Manifold vacuum vs performance

Question

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.

Answer 1

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.

Answer 2

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.