Vacuum pressure
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Northstar Engines and System Technical Discussion Discussion, Vacuum pressure in Cadillac Engine Technical Discussion; On my 1997 Concours, when you accelerate the vacuum pressure falls and the A/C vents close. They will open back ...
  1. #1
    ccriderxxx is offline Cadillac Owners Member
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    Vacuum pressure

    On my 1997 Concours, when you accelerate the vacuum pressure falls and the A/C vents close. They will open back up on a lite throttle. I have cheked the PCV valve and see no problem there What gives?

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    lry99eldo is offline Cadillac Owners Fanatic
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    Re: Vacuum pressure

    First off, vacuum pressure is an oxymoron of sorts. Vacuum is abscent of pressure. Normal atmopsheriic pressure is ambient 15psi, + or - depending on where you live. When your engine is running at idle it actually pulls a vacuum, and as you accelerate, this vacuum decreases to or near zero and that is what you are seeing. The only thing left is atmospheric pressure to provide air to your engine, which at 15 psi is still substantial enough to continue accelerating. So at idle, your vacuum "manifold pressure" is high at 15 inches Mercury, or vacuum enough to pull a reserve of Mercury in a tube to 15" in height. So it's normal to see this drop as you accelerate. The higher you keep that vacuum the more economy you see, lower the vacuum and you lower your MPG. It's also a way of indicating a various amount of ills your engine may have or develop, like a tune up meter. Think of it as this. Your power brake system is a good example. That large flat canister thingy has a diaphram in it. On one side is a vacuum and the other side ambient pressure at 15 psi. As you apply the brakes the assist comes from the ambient side times however many square inches of surface the diaphram represents. So, 100 square inches surface area = 1500 lbs of assist pressure to apply to you brake master cylinder. Does that help?
    As for the AC, I believe the AC shuts off to provide the additional power during hard acceleration and that in turn, depending on system design, closes off your charge air.
    The PCV has nothing to do with this matter, but it's good to know that it's OK!
    lry99eldo

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    eldorado1 is offline Cadillac Owners Connoisseur
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    Re: Vacuum pressure

    Sounds like you might need a new check valve on your vacuum resevoir..... You'll have to ask someone else where it is, and what it looks like. I'm not familiar with northstars in their stock cars

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    Re: Vacuum pressure

    You may also have a crack in the vacuum reservoir or a vacuum line.

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    Re: Vacuum pressure

    The AC compressor shuts off during wide open throttle acceleration to improve performance but that does not affect the air delivery inside the car.

    The air delivery door is actuated by vacuum. There is a vacuum supply hose that feeds the HVAC system. The HVAC vacuum comes from a vacuum reservoir under the hood. The vacuum reservoir is fed engine vacuum thru a one way check valve so that the check valve "checks" and holds vacuum in the reservoir for the HVAC and cruise operation.

    I think the vacuum reservoir is either inside the fender on the right side or under the front cradle extension (out toward the bumper) on the right side....not sure. Follow the vacuum lines from the fitting on the throttle body and one will go to the reservoir with a check valve and a tee to the HVAC system in line with it.

    I'm guessing it is the check valve....or a leaking vacuum hose to the HVAC possibly that is depleting the vacuum when you are in the throttle.

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    Re: Vacuum pressure

    Yea...LOL...."vacuum pressure" is an oxymoron alright.

    While I basically agree with Iry99eldo there is a little bit of additional information regarding this.

    Vacuum is the absence of pressure. Atmospheric pressure is around 15 PSI as stated. If the pressure is less than 15 PSI then the difference is vacuum. An engine is throttled by restricting the flow of air into the intake manifold. The amount of throttling can be quantified by measuring the vacuum level in the manifold. The higher the vacuum the greater the restriction or throttling and the less power the engine is making.

    When the piston moves downward and the intake valve is open the piston is creating a void which the atmosphere tries to rush in and fill. So, instead of thinking of the piston sucking air in think of the atmosphere trying to push into the void created by the piston moving downward. The throttle blades restrict the flow of air limiting how much air can push in. The air pressure outside the throttle blades is the 15 PSI of the atmosphere. Inside the manifold there is "pressure" but it is less than 15 PSI...depending on how much restriction the throttle body is creating. When the throttle is wide open there is no restriction, the atmosphere is free to push in and the pressure inside the manifold is the same as the atmosphere....15 PSI.

