Ignition Timing with RPM cam

[DJCarbine]

Finally got my 289 all sorted out with a new edelbrock carb, performer RPM cam, and gt40p heads. The only issue left is that of ignition timing. The car seems to run fine at around 14* initial, but there is 0 power from idle which is expected with this cam. However, if I advance the initial to something like 20*, the low end is a lot stronger and the car seems to pull great. The added initial timing also smoothes out the idle, and I put the vacuum advance on manifold, which gives it a lot of initial timing at idle.

How much timing is too much? I've gotten this car started with 40* initial and it ran ok. I always use premium, and the engine is a C code so I think the compression is a little less than the hotter variety 289's.

Whatever the dizzy mechanical advance is on (13L) should give me another 26 degrees of timing, for a total of around 46 total. Car seems to run fine up to 5500 rpm, no detonation or pinging or anything

The harmonic balancer is new, the cam was installed so that the timing marks on the gears lined up, and the distributor is a remanufactured 289 stock replacement with a Pertronix drop in and coil.

 

[D.Hearne]

Move the vaccum advance to ported vacuum. Set the initial to 12 degrees BTDC. If that doesn't work, something is amiss. Like the balancer has slipped. What intake do you have on it?

 

[Max Power]

Well, one frame of mind is that you can run whatever provides the most power without detonation, starting or overheating issues. Seems like you may have found that.

Caveat: Probably you shouldn't run manifold vacuum to the vacuum advance. With performance cams, quite often when you put it in gear or let out the clutch (ie: create a load), the rpms drop, causing vacuum to drop, retarding the timing at the very time you don't want to. It would probably be better to re-curve so you have more initial advance mechanically.

 

[coolblue65]

I ran 14 initial with 34 or 36 as my total timing. The last time I had it dynoed, the operator felt I had to much total advance...but I didn't mess with it at that time so who knows. 20 initial & 46 degrees total seems like a lot of timing. IMO take it to a dyno shop spend the $150 for an hour of tuning- it will take all the guess work out of it.

With that cam my 289 was never a torque monster down low, but once you got past 3000rpm the power came on the car would pull real hard and I used to shift just a tick past 6000rpm.

 

[DJCarbine]

For now I have the stock manifolds and a performer 289 intake manifold. Getting gt40p headers and a performer RPM manifold in the mail soon. Poor college student, butt dyno is all I can afford now. I will run timing in several different places and see which seems correct till I can get it dyno tuned.