How to calculate a base ignition table
 

Here's the approach I'm taking to calculating my base ignition table. That is, my starting point to get the car driveable enough to whizz it around the block a few times and eventually to a tuner.

I need a sanity check, because it doesn't seem to be working out for me. Please check my logic and let me know if I'm completely off base or if there's an easier way.

I'm using the centrifugal advance and vacuum retard curves published in my workshop manual for Euro '78 and later 930s.

Its basic freshman year algebra 1 stuff to calculate the formulae for these (f=mx+b). For example the advance is

• Below 600rpm, 1.25 degrees
• From 600-1,600rpm advance=1.25+0.0055*(rpm-600)
• From 1,600rpm-3,400rpm advance=6.75-0.000833*(rpm-1,600)
• Above 3,400rpm advance=5.25 degrees

Similar sort of thing for the vacuum retard.

So my plan was to build an Excel table with the same rpm and load cells matching what is in Tunerstudio, calculate the cell values in Excel, then transfer to the ignition table in TS.

The Euro ignition timing is set at 29 degrees BTDT @ 4,000 rpm with the vacuum hose disconnected. So that is, with centrifugal advance maxed out and no vacuum retard.

With that fixed point, and then adding in advance, and subtracting out retard, I should be able to calculate the actual advance for each cell.

Am I interpreting these charts correctly? For example, the advance chart. I interpret that to mean ADDITIONAL advance on top of whatever static timing is set, not ABSOLUTE advance.

In other words, my base ignition timing is 29 BTDC minus what the centrifugal advance is at 4,000 rpm which is ~5.25 according to chart, or 23.75 degrees BTDC at 600 rpm. Using the above formula I'd get 3.175 degrees advance at 950 rpm + 23.75 = 26.925 at idle.

I can't see where my logic is wrong, but the numbers I end up with don't sound right to me. Other ignition tables I've seen show around 18-20 BTDC at idle.

Please see my response in this thread on how to understand the ignition curves:

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/961515-does-anyone-understand-ignition-curves-wsm.html

TL;DR,

You can make a simple map by putting known degrees at given RPM's and loads.

Simply click on one cell that is known good, then the last cell that is known good linearly, followed by the interpolate button.

It'll fill in the cells in between with a nice linear cell value.

Do it across all known 'good' values to other known 'good' values until the map is complete.

tl-dr = doesn't answer the question :D

Thanks speedy.

My logic is pretty much on, but I totally missed distributor speed/angle vs crank speed/angle conversion.

I was using Euro initially, but I think I'll switch to the non-Cali USA curves due to the slight octane difference between fuels in EU vs US.

This is a Cali car, but everything's been ripped out and its SC cams and bigger turbo so who cares, the basic US curves are similar.

Speedy, given that you've apparently been down this road before, mind sharing what you ended up with?

Did you build a timing table based on map and rpm, or is it something different for MSD?

Also, I'd love to hear how it turned out. Was it just a starting point for additional tuning, or did you stick with it?

I may go to 1.2 bar, how much additional retard is reasonable going from 0.8 to 1.2 bar?

The problem is that the motor responds very well to advance and generally loves more timing. Another 6 degrees over stock makes a world of difference - especially with SC cams and a half-way decent turbo.

But, at least in my experience, even a stock US ignition curve with factory timing can/will cause knock at certain places in the map. For me it was 100% reproducible; 3000 RPM with light load/part throttle would do it every time. Even with race gas. The Euro curve was more aggressive.

Dynamic knock control works very well to solve this conundrum and tame these little dark corners of the map. The motor really loves the extra timing the other 99.9999% of the time. And you're protected against something untoward happening with an injector, filling up with junk fuel - or your dizzy plate sticking fully-advanced. For example.

And then once you're convinced that all this voodoo actually works, you can add extra advance and enjoy.

I was running the my Safeguard (dizzy/CDI unit) when I wrote this, which describes exactly the circumstances under which the Safeguard would detect knock for me: http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/611409-my-j-s-safeguard-paid-itself-last-week.html - and how it likely saved my a$$...

Replaced the Safeguard with John's Interceptor (for smart coilpacks) when went Motec/twin/plug/Cayenne COP (some might find this handout interesting www.beru.com/download/produkte/fachaufsatz_zsp_e.pdf - Porsche spec'd these to make 25% more spark energy, and my tuner loves them).

John was, as usual, great to work with - even bench-testing a Beru to confirm wiring/that they worked with the Interceptor w/ no changes, and the correct settings. His units work perfectly - and he is, IMHO, another example of a small outfit providing top-shelf products/service/support.

Since motor was twin-plugged, I have seen no knock, BTW - just sayin'...

