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I have a .484 in my 383 with power brakes, no issues. It idles pretty well too.
I'm running 0.541/0.537 lift 230/236° duration at 0.050 on my 496, w 16° base timing, I idle at 850 in gear at 14" vacuum.
Sure! This one would be just fine
It does sound like there are other issues with the vacuum being that low? What year type power brakes? Drum or disk? My '71 Charger Power Disc brakes seem to work fine with 12"+ vacuum. Just using a vacuum resvoir/can with a check valve. A few blips of the throttle builds up the vacuum after startup. The engine has very low vacuum, maybe 4-5" at idle, 270+ duration @ 0.050" cam, 0.726" lift
Seems there are more mystery cams around that seem to pull less vacuum than expected. I am running an "oldie" Hughes engines 3844BL cam (238/244* @ .050") that only pulls about 7" vacuum at idle of 850 rpm. Believe me, i've tried everything to ensure i have no vacuum leak, it does the same with the old carbs i used and with the current Holly Sniper EFI. My power brakes do work, although during idle there is some more effort required to brake but it is managable. My future plan is to swap to a 2832BL cam to get a little more vacuum and better drivability.
from my experience the comp 21-305-4 was pretty miserable in my 440. the mopar 272-.455 went 3mph faster in the 1/4 mile than the 21-305-4. i have no experience with the crower 271. the summit 6401 has good mid-range and reasonable vacuum at a low cost. i liked that summit cam using cast manifolds. but, i'd look at at a comp xs268.