So what you will find is you can set your timing at 43 46 but under load it will back up. sometimes a lot. I have personally seen from 2 (pretty good) to 8 thats bad.
I too run a gear drive
A few years back when I decided to figure out how a shaft collar works and why I found my timing was backing up 7/8 degrees under load so when I thought I had 39 I had 31/32 going down the track. the shaft collar limited that to 2/3 degrees and so setting at 39 still I ran almost 2 tenths quicker now it was 36/37 under load. later that year I went all the way up to 42 which seemed to back up to 38/39 and ran quicker yet. This was with 11.85 to 1 compression Sunoco 94 which I tested last year and is 10% ethanol.
This year mine seems to be backing up 5 6 under load (I think the brass gear has some wear again) so last time out set it at 43 and have it at 45 for next time out.
Was explaining this timing roll back to a friend and next time I seen him he told me they checked the timing on his dads 500 and when they thought they had 36 they had 33 under load so they went up to 39 and now have 36 and picked up a tenth.
So this came up when I picked my balancing up for my 574 and he said oh yea we have found this even on locked out dizzy's not sure if it is all slack being taken out or some right in the electronics as rpms come up but they now like to set timing as they run on the dyno to see where it really goes and set accordingly. They said one customer they have with locked out timing they showed him by 6000 his was back 12 degree. when they put it up it gained about 40 hp.
They made an ingenious rig to run motors on their chassis dyno. rear end with slicks, short drive shaft, tranny (5 speed i think) and then set up that any motor can be bolted into the rig powering through the clutch.
Anyways according to him the best way to get a true rock solid timing is to go to a crank pick up. Then your locked out dizzy is only aiming the spark and your timing chain (timing gears) cam walk, distributor drive (wear) are all taken out of the equation.
so this is on my wish list
http://www.summitracing.com/int/parts/msd-8636/overview/make/chryslerThey said if you have advanced till you got your best ET's you will not run any faster but timing is rock solid and not moving all over so easier to dial in.
PS the reason the shaft collar works is the pump/distributor drive gear can move up and down thus advancing and retarding the distributor as we know you turn it CW to get it to go down in and CCW to get it to come out or opposite directions big block to small. the shaft collar helps to limit this. A LOT in some cases. (think it is how deep you dizzy shaft tang is compared to depth of gear slot???)
