Copy & paste from another thread... More timing will give you a higher vacuum reading. I run my timing locked at 35*. It will idle in Park at 15", and 12" in gear. If you used a vacuum gauge to determine best idle vacuum, you would almost certainly end up with more higher RPM timing than you wanted. What is timing locked.
Disabling the mechanical advance and running constant timing. It is an option on MSD distributors. Instead of using the advance stop bushings, you can remove the gear, pull the shaft partially out, and turn it 180*. When you do that, the bushing pin fits into a hole in the advance plate, and locks out any mechanical advance. Locked timing can be desirable with a big cam. The engine idles better and makes more vacuum. It is also a requirement for some EFI systems that control ignition timing. My 470 seems to like it a lot. I use it in conjunction with a select able 20* start retard on my MSD box. To lock the timing on my Dave Ray converted points distributor, I fabricated a metal plate that fits over the weight pins. I removed the weights and springs, moved the distributor to full advance, put the plate on, and installed the rotor, and it locks out the mechanical advance.
The advance mechanisms are locked so their is no centrifugal or vacuum advance. The timing is set at the high limit and doesn't change. there is a timing retarder that allows the engine to crank. Usually this is race car stuff but some guys run it on the street.
Thanks... I did a search on the web.... I came up with more comedy answers.. After a while time to toss it back to where it came from...Cool thing hit a home run with bases loaded...
Guys like to lock their distributors in to one set degree, eliminating the mechanical and vacuum advance, mostly for drag racing where the stall of the converter or the rpm where you drop the clutch is above where the curve would come in anyway to eliminate the possibility of the mechanical advance failing....example is, most performance engines like to have the timing "all in" by about 2500 rpm. If your converter stalls or you drop your clutch at 3500 rpm on your drag car, you're already above that 2500 rpm in the timing curve, so you just eliminate the mechanical advance because it's not needed....You can put an inline ignition kill that cuts spark for starting if needed. You simply turn off spark, get the engine cranking, then flick the switch to fire it up. Most engines that a guy locks the distributor out don't need to do this because they have a large enough cam with enough overlap to bleed off compression at low rpms, but some still need it, or you can get ignition retard for starting as Larry said.
^^^ Yeah, you want to see a car with a performance engine become a dog, have it come out of the hole and drop RPM, losing mechanical and or vacuum timing and it lays down and takes a nap. (usually with stock distributor and initial timing set per book). And in bracket racing, non-locked distributors can have you chasing numbers until you chase your trailer. Lock it out, set it and forget it (mostly).
