I've been reading around looking for data on how to interpret manifold vacuum. Why? Well, my original thought was that 0.0hg meant that there was no restriction in the intake system and would mean you're getting the most potential. I soon found that this is not the case. Since there has to be a pressure difference in order for air to move, 0.0hg would mean no air is moving.

If vacuum is reading 20hg with closed throttle meaning it has massive restriction, and 0.0hg at WOT means no air is moving, what would the optimal reading be? Is 0.1 better than 1.0 or vice versa? What are the variables for determining optimal vacuum at WOT?

I have only found data from carb'd setups in which vacuum has different meanings. I'm specifically looking for fuel injection MPI and GDi setups.

Please correct any of my assumptions/statements as necessary.


I remember that gauges were available for tuning / performance fitment : id est : boy racer / petrol head applications and the gauges came with an explanation sheet which explained the readings or fluctuations, but this was, as you point out, for carburettor applications.

I have not seen any documentation that refers to the use with fuel injection setups - mainly I think, because the "throttle" as was gave a restriction to the airflow and most, if not all, injection setups either with or without turbo rely on a "fly-by-wire" throttle and have a restriction-free air intake with sensors to measure temperature etc.

  • 1
    "throttle by wire" still has a good old throttle plate. Its just operated electrically instead of by cable. – agentp Mar 21 '17 at 20:14

Manifold gauge vacuum is the difference between manifold absolute pressure and barometric pressure. The reason you see 0 inHG at WOT is because manifold pressure is equalized to barometric pressure in the manifold on a naturally aspirated car. On the other hand when the throttle is closed you'll see roughly 20 inHG. Engine vacuum tends to differ between engines and I'm unsure of how exactly this is determined. I'd suppose that it depends on the size/number of the intake and exhaust valves as well as where you're tapping into the manifold to determine gauge vacuum. In general engine vacuum at idle is anywhere from 18-20 inHG. You'd be more interested in doing an in cylinder pressure test with a scope to determine how much air the intake valves are allowing in.

You should never see anything but 0 inHG at a manifold vacuum source at WOT. older Chryslers used to have a performance gauge which was basically a manifold absolute pressure gauge.

  • I'm not asking why I see 0gh, I'm asking 1) does 0hg mean that there are really no restrictions 2) doesn't 0hg (which should equal baro as you stated) mean that there is no air flowing due to equalization of pressure on both sides of the throttle plate, so there should always be < or > 0hg in order for air to be flowing, correct? If that's valid, then is 0.1 more optimal than 1.0 since there is a bigger pressure difference, wouldn't 1.0 suggest more air flow? Or is my understanding flawed? – DustinDavis Mar 21 '17 at 20:49
  • @DustinDavis You're misunderstanding I think. The only real restriction is in the throttle plate and if it's wide open than pressure is equalized. This isn't taking into account the pressure difference between in cylinder and manifold pressure. As the piston goes down before the intake stroke it creates a vacuum in the cylinder. Realistically you'll never see 1 inHG or 0.1 inHG. If you had a plugged exhaust or something else that restricted engine airflow you'd really only see it at idle. – Ben Mar 21 '17 at 20:57
  • possibly, which is why I'm asking. A real world example would be a change made to intake manifold (let's say a mild port), the fuel trims are now reading lower (meaning it's trimming less fuel) suggesting more air going into the cylinders, causing AFRs to lean out. The MAP was reading 0.9 before and is now reading 1.2 at WOT. So in this instance, 1.2 "seems" to be better than 0.9 – DustinDavis Mar 21 '17 at 21:02
  • @DustinDavis I'm no expert on porting, I thought that porting was a way smooth the airflow inside the manifold to the cylinder as that has an effect on combustion. The only way I can see more air entering into the cylinders would be with bigger valves or a change in the cam profile or VVT. – Ben Mar 21 '17 at 21:13
  • well, if vacuum readings are due to restriction, let's say in the intake runners or even in the air filter, and you remove those restrictions (porting or eliminating the filter), wouldn't the vacuum readings get closer to 0? If that's true, then 0 vacuum would be the optimal point, correct? That's the ultimately the basis for my question. – DustinDavis Mar 21 '17 at 21:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.