Setting up an initial ignition timing table

[wmahaffey]

I have finished my MSnEDIS unit and am almost ready to try to start my engine. It is a 1994 Lincoln 4.6L 4 cam. When can I obtain good starting values for the fuel and spark tables?

Thanks,

William Mahaffey

[Bernard Fife]

William,

You can get a base starting point for the VE table by using the 'Tables/VE Table/Tools/VE Specific/Generate Table' function of MegaTune, using the peak horsepower and torque figures for your engine.

For timing, we don't have a generator written yet (it's on my list, but unfortunately not near the top!). The basic principles are to determine a maximum advance for your engine and work backwards from there with heuristics:

- older engines (1960s up to 1990 or so) with two valves - max advance = 36°
- newer two-valve engines - max advance = 30°
- three or four valve engines - max advance = 26°

then adjust for bore size:

- under 3.5" (89mm) - subtract 3°
- between 3.5" and 4.000" (101.6mm) - no adjustment
- over 4.001" (+101.6mm) - add 3°

then adjust for the fuel:

- regular - subtract 2°
- mid-grade - subtract 1°
- premium - no adjustment

That gives us a maximum advance figure. It you have an aftermarket combination with a good squish area and optimized quench, subtract another 2°. If you have a flathead, add 3° or 4° or more.

We will use this to fill in the table at 100 kPa from 3000 rpm to the redline.

From idle to 3000 rpm, we want the advance (@100kPa) to increase fairly linearly from the idle advance to the maximum advance. idle advance is really a matter of tuning, but assume 8° to 16° in most cases, with stock engines being on the lower end, and 'hotter' engines being on the upper end.

So if we have a hot engine with 36° maximum advance and 16° idle advance (at 800rpm), the spark table might look like this for 100kPa:
<pre>
100 16° 16° 18° 24° 28° 36°
rpm 600 800 1000 1500 2000 3000
</pre>
Below 100 kPa, we add 0.3° per 1 kPa drop. So for example, if our total spark at 100kPa and 4000 rpm was 36°, the advance at 50 kPa would be:

36° + 0.3° x (100-50) = 51°

and the advance at 45 kPa and 800 rpm would be:

16° + 0.3° x (100-45) = 32.5°

However all of these would need to be tuned, and it often helps idle stability to limit the advance at idle to under 20°.

Finally, for boosted engines, you subtract 0.3° of advance for every kPa above 100 (it's not a coincidence that this is the same factor as for the 'vacuum' adjustments). Because 101.3kPa=~14.7psi, this works out to ~2° per pound of boost. It is often the case that you want to limit the retard under boost as well, typically so that it takes out no more than about ½ of the maximum advance at 100 kPa.

None of these will give you the 'right' values for your engine though, and like the VE table calculator, are just a relatively safe starting point. They should be somewhat closer than starting with an empty table, though!

Lance.

[wmahaffey]

Lance,

Thank you so very much.

William

[Scott]

Very interesting! I've never seen anyone attempt to formulate an advance curve.

Just out of curiosity, I plugged the method into a spreadsheet and found some similarity to the rough Seat-O-Pants spark advance tuning that I came up with some time ago.

Here is the file for those who may be interested in modifying it for their own engine combination.
http://planetcampbell.us/IgnitionAdvanc ... timate.sxc (OpenOffice.org file format)

Just enter the appropriate values in the boxes in column C.

[mbohn]

My pappy said, "Son, you're gonna drive me to drinkin..."

[eliotmansfield]

For the benefit of those running Mr Gate's products (and to save you downloading openoffice!) Ive converted Scotts spreadsheet into Excel format and also added the "boosted" calculations.
I'm no excel wiz, but i'm sure there's a way to add the max retard function in also so it never goes over/under a certain amount.

[wmahaffey]

Eliot,

Thank you very much.

William

[kbracing96]

This is realy cool, Thank for your hard work guys

Kyle

[eliotmansfield]

Its handy starting point i think. the figures at the edges are a bit wacked out though, your actual curve would be a roughly diagonal line from the bottom left to top right.

I would try to smooth out and flatline the edges, as you are clearly not going to have -22 advance (retard)

[ronchinoy]

Briliant guys.
Though you lost me when I read this

Below 100 kPa, we add 0.3° per 1 kPa drop. So for example, if our total
spark at 100kPa and 4000 rpm was 36°, the advance at 50 kPa would be:
36° + 0.3° x (100-50) = 51°

I dont see how an engine can run 51° under any conditions.