Anyone know what vacuum a newly-rebuilt 3.8L engine should pull at idle? Thx.

 

16 to 20 inches Hg. Once run in and no manifold leaks etc. typically 20 in Hg at sea level.

EDIT: As reminded below drops with altitude. As a reference 15/16 at 5000ft.

The differential also drops with rising altitude. But for our purposes and range drops approx 1 in Hg (inch of mercury) for every 1000 feet is an OK rule of thumb.

If you want more accurate than that Google one of the many tables available.

 

Check points, carb adjustments and timing, each time you change something vacuum can change. When all is adjusted maximum vacuum at idle.

 

It will depend on your elevation too. At my elevation the atmospheric pressure is about 4 in Hg less than at sea level, so that affects the maximum an engine can develop.

 

Absolutely. I should have stated Sea Level. Reduces with altitude. Corrected & embellished above with example. Thanks.