Timing advance guru input needed

[racerjoe]

I'm running sequential EFI and spark on a 408ci w/ aluminum heads and approximately 9.5:1 compression. I can set the timing advance to anything I want at any RPM, and any load level. I know it would be best to tune on a dyno, but I don't have access to one at the moment. Below is my current timing table. My cam RPM range is 2500-6800. Initially I didn't have all the timing in (36 degrees) until about 3200RPM and it was a little sluggish during acceleration. I changed it to what you see below and it made a huge difference. I was wondering if anyone has input on when full advance should come in. Should it be all in (36) at 2500, or can I bring it all in sooner than 2500RPM? Does anyone run more than 36 degrees? FYI, this is a street car.

The engine idles at about 850RPM and 55-60kpa. You can ignore those cells as they are only used during decel, or warmup. It follows a different timing table at idle.

 

[67Dart273]

I say all you can do is try it. To some extend this varies with compression ratio, heat in the engine and even OD temp, gearing, etc.

 

[flyfish]

I would try all in at 2500, see how it goes.

For what its worth, I have my timing locked at 34 on my Eddy headed 408 and its happy...granted, my car is a race car first, and a street car second.

 

[famous bob]

I say all you can do is try it. To some extend this varies with compression ratio, heat in the engine and even OD temp, gearing, etc.

And fue octane rating ----------

 

[toolmanmike]

Just for clarification, what's your build? Cam, transmission, gears, tire size?

 

[nm9stheham]

Just curious: Do you have a knock sensor on the block to detect detonation, and some tactic programmed in to handle detonation? That would be useful as you get more agressive on timing. Otherwise, keep a sharp ear out for knocking and learn the signs of that on your plugs.

 

[BigBlockMopar]

Only the engine itself can tell you what curve and timing wants.
Listen for ping or better, use a knock sensor.

Why aren't you using the 'vacuum advance' fields properly?
Values at the top end (row) of the able should always be lower the values in the fields below them.

 

[Mattax]

Why aren't you using the 'vacuum advance' fields properly?
Values at the top end (row) of the able should always be lower the values in the fields below them.

^^^^^^^^This^^^^^^^^^^^

I don't trouble myself with EFI so forgive me a little here.
All the same principles apply.
As rpms climb, lead time in degrees is needed because the engine is turning faster and there's less time (seconds).
As efficiencies build, the combustion itself speeds up, reducing the amount of additional lead needed in the upper rpms.

from MTSC 1959

On your table, the left is presumably MAP.
So when looking at timing from traditional analog control, it will be discussed as vacuum.

So, 105 kPa is atmospheric or for purposes, wide open throttle.
and 40 to 60 kPa should be low throttle, steady cruising on a flat highway or interstate.
The engine should be running leaner than at idle or WOT.
Finally between 80 and 60 kPa is throttle is opened further, and in this range will be when the leanest mixture will make the most power.

Lean mixtures take more time for the flame to travel. So this is when the most lead time is needed.

from MTSC 259. Ignition system analysis. 1969

This lead time for lean mixtures (vacuum advance) needs to be removed as the mixtures go rich again approachin gwide open throttle.

That's the basics of it.

 

[Mattax]

If you want to compare with a distributor-
The boxes outlined in white are timing from a distributor's initial and mechanical advance.
The boxes outlined in green are approximately the timing with vacuum advance added in.
In between is in between.

Comparing with a factory iron head is all I've got. But assuming your engine's combustion efficiency is similar or better, might want a little less advance from 1800 to 3600 rpm.
Again using a stockish engine as a some guidance, probably want around 50* when MAP is 40 to 50 kPa

All those things 67Dart273 posted play into it so this is just to give you some ballpark.

 

[racerjoe]

Just for clarification, what's your build? Cam, transmission, gears, tire size?

It's a custom solid roller, .242 intake, .249 exhaust at .050", .278in, .286 adv., .594 in/ex lift, 112LSA. Tremec TKO-500, 3.91 gear, 26" tall tire.

 

[racerjoe]

Wow! Thanks for all the information guys. While I do consider myself pretty educated about engines and their functionality, I've never been one to learn about the ultra finite details to edging out the perfect tune, via carb or efi. Prior to the efi conversion, I always ran an MSD dizzy without a vacuum advance, so it wasn't even there for me to tune.
I've had intentions of adding in "vacuum advance", but have been focusing more on getting fueling right first. Now that I have that done I'm working on the timing curve. This is where I started. While the intent of my initial question wasn't about the cruising curve, I appreciate the additional feedback. Maybe I'll start getting some better MPG with the "vacuum advance" in there.