Ignition Timing?

[ralmo94]

Years of searching the internet has produced a consensus that SBC's like 33° to 36° when carbureted, this seems to be high octane, Vortec Heads, like less, most say around 30° to 33°. The internet is about the same consensus about Big Blocks.
The Least I have seen reported on a carbed engine with an HEI dizzy is about 25 all in mechanical.

The question, ( I'm getting to it ) in a computer controlled ignition system, wouldn't say the 85kpa to 100kpa cells be the mechanical timing if you just put a dizzy in, then the rest be thought of as a vacuum advance?

If you set the initial timing at 8° or even as low as 6°, then that's the lowest the timing would ever go, and always more with RPM, until you get to all in RPM.
SO why is there 0° cells in the stock timing maps? Even at say 25° all in by 3200rpm, that is a lot more than stock timing maps I have seen.
Is the OE just leaving that much on the table? Are people over advancing carbed engines?

More internet searching on tuning, produces the just advance until Knock Retard, then back off, but more in depth searching says not enough timing can cause knock, and knock sensors can be fooled, by old style starter rattling, exhaust leaks, or even other vehicle noises?

I have been searching this forum for info on spark timing, and found a link 84elky posted to the thirdgen site, with a spreadsheet and how to basically.

Stock L29 advance maps only have about 12° max or so, about the same maybe for LS stock even High Octane table, (even though P01 & P59 is air mass based) TBI 350 Seems along the same lines until about 4krpm, Vortec 350 is a little more generous, with 18° by 2400rpm, but the internet say vortec heads don't need as much timing?

I KNOW every combination is different, vehicle weight gear ratio etc.

I have attempted to tune spark curves before, but am a little scared of not being able to hear knock, and causing damage if KR doesn't take enough out, and should the knock sensor be trusted?

I know there is also PE adder on top of all this also.

Long Post I know.
I appreciate everyone who reads the whole thing,
ANY thoughts are also welcomed.


Hope I posted in the correct section

[Sledhead2]

I am far from even being able to say I know tuning, but recently while messing with a Sniper EFI unit with Hyper Spark and trouble shooting a high idle issue. We noticed that the sniper in trying to lower the idle it was pulling timing out. So this car had initial at 16* and idle was high, to try and lower idle. computer was reducing timing down to 11* or so. High idle issue turned out to be secondary throttle blades being open a touch, thus the IAC was at 0.

But back to your question, are the other cells in there to allow computer to adjust as needed when something goes out of whack?

[ralmo94]

But back to your question, are the other cells in there to allow computer to adjust as needed when something goes out of whack?

I know there are altitude compensations, IAT compensations, but mostly what I have seen, they take timing out from the base map.
What I have seen for idle timing is, you have a value you set for idle, it shouldn't be at peak vacuum, but about 5° less, then you have a table that if rpm drop off a certain amount, it adds this much timing, and if too fast, it retards this much. The only thing "I" have noticed that adds timing is PE adder, cause richer mix needs more time to form pressure wave.

Thanks for the Post

[kur4o]

For ls1 stuff and up there is one table MBT spark. That is the ideal spark for that engine under lab conditions.

Usually optimal spark depends on combustion chamber, compression ratio and cam overlap.
Newer engine needs less advance to make power. The less advance needed the more efficient is the engine.

The time needed to ignite the mixture is the main pointer. Some Gm heads known as fast burn run on very small advance.
Heavier vehicles also run much less advance for the same engine

Carburators have wide range of AFR fluctuations and also the dizzy have some spark scatter, and the cam profiles make unique combo not applicable to later FI engines.
Gm controlling idle speed control also depends heavily on advance/retard. Much faster that the IAC

[Fast355]

Years of searching the internet has produced a consensus that SBC's like 33° to 36° when carbureted, this seems to be high octane, Vortec Heads, like less, most say around 30° to 33°. The internet is about the same consensus about Big Blocks.
The Least I have seen reported on a carbed engine with an HEI dizzy is about 25 all in mechanical.

The question, ( I'm getting to it ) in a computer controlled ignition system, wouldn't say the 85kpa to 100kpa cells be the mechanical timing if you just put a dizzy in, then the rest be thought of as a vacuum advance?

If you set the initial timing at 8° or even as low as 6°, then that's the lowest the timing would ever go, and always more with RPM, until you get to all in RPM.
SO why is there 0° cells in the stock timing maps? Even at say 25° all in by 3200rpm, that is a lot more than stock timing maps I have seen.
Is the OE just leaving that much on the table? Are people over advancing carbed engines?

More internet searching on tuning, produces the just advance until Knock Retard, then back off, but more in depth searching says not enough timing can cause knock, and knock sensors can be fooled, by old style starter rattling, exhaust leaks, or even other vehicle noises?

I have been searching this forum for info on spark timing, and found a link 84elky posted to the thirdgen site, with a spreadsheet and how to basically.

Stock L29 advance maps only have about 12° max or so, about the same maybe for LS stock even High Octane table, (even though P01 & P59 is air mass based) TBI 350 Seems along the same lines until about 4krpm, Vortec 350 is a little more generous, with 18° by 2400rpm, but the internet say vortec heads don't need as much timing?

