Originally Posted by Slidin'Miata916
Ok, well then it's not quite as precise, but same concept...
When you say "Beyond that it will not be able to pull any more timing" Do you mean it will stop retarding?
Yes, but read further.
Lets say it retards six, then four, and by four your at 10psi, and then it just stays at four?
I think you're approaching this from the wrong standpoint.
We tend to think of the ignition timing in terms of the "base" timing, which is 10 degrees BTDC. But even in a bone-stock vehicle, the timing does not remain at 10 BTDC all the time- it advances based upon RPM and retards based upon load. 10 degrees BTDC is just where it happens to be at no-load idle, which is the point where we calibrate CAS.
So hypothetically if you rev the engine up to 5000 RPM with no load the timing might now be 25 degrees BTDC, and if you then start lugging up a hill in fifth gear it might retard back down to 20 degrees BTDC, or maybe 15 degrees or even less. (I'm making all these numbers up since I don't have the actual data in front of me, but it's the concept I'm trying to convey here, not the precise figures.)
When I read your message saying "Lets say it retards six, then four...
" I get the impression that you mean "The actual ignition timing goes from 10 degrees BTDC, down to 6 degrees BTDC, down to 4 degrees BTDC...
" but this is fallacious because the timing wasn't at 10 degrees to start with- the stock ECU has already advanced it somewhat because it's no longer at idle.
Since the ignition curve of the stock ECU is shrouded in some degree of mystery, we typically do not deal with absolute ignition timing when working with piggyback controllers. Instead we deal with a certain amount of retard relative to whatever the ECU ignition angle happens to be.
I realise you're using AFM, but for the sake of simplicity let's say hypothetically that you are running MAP, and that you have the ACU set to 1.4 degrees per PSI of retard.
At 1 PSI, the ACU will delay the ignition by 1.4 degrees. At 2 PSI it will delay the ignition by 2.8 degrees. At 5 PSI it will delay by 7 degrees, and at 10 PSI it will delay by 14 degrees.
At this point we don't know what the actual ignition timing is, because we don't know exactly what the stock ECU is doing. All we care about is that we are delaying the ignition by a certain amount based on pressure, relative to whatever the stock ECU thinks it should be, because we are accounting for a variable that the stock ECU does not understand- boost. The ignition tables in the stock ECU were written based upon the assumption that the pressure outside the throttle plate is always atmospheric. Since we are changing that assumed constant, we must provide our own compensation for it.
14 degrees is the maximum amount of delay that the ACU will provide, so whether you are using MAP or AFM, by the time you get to 14 degrees of retard, there will be no further retard applied for any additional increase in pressure. Thus, it is important that your engine never run in a condition that requires more then 14 degrees of retard, because we cannot provide it. Based upon MiataNuTca's observations for his engine, this point was reached at 10PSI. Bear in mind also that he has an intercooler. I didn't see one in your list of goodies.