    Most engine development and calibration engineers think of manifold pressure instead of vacuum. That is what the MAP sensor measures....manifold pressure...i.e...Manifold Absolute Pressure. If you take the MAP and subtract it from the barometer (atmospheric pressure) the result is the vacuum in the manifold.

    Vacuum works fine for calibration work as long as the engine always operates in the same location and altitude and the barometer never changes. When the throttle is wide open the vacuum will always be zero since there is no restriction but the resulting pressure inside the manifold (that is really what is determining the power the engine will make) but the pressure or MAP will vary depending on the barometer or atmospheric pressure.

    At sea level the barometer is 15 PSI so there is 15 PSI trying to rush into the void created by the piston moving down. At the top of Pikes Peak the atmospheric pressure is about 8 PSI....so there is only 8 PSI of manifold pressure. That is why the engine makes less power. The manifold vacuum is zero at full throttle in either case so it is important to know the manifold absolute pressure to properly fuel the engine.

    This is also important when supercharging or turbocharging an engine as the manifold pressure is well above 15 PSI absolute. If the blower is rated as making 15 PSI of boost then the MAP is 30 PSI.... The blower rating is 15 PSI gauge which is measured above atmospheric pressure so the two are additive.

    So....there is "pressure" in the intake manifold...just less pressure than in the atmosphere due to throttling by the throttle plates. That "less pressure" is vacuum.

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    Thumbs up Re: Vacuum pressure

    Quote Originally Posted by bbob
    Yea...LOL...."vacuum pressure" is an oxymoron alright.

    While I basically agree with Iry99eldo there is a little bit of additional information regarding this.

    Vacuum is the absence of pressure. Atmospheric pressure is around 15 PSI as stated. If the pressure is less than 15 PSI then the difference is vacuum. An engine is throttled by restricting the flow of air into the intake manifold. The amount of throttling can be quantified by measuring the vacuum level in the manifold. The higher the vacuum the greater the restriction or throttling and the less power the engine is making.

    When the piston moves downward and the intake valve is open the piston is creating a void which the atmosphere tries to rush in and fill. So, instead of thinking of the piston sucking air in think of the atmosphere trying to push into the void created by the piston moving downward. The throttle blades restrict the flow of air limiting how much air can push in. The air pressure outside the throttle blades is the 15 PSI of the atmosphere. Inside the manifold there is "pressure" but it is less than 15 PSI...depending on how much restriction the throttle body is creating. When the throttle is wide open there is no restriction, the atmosphere is free to push in and the pressure inside the manifold is the same as the atmosphere....15 PSI.

    Most engine development and calibration engineers think of manifold pressure instead of vacuum. That is what the MAP sensor measures....manifold pressure...i.e...Manifold Absolute Pressure. If you take the MAP and subtract it from the barometer (atmospheric pressure) the result is the vacuum in the manifold.

    Vacuum works fine for calibration work as long as the engine always operates in the same location and altitude and the barometer never changes. When the throttle is wide open the vacuum will always be zero since there is no restriction but the resulting pressure inside the manifold (that is really what is determining the power the engine will make) but the pressure or MAP will vary depending on the barometer or atmospheric pressure.

    At sea level the barometer is 15 PSI so there is 15 PSI trying to rush into the void created by the piston moving down. At the top of Pikes Peak the atmospheric pressure is about 8 PSI....so there is only 8 PSI of manifold pressure. That is why the engine makes less power. The manifold vacuum is zero at full throttle in either case so it is important to know the manifold absolute pressure to properly fuel the engine.

    This is also important when supercharging or turbocharging an engine as the manifold pressure is well above 15 PSI absolute. If the blower is rated as making 15 PSI of boost then the MAP is 30 PSI.... The blower rating is 15 PSI gauge which is measured above atmospheric pressure so the two are additive.

    So....there is "pressure" in the intake manifold...just less pressure than in the atmosphere due to throttling by the throttle plates. That "less pressure" is vacuum.

    Very nice, enjoyable read. Thanks!

  9. #8
    dkozloski's Avatar
    dkozloski is offline Cadillac Owners 10000+ Posts
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    Re: Vacuum pressure

    Bbob, The engine responds to the pressure across the total engine because of the decreased exhaust back pressure at altitude. 30 lbs of MAP or 15lbs of boost at sea level results in 22lbs of boost on Pikes Peak across the engine with a resulting increase in power. This is why engine power does not drop off as radically at altitude on a normally aspirated engine as it might seem. This is of little consequence with automobiles but is important when figuring performance of aircraft.

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