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?
Both, I hate this; "but it all depends". Sounds like Einstein talking about relativity doesn't it?
It gets down to more than one way to set up the carb. To simplify; if the main metering is rich then you can delay the power circuit to a lower vacuum before it's necessary to turn it on. If you're running on the lean side say for good cruise gas mileage, then you may want the power system to come on earlier to cover power and anti-detonation requirements say when pulling a trailer or heavy load in this case having it come on earlier but not under cruise vacuum would be desirable.
If your idle vacuum is only 10 inches then this speaks to a large cam. Large cams speak to high performance so you want the power enrichment to come on earlier. But not at cruise if this is a street strip machine; so you need to get your cruise vacuum reading as well, which might be higher than idle vacuum. This is an effect of gearing which is a big player in this, therefore, a major reason why hard, fast rules are dangerous to apply.
A full up race engine that never sees the street tends to block off the power system and use really rich main metering instead, but these engine are either idling or at WOT so in-between metering like cruise isn't a consideration while eliminating the power circuit is one less potential failure point.
Certainly if you can do all of this tuning with an exhaust gas analyzer all the better?