Alot of iron headed engines built from the 60's to the 90's (no aluminum out there yet for most mopars) were built with open chamber iron heads and no quench. When these guys added alot of mechanical advance they were getting pinging at part throttle with the vacum advance hooked up as they could be seeing up to almost 60 degrees advance at part throttle about 3000 rpm. Problem was with iron heads and no quench and the cheap gas many people just unhooked the vacum advance to stop the pinging. But today with aluminum heads and quench and with custom cams to keep cyl press low enough for pump most can get away with 50 or more degrees at part throttle even on pump and of course at wide open throttle they only have the mech advance around 32 to 38. I saw that alot. As for me I got a great deal on a race dist with no vacum advance so I cant run one or I would if I had it. But my car runs good since I have alot of very fast mech advance but I could get better part throttle milage with a vacum advance as I would just have to tune it in. Also race cars dont use the vacum advance since they are mainly at full throttle most of the time and many saw that race cars did not use the vacum advance so they thought they would be faster without it. Ron