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Ignition timing on big solid cammed 383 sbc

5K views 7 replies 4 participants last post by  v8hed 
#1 ·
I started talking about the ignition timing in my other thread (Victor Jr street manners), but thought I'd start a new thread, as it was going off-topic.

I've been following the advice of F'BIRD and set my initial timing at 22*. He actually suggested 24*, but I thought I'd be slightly conservative at first. I've also hooked a vacuum gauge up. Idle vacuum @ 22* was 7.5" hg. I limited the mech advance to add another 12* all-in by around 2,500-3000rpm for 34* total (this is what made best power on the dyno). Car seemed to drive fine and I couldn't detect any pinging (although not easy to hear with sidepipes and an open cockpit). However, I was curious to see if I could get any more idle vacuum, so tried increasing the initial... managed to get around 10" hg, but that was at 44*. Backed-off to 34* and got 8.5" (jumping between 8 and 9) @ 950rpm. So, I've now locked-out the mech advance and have 34* total across the board. Car drives fine (didn't notice any difference from when the initial was at 22*).

Does the above sound about right? FWIW, it's a street car (Cobra rep) weighing 2,400lbs with 3.54:1 rear and Tremec TKO500 trans.

PS. I did a very rough estimate (counting in my head!) of 30-50mph in 4th, which took 3 seconds. 30mph in 4th is pretty-much idle RPM.

Cam is a solid FT 248*/248* @ 0.050. Carb is a 750DP Holley.
 
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#2 ·
If its too loud to hear pinging, look at your plugs with some sort of magnification device and look for small black specs(aluminum from your pistons) indicating detonation or ping. I don't know what your CR is or your fuel octane, not that they are absolute indicators of maximum timing, but they can give an idea of where to start for engine safety. I'm sure others can tell you alot more than I can.
Good Luck
ssmonty
 
#3 ·
FBird knows his stuff. Ideally you'll be better off with 24' which would give you a slightly higher rpm and higher vac at idle but youd need to adjust the total timing to compensate.

It seems like you were idle timed at 32' when you got the highest vacuum and that also increased you overall timing. My opinion is that its not unheard of but I bet the idle rpm was higher too and maybe too high to sit in gear? I have my 383 stroker at roughly 28-30' at idle which gives my 1000rpm and about 10-12vac. If Fbird says 24 only then Id say he's correct.

There a ton of posts where they talk about the cars with the choppy rump rump idle and it comes down to poor timing overall. So a higher idle timing will smooth out the roughness, give you a better vac reading, and then just limit the overall.
 
#4 ·
bubbahotep said:
FBird knows his stuff. Ideally you'll be better off with 24' which would give you a slightly higher rpm and higher vac at idle but youd need to adjust the total timing to compensate.

It seems like you were idle timed at 32' when you got the highest vacuum and that also increased you overall timing. My opinion is that its not unheard of but I bet the idle rpm was higher too and maybe too high to sit in gear? I have my 383 stroker at roughly 28-30' at idle which gives my 1000rpm and about 10-12vac. If Fbird says 24 only then Id say he's correct.

There a ton of posts where they talk about the cars with the choppy rump rump idle and it comes down to poor timing overall. So a higher idle timing will smooth out the roughness, give you a better vac reading, and then just limit the overall.
Total timing was 34*. I ended-up with 34* initial and the advance locked-out. May go back down to 22/24* with 12* advance for a bit of a safety margin, as I couldn't feel any difference between 34* initial and 22* initial. The only reason I went to 34* initial was it gave me another 1" hg vacuum @ idle.
 
#8 ·
F-BIRD'88 said:
The basic power range is 3000 to 6500+rpm. Why you are making power test runs from idle rpm in 4th gear I don;t know....
The engine wants to rev. Rev it up... use the gears.
1. Because it's a street car
2. Because 30-50mph in 4th is an accepted performance benchmark
3. Most importantly, to see if it pings under high load/low RPM

Testing the car from 3,000rpm up would not tell me anything about the timing adjustments I'm making. As the timing is all-in by around 2,500rpm, whether I'm using 14* initial, 22* initial or 34* initial with advance locked-out, they'd all test the same from 3,000rpm up. It's the performance/behaviour of the motor under 2,500rpm that is being altered by all this timing adjustments.

The more I think about this and the more I learn about the subject from a whole host of sources, the more I think that setting an initial of 14* + full manifold vacuum advance with a vac can that starts at low vac (<6" hg) would be a better option for me. This would also take away any fear that the motor could be pinging or generating reverse torque under wide throttle openings at lower RPM levels. The vac advance will deliver more advance when the motor can deal with it better (under light load, small throttle openings) and take it away when the motor can't deal with it (high load, wide throttle openings). As I said, all scenarios are the same after 2,500rpm anyway.

Appreciate the input fellas... it's all a learning experience.
 
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