Originally Posted by Zerocyde
Half the people I talk too say you cut your idle manifold vacuum in half, and round down to the nearest .5, so with 10hg you want a 4.5pv, etc. The other half of the people I talk to say you want 2 hg below your idle manifold vacuum, so with 10hg you would want a 7.5pv.
Who is correct, and why?
If you use example #2 what if your engine vacuum is 15 hg at idle and you -2 hg the largest PV number you can get is 10.5PV Everytime your engine vacuum drops below 10.5 the power valve opens feeding more fuel. If driving in a hilly countryside and the vacuum stays at 8.5 cruising this would be rough on gas mileage. If using example #1 with the same engine vacuum and 15 hg*2 = 7.5PV. If driving in the same conditions the PV never opens until 7.5 hg so you have a lean condition which could cause severe complications.