Thanks Spuggy

Since the graphs show an acceptable band, I'm using the most advanced part of the acceptable band. It's a diff of 1-2 degrees. I'm sure I'll leave a lot on the table, but this is just to get the car started and run it around the block and check things out before heading to a tuner.

I'm being convinced some sort of knock sensing and control is important, whether thats some sort of stand alone system like the J&S or built in with the Microsquirt.

ok here's what I've ended up with at this point. I started with the '78 and later USA graph for centrifugal advance and the California vacuum retard. Since the graphs show an acceptable band, I used the top edge (more advanced) value.

I did modify the vacuum retard calculation slightly. I increased max retard to 5 degrees at .5bar of boost. I'm considering adjusting that to slow the retardation, say 5 degrees at .8bar boost. Thoughts anyone?

Also, there's a step in the graph around 150-180 mbar of vacuum (throttle partially closed and off boost) that I eliminated. I'm not sure of the purpose of that step.
http://forums.pelicanparts.com/uploa...1546736313.png

http://forums.pelicanparts.com/uploa...1546736313.png

I just built a Tunerstudio style ignition table rpm+map from the above. I'll share it later. But if its correct (and I'm pretty sure it is), it bears little resemblance to any other similar ignition table I've seen and is, to my untrained eye, bloody awful!

I modified the formula for the vacuum retard to have a smooth slope down from a retard of -1 (iow an advance of 1) at .165 bar over atmospheric to 5 degrees of retard at 1 bar over atmospheric.

The Porsche diagrams and specs are pressure relative to atmospheric, mine and the way you build in Tunerstudio is absolute. So -400mbar on the Porsche is +600 mbar absolute.

Here's what I ended up with on the vacuum retard curve.

http://forums.pelicanparts.com/uploa...1547064415.png

And here's the map these come up with.

Can anyone explain the drastic vacuum retard at MAP below a few hundred mbar below absolute (chart above, e.g idling or overrun)?

Should I implement a min/max function in these cells to limit the values between two bounds?

http://forums.pelicanparts.com/uploa...1547064648.png

The high retard at idle and low rpm's was to keep the reactor cans hot for HC control.

Ah right, of course. But almost 6 degrees ATDC!

Yes, crazy. I would just run 10 BTDC all the way across.

Replace anything below 70 kPa (so bottom 3 rows on my chart) and up to 80 kPa for rpms below whatever the crossing point is (2000 in the chart, but really some number in between)?

I think previous to mods the car idled at around 65 kPa, probably not going to change much if at all with SC cams cs 930 cams.

Yes, there is no need for it. Unless you want to get into two step for building boost at the tree..

It did an excel ignition base map configurator some 10 years ago. I used it at that time for start-up mapping, or cars that had to drive in safe-mode to the dyno for full tuning. It was also shared on a local forum, and several other cars used it for doing their own mapping on the road (back in the days), and found it usefull without detonation and still not too conservative as a base map. However should only be used carefully for limited transport and close attention to detonation as all ignition should be tuned on a dyno for MBT (Minimal Advance for Best Tourqe). But it will get your engine started.

For fun I just entered the Porsche 930 engine spec.

Inputs are:
Number of valves: 2
Compression: <8:1
Bore: >88 < 102mm
Octane: 93 US / 98 Europe
Inlet air temperature at idle: <40 celsius
Squish: Average 2 valve
Rpm & boost level: 7200 rpm, 1 bar

Then it automatically spits out a base ign. map.

(930 ign. base map example)
http://forums.pelicanparts.com/uploa...1547209245.jpg

http://forums.pelicanparts.com/uploa...1547209555.jpg

A few comments:

A flat curve from 3000 rpm and out is normally only by fixed mechanical vacuum/boost controled actuators. You will see this example base map advance as rpm still increases beyond 3000 rpm.

This base map is more conservative (less advance) up to 3000 rpm than the map discussed above. If I was to change the squish to 2 valve hemi, then it might be more the same. But I went conservative on the inputs, as this is aircooled. And I only tested the base maps on watercooled engines before.

A jump in ignition below rpm target, to create that "cradle" for setting idle by ign. advance

Ign. advance should increase in "NA" mode below 100 kpa

Output is set down to 20 kpa, as vacuum between 20 and 50 can be reached in inlet plenum/engine when you close the throttle hard between gear shift. 30-50 kpa at idle.

Just for fun inspiration and comparison;)

I reread your OP.

I don't understand why all of the brain power you are putting into this if you are simply dumping off at a tuner?

None of your numbers will be reused if using a load dyno and dialing in MBT?

I gave you my method to get a quick rough map in that is actually a great way to tune it all and just check for knock afterwards and adjust accordingly.

Interesting.

Care to share that file?

Would like to see the formula it uses to calculate the timing.