70 LS1

iTrader: (1)

How quickly does vacuum build up when starting the engine? Is it higher when cranking or at idle?

Adrenaline_Z

Vacuum (low pressure) within the intake manifold will occur as soon as one of
the intake valves begin to open and the piston draws downward. Being an 8
banger, it's almost certain to occur immediately.

The amount of vacuum during cranking? Not really sure...but I can tell you
there is more vacuum (lower pressure) during idle.

white2001s10

iTrader: (4)

Vacuum surges hard during cranking and then smoothes out at idle. Idle pulls more vacuum due to higher engine speed with closed throttle. The minimum air rate and IAC settings will have an effect on cranking vacuum. A lower idle setting would create more cranking vacuum.
If you open the throttle while cranking then you will get no vacuum during cranking.

70 LS1

iTrader: (1)

Thanks

P Mack

iTrader: (6)

Looking back at an old log file, map starts at 100 kpa before cranking (obviously), as the starter spins the engine to ~450 rpm MAP is at 65 kpa, and at normal idle MAP is about 45. That's with my old c1 cam. So yes, more vacuum at idle than cranking. Makes sense, there's more air through a same sized (or smaller) hole at idle.

wrencher

iTrader: (2)

Ya a typical rule of thumb is to look for at least 2 hg" of vacuum cranking. Otherwise there is a problem.