crazymjb

My friend, a diehard saab fan, is going to be getting an 89 SPG. Anyway, we will likely be rebuilding the engine just for mileage sake and so we can comfortably do some other "modifications" for more power...

Anyway, he was showing me some Vacuum/Boost combo gauges, which is basically a continuous gauge from negative to positive pressure. I got to wondering, when the manifold is under positive pressure where is the vaccume coming from to run the accessories? Is it pre-turbo? If so, is it always pre turbo or are the sources changed through valving based off manifold pressure? Wouldn't one have NO vaccum if they were using a ram air intake in conjunction with their turbo?

Mike

teranfon

Hi Mike

I'm not really sure what you are asking, but are you referring to something as a vacuum canister? Some automobiles use a canister for short term vacuum. You've probably seen these basic canisters under the hoods of some cars. In the older T-birds, canisters such as these often "powered" the headlights even while the car was turned off. Some cars which used vacuum assist for windshield wipers also used this feature as well.

I'm not sure what you're referring to in regards to ram air with a turbocharged system. As you know, its the pistons creating vacuum which controls the amount of air being fed into the engine. The compressor of a turbocharger is of course a direct part of the intake system. Air is being forced into the engine, regardless of the engine's vacuum, but limited to the compressor itself. Most modern systems have sensors which monitor this and supply the engine with additional fuel. If not, detonation occurs and the results often catastrophic. Because of this I'm not even sure if a ram air system is even usable or even beneficial on a turbocharged engine.

If I not completely understanding your questions or haven't answered your concerns satisfactory I apologize. All of my GN's have always used electrically driven servos for their assist of the braking systems.

Terry

crazymjb

Thanks... Basically what I was wondering was this: With a forced induction setup how exactly are the vaccum actuated accessories operated; IE brake assist, climate control vents?

My line of thinking tells me that positive manifold pressure from a turbo setup for example would not allow for that, unless the car used the vaccum pulled in front of the turbo. That where my Ram Air comment came in... If a ram air setup was being used in conjunction with turbo or supercharger setup, where could a steady amount of engine vacuum be found to operate the various items which regularly use vacuum.

Hope that is more clear.

Thanks,

Mike

teranfon

Hmmmm...........I'm not entirely sure. What does a car normally pull at idle? I think its about 20, maybe 22 inches? Not sure of that either. But lets say for the sake of argument that any car at idle will have, or obviously should, have the greatest vacuum. With the throttle plates closed manifold vacuum should be the greatest at this point since the pistons are obviously trying to suck in air past the plates. I would think this would be the same whether the engine is normally aspirated or turbocharged. Now, lets assume you put your foot into it and start making boost. And of course this is greater than atmospheric pressure. I can certainly see your point that there wouldn't be any vacuum left to assist such things as vacuum brakes. The most immediate thing that I can see is a vacuum canister to assist while the engine is under boost. Once you take your foot off the throttle, and the wastegate snaps open, it seems reasonable to assume that vacuum would be created once more. The reciprocating mass would be turning at a high speed (assumably), the throttle plates would close, and the engine would obviously be creating vacuum once more. I think its logical to assume that the driver of a turbocharged vehicle would not be creating boost under throttle and attempting to use the brakes at the same time. Once on the brakes, vacuum assist would once more be present as vacuum is now present while the throttle is closed.

At least I think this is how it would work. I know some cars now use variable valve timing to create vacuum, but I'm not sure how this would work with a turbocharged engine. My Grand National does not use any sort of vacuum assist. The servo for the braking system is actually electrically controlled.

Terry

Accord_V6_400m

iTrader: (1)

Bernoullis principle most likely, the boost pressure passes vacuum lines and there you have your vacuum.

crazymjb

His clutch would also, I assume, have a master cylinder, also the air vents within the cabin also need to be able to operate under boost, unless the doors are electronically controlled, which I suppose is very possible.

Is their enough air flow to generate the amount of vacuum needed if it was using some system based of Bernoullis principle?

Mike

teranfon

Mike:

I forgot one last thing. Remember that a idle the wastegate will be open. Your are not creating boost of any sort. This is why, at idle, vacuum is similar to a normally aspirated car. In fact, all things considered, a normally aspirated car is not creating much vacuum to speak of under load as well.

Terry

teranfon

The clutch is likely a complete mechanical item. The air vents, I don't really know. In many ways a turbo and non turbo car basically function the same way as far as vacuum is concerned. Granted, when boost is created in a turbo car its there isn't vacuum of ANY kind, but as you know you don't really have any vacuum when accelerating in a non-turbo car as well. Whether turbo or non-turbo, how often is vacuum needed for accessories under load when accelerating? For the brief moments, maybe, then perhaps a vacuum canister is used in these situations. You've probably seen them under the hood, a juice looking thing with a hose attached to it.

Bernoullis principle, if I can recall university physics correctly, is merely the principle that as a FLUID rates increase, pressure decreases. Has nothing to do in this situation.

Terry

Edr0e

Ram air tuning simply takes advantage of the inertia contained within a moving column of air/fuel mixture as it comes to a stop against the closed intake valve.
In the 50's they realized by adjusting the length of the intake runners, this energy can be used to improve cylinder filling when the valve opens again. The more pent-up energy waiting to crash the gate, the more air to pack the chamber.

Edr0e

An easily-accessible source of power from an internal combustion engine is partial vacuum, available by tapping the inlet manifold. The piston engine is fundamentally an air pump, and it produces suction and partial manifold vacuum on the inlet side. This can be used to power accessories or advance the ignition system spark timing.

Inlet manifold vacuum varies depending on engine load and throttle position. Therefore, rather than being connected directly, "vacuum canisters" are sometimes used to smooth things out, also to have a vacuum 'reserve' on turbo charged engines where, when the turbo is active, there exists no vacuum but an overpressure in the inlet manifold.

These cannisters are connected to the inlet manifold through one-way valves that allow air to be sucked out when the engine is generating a lot of vacuum, but do not allow air to flow in. They can be viewed as, effectively, a "vacuum battery". Vacuum canisters allow vacuum accessories to be operated for a limited time even when the engine is turned off. (Electrical systems could not do this until the development of retained accessory power allowed power accessories to function after the ignition is turned off.)

The most common vacuum-powered ancillary is the power-assisted braking system. This is often vacuum powered even on modern cars, and is generally connected directly to manifold vacuum for increased reliability. In this case, the vacuum is only an assist; the brakes function (albeit requiring greater force) if vacuum power is lost.

Many older cars used vacuum-powered windshield wipers; loss of manifold vacuum when the engine was working hard, with wide open throttle, caused these to slow down or even stop when climbing hills. Even in ordinary conditions, their speed varied inversely with the speed of the car--the wipers ran slower at higher vehicle speeds, an inconveient and even unsafe characteristic.

Hope this helps

crazymjb

Well according to Bernoullis principle if there was a moving column of air, say right after the turbo, if you had a hose going perpendicular to it, their would be some vacuum created as that moving air is a low pressure zone. However, I think that any moving air from the turbo would be countered by the high pressure zone it has created.

I was also thinking that air could be drawn from infront of the turbo, possibly a low pressure zone, but any car that had any decent filter would render that useless.

Mike

crazymjb

Edroe, thanks. So it seems it would be the "stored vacuum" afterall, which hypothetically could be expended if someone was under boost for an extended period of time.

Mike