Trouble with my timing light
- Thread starter johng
- Start date
Forrest Gump
CCCUK regional rep
John, the HEI distributor normally has 22 degrees of mechanical advance so you can set 16 degrees initial (no vac) for 38 total.
johng
CCCUK Member
Thanks Andy, at the moment I just wanted to get it running to check things out (eg I now know that my headlamps pop up and down properly and my vacuum advance works). With no seats, instruments or winsdcreen in it I can't really take it for a drive to see if it's pinking. When my new damper arrives I'll either add a few extra timing marks (36 degrees for example) or stick a tape on it and set the timing more aggressively once it's on the road. From what I've read I might need to restrict the amount of vacuum advance if I set it for 36 or 38 degrees without the vacuum and also might need to alter the centrifugal to come in earlier. What have you done on yours?
Forrest Gump
CCCUK regional rep
On my old motor when it was still 350cu in I had 38 degrees all in at 2500rpm. I think the vac added something like 16 degs at cruise, It never pinked.
Roscobbc
Moderator
Ignore the effect of vacuum advance - it's only ever operational when you have high vacuum conditions (like cruising or idle) depressing accelerator will instantly reduce vacuum and correspondingly retard ignition to whatever centrifugal advance is for specific engine rpm. Vacuum advance should never (on its own) create a 'pinking' issue (if functioning correctly). On cruise you could have up to 52/53 degrees total advance. I did some additional research on this earlier this week - a rich mixture ignites far easier and quicker than a weak mixture. Therefore a weak mixture takes longer to ignite - ergo it needs to begin ignition sequence earlier to avoid being retarded (so additional ignition advance) to counter the longer ignition sequence of the weak mixture. I didn't know that.......which might explain why when using nitrous (which is an extremely rich mixture) you need to significantly retard the ignition. Perhaps the same reason with turbo's and blowers?
johng
CCCUK Member
Sounds like yours was all in at quite low rpm, did you get new springs or weights to achieve this? If so, which ones?
Forrest Gump
CCCUK regional rep
I think getting the timing “all-in” anywhere between 2500 and 3000 is the aim to give the best performance. The timing advance kits like from Moroso have new weights and springs but I’d keep the weights you‘ve got and just fit the lightest springs or one light and one medium. You never know, the prevoius owner might have already done it on your car.
Roscobbc
Moderator
That was the very first thing I did 20 years ago when I first bought my Vette. So, Morose kit with lightest springs, heaviest bob weights and advance was all-in (believe it or not) at 2000 rpm with the advance curve starting at 800 rpm. Never any pinking - ran smooth as anything - would pull from 700 rpm. But is was a stock 427 big block and a manual and larger engines can use far more timing - wouldn't expect to use the same timing curve on a small block automatic.
Current 489 engine's timing is all-in at 3200 rpm.