After hours of searching on timing light accuracy, msd box scatter, dial back light issues, timing light brands etc....I decided to test out a couple today.
1)My car is running locked out timing(no vacuum or centrifugal advance) so its really simple to check.
2)The timing light I have always used is a Sunpro(Actron #7519) dial back style. I have had this thing 15yrs and its always worked well, but the reason I was questioning this is now my car has a MSD 6AL and everyone says the dial back lights are not accurate, especially on a MSD ignitions. I really had no idea where I was at. On the dyno, we set the timing at 31* with a Snap On digital light. Upon installation in the car after stabbing the distributor, I set the timing at 32* with my Sunpro light and thats where I have been running it.
So today I go to Sears to buy a Craftsman non advance light, only to find out they are not selling them anymore...All they had was a Craftsman dial back style and the Craftsman Professional series (digital) with the tach and other stuff. Well since I already have a regular dial back, I decided to roll the dice on the digital model.
My balancer is already degreed so I figured the standard style light would be easy to test against the dial back method but....since I couldnt find the non dial back light, I will just have to hope the dial back method is accurate.
I came home tested both at idle, 1500 rpms and at 3000 rpms(since MSDs are supposed to only fire single sparks after 3000).
Both lights read IDENTICAL at all engine speeds and degrees.....Both light lights had about the same about of reading fluctionation at idle(I attribute this to the MSD multiple sparks but this was minimal) at 1500 rpms, both lights were reading the same smoothness at "0"(using the dial back set at 33*) and at 3000rpms, the line was rock steady at "0".
So for me, I consider two timing lights that are reading IDENTICAL to be pretty accurate, eventhough neither one I am supposed to be using due to their dial back feature and the MSD interference. I will say the digital light is pretty bad ass, the strobe is much brighter then the old Sunpro light and the digital action makes it idiot proof to add a degree or two with great accuracy. Lastly the tach feature is nice when you are laying over the fan revving the piss out of the motor thinking your are "up there pretty good" only to find out you are at 2k or less....LOL
I will keep both and continue to benchmark them against each other but, I guess this is close enough for me. As long as they are consistant, the absolute numbers shouldn't normally matter TOO much, but my motor is on the edge of detonation so I need to keep the timing in check at all times.
Sorry for the long post, just thought I would throw my experience out there.