hi david,
i have been gazing at your graph, and i dont see that it is so hard to explain (nothing like bravado is there?)
as brent touched on, and you have confirmed, you were using a ported vacuum takeoff,
which once was for the dist vacuum advance. for the benefit of other readers too,
ported vacuum simply means that the vaccum is taken from a hole which is covered
by the throttle plate, when it is fully shut. at that point, vacuum will be 1 bar, as the
hole is no longer on the engine side of the throttle plate. if the throttle is opened even
a small amount, the hole is then on the engine side of the plate, and seeing manifold vacuum.
implication: if you foot is off the throttle, measured vacuum will be 1 bar.
note: ported vac shouldnt give the opposite to manfold MAP. it is only different when the throttle is closed but the motor is revving. afaik, the main purpose, if indeed there is any other, of ported vacuum, is to remove all the vacuum advance at idle, as high advance at idle makes it difficult to set a steady idle speed. if you want to see how much of a difference that makes, just twist a distributor in the block at idle, and see how much the car revs up when you twist in 30deg advance. that is all assuming anyone on this forum actually remembers what a distributor is....
on your chart, you would expect to see measured MAP (from your ported outlet)
at a maximum, as soon as your foot is off the accelerator pedal. that is represented
by either idle rpm, or rapidly falling rpm. i think your chart shows high MAP in such circumstances.
next, you would expect to see low MAP ie high vacuum, when you have your foot lightly
on the pedal and the engine turning over at significant rpm. i think the chart shows that too.
every time you rev the engine up, MAP goes down. that would be counterintuitive if it were
on the road, because you would likely have the throttle wide(ly) open, which should lead
to high MAP, right? i am guessing however, that this was measured in the garage, in which
case the throttle is never really very wide open when reving the motor. so what you have is
part throttle and high engine speed, which should cause low MAP/high vacuum.
was this measured with the car stationary? i think that would explain it all. if not, let us know
so i can publicly disavow most of what i have said above
.
regards
alexander.