I KNOW every combination is different, vehicle weight gear ratio etc.

I have attempted to tune spark curves before, but am a little scared of not being able to hear knock, and causing damage if KR doesn't take enough out, and should the knock sensor be trusted?



I know there is also PE adder on top of all this also.

Long Post I know.
I appreciate everyone who reads the whole thing,
ANY thoughts are also welcomed.


Hope I posted in the correct section

TBI heads require less timing than Vortecs. The swirlies will make peak power in the 25-29° BTDC range. Vortecs in the 29-34° range generally speaking. My 91+ octane timing map with Vortecs gives 24° as early as 2,400 rpm and starts out no lower than 12°. Cruise timing is about 44° but that is the limit for a Vortec distributor with the CMR pushed to the upper limit. With 24x and coil near plug, I have run as much as 52° of cruise timing in lean cruise at ~3,000 rpm on a L31 with a small cam.

My Express van was all in at about 22-23° at WOT and 40° at cruising speeds on the stock timing table.

[Fast355]

My L31 based aluminum head 383 likes the same kind of advance I ran when it had a carb with the P59 running it. 16° at WOT up to 1,200 rpm, smoothly advancing to 31° by 3,600. On the HEI, vacuum advance held off until 8 in/hg and was full on at 15° by 15 in/hg. Idle timing was also 31°. At 3,600 light throttle 46°.

[ralmo94]

My L31 based aluminum head 383 likes the same kind of advance I ran when it had a carb with the P59 running it. 16° at WOT up to 1,200 rpm, smoothly advancing to 31° by 3,600. On the HEI, vacuum advance held off until 8 in/hg and was full on at 15° by 15 in/hg. Idle timing was also 31°. At 3,600 light throttle 46°.

Fast,
Do you run any 87 octane?

Kur4o,
Wouldn't you still get scatter with a computer control dizzy?

[steveo]

traditionally you advance timing until your torque peak then level off, and that usually kind of works. usually in the low 30 degrees is a good target for most engines, but some iron headed engines like it a bit lower

my experience has been than ideal timing advance vs power looks like this on every engine (under load)

with no advance, you are not burning much fuel and it runs like crap

as you increase timing advance, power output and efficiency both ramp up, and its fairly linear

you reach a point i like to think of as the 'shelf', where you are adding more timing, but no gain is seen.

things start getting a bit hotter and the engine 'feels' a bit high strung when you are on the shelf. it's like you're tightening a string, but it hasn't snapped yet, and it looks the same as you keep tightening it, but please stop turning the knob.

it's best to have your timing dialed in at the start of this shelf, or even just a hair before it, because a few horsepower is worth the longevity.

past that 'shelf' you reach a point where nasty things start happening, power drops off, you are pushing the piston down while it is still moving up. power drops off and shortly after that you start pinging, knocking, and then burning holes in stuff.

because knock kind of occurs near the end of the 'shelf', when you reach the point of knock, i find it best to back off a decent amount.

the 'shelf' can be 1-2 degrees on some engines and 5+ degrees on others.

there are so many factors in good timing tables. fuel has a symbiotic relationship with spark. in general you need more advance to burn more fuel, but you also need more fuel to cool things off and handle more advance.

[ralmo94]

Thanks all for the read, and we'll thought out replies.

traditionally you advance timing until your torque peak then level off, and that usually kind of works. usually in the low 30 degrees is a good target for most engines, but some iron headed engines like it a bit lower

my experience has been than ideal timing advance vs power looks like this on every engine (under load)

with no advance, you are not burning much fuel and it runs like crap

as you increase timing advance, power output and efficiency both ramp up, and its fairly linear

you reach a point i like to think of as the 'shelf', where you are adding more timing, but no gain is seen.

things start getting a bit hotter and the engine 'feels' a bit high strung when you are on the shelf. it's like you're tightening a string, but it hasn't snapped yet, and it looks the same as you keep tightening it, but please stop turning the knob.

it's best to have your timing dialed in at the start of this shelf, or even just a hair before it, because a few horsepower is worth the longevity.

past that 'shelf' you reach a point where nasty things start happening, power drops off, you are pushing the piston down while it is still moving up. power drops off and shortly after that you start pinging, knocking, and then burning holes in stuff.

because knock kind of occurs near the end of the 'shelf', when you reach the point of knock, i find it best to back off a decent amount.

the 'shelf' can be 1-2 degrees on some engines and 5+ degrees on others.

there are so many factors in good timing tables. fuel has a symbiotic relationship with spark. in general you need more advance to burn more fuel, but you also need more fuel to cool things off and handle more advance.

I've heard of the "shelf" term before.

The question is, is that 30° for most engines, maybe a little lower for Detroit iron, for 87, or good gas?

I've read tons of threads that say "a tune" really wakes up a stock engine.
When I first got my HP tuners, I read my 4.7 dodge with a superchips tune in it, the only thing I could see different at all through the entire tune was the spark table. They didn't touch anything else that was visible with HP.