I am foot braking as long as I don't readjust my foot on the brake to inch the car ahead while under boost - then the brakes go to hell in a handbasket in a hurry.