|
The premise behind watching manifold pressure/vacuum is to increase engine efficiency. That's not the same as the absolute quantity of fuel being used.
That manifold vacuum drops when climbing a hill, at least without getting into enrichment, is a good thing. Low(er) engine speed and low manifold vacuum/high manifold pressure reduces two of the sources of wasted work within the engine.
For insight, find a BSFC map that also has lines of constant power on it and you can trace the effects of engine rpm - determined by gearing for a given road speed - vs manifold pressure for the given power output required to maintain that road speed.
Sometimes it's possible to trade off the increased vehicle drag associated with a higher road speed against the benefit of the engine operating at a higher efficiency combination of load (manifold pressure) and rpm. That can mean even though the power requirement is higher, the quantity of fuel used is similar or lower.
Knowing if you are in enrichment or not is really useful. There are any number of simple LM3914 based gauges that will show you that at very low cost. The circuit I described and provided a schematic for, for indicating TPS or MAP voltage on a string of LEDs is based on one. For an O2 sensor application, merely delete the 330k resistor that is part of the voltage divider for the TPS or MAP input.
The components for that are ~$20. Alternatively find one of the pre-packaged kits for only a little more or an assembled version for about twice that.
With the vacuum gauge and O2 LED meter, aim for the lowest possible vacuum achievable without losing the closed loop oscillation on the LEDs i.e. without entering into the enrichment. Just when that occurs will vary with rpm and load.
|