Hey gang...

I was curious on the range of vacuum required for normal/safe operation of the power brake booster ( bendix ). Yesterday I got the 426 HEMI running and I'm drawing about 10-11 inches of vacuum. I am going to try and fine tune the timing and fuel-air mixture some more.......

Cheers,
Troy

iirc I heard 12 or 14 but I did not pay attention since I never run PB

Call it coincidence, but I just checked my vacuum on my HEMI. It is 540ci, 10.78 CR, cam is roller 246I and 254E @.050 on 112 lobe separation. Intake is StageV (single plane) with two Holly 750s. 17" vacuum @ 900 rpm idle. (initial advance w/o vacuum advance hooked up is 14 degrees.)
My power disk brakes work great with factory HEMI dual diaphragm and large bore master cylinder. My car is a 727 with 3.54 gear, 4,175lb. without driver. Note, I wouldn't expect same results with cheap GM based aftermarket single diaphragm,small bore master cylinder from Camaro collection, owner brake company.

Just spoke with a friend of mine. He is old-school Mopar and former car-chief for Michael Waltrip. He echoed what a lot have already mentioned over the years and I probably need to go down the same path. I am using a Chrysler electronic distributor as my trigger for the MSD-6A ignition. He proposed I remove the springs, install both light springs, bee-bee the vacuum line ( I want to keep the OEM look ) and set the total degrees at about 32 as a starting point. I mentioned this will gain my more initial timing and hence vacuum. If you have 17, I should get more than 11......

Now that I have the motor started, the next phase is test and tune for THIS particular motor...

My 496 started at around 10" before tuning. After it was around 13" and PB work fine. Some folks have reported decent PB as low as 10" vacuum but that is probably marginal.

I got my idle vacuum up with more advance... 18-20 degrees at idle and fine tuning the idle AF ratio. In my case it was data logging and laptop time (FAST XFI).