[anjuna]

IGNITION

For all the topics we have covered, ignition is generally the one that most everyone is familiar with. So, i'll discuss a few general basic ideas around ignition AND touch on some of the more specific concepts to the N55 DI platform.

First, ignition timing follows the value in the cell - a positive number is an "advance" and a negative number is a "retard" in timing. If our cell value is 20, then we are firing the coil for the spark plug as close to 20° BTDC (Before Top Dead Center) as possible.

The reason for ignition advance has a lot to do with a lot of things, but some of the most basic are flame front velocity, air fuel mixture (both how well the air and fuel is mixed as well as the Air Fuel Ratio [AFR]), engine load, and VANOS... Those are just a few, but also the ones that we are in control of if we are tuning our own. I'll walk through those.

Flame Front Velocity - This is a fancy way to say "how quickly the fuel burns" which can be affected by fuel octane, fuel type, and general temperature in the chamber.
- Lower octane fuel has a faster flame front velocity, but consequently is more prone to pre-ignition (spontaneous ignition that does not occur because of the spark - often more advanced than our spark - this is what knock sensors try to capture) - Additionally, this is a radical oversimplification.
- Higher octane fuel has a slower burn rate, but is not as prone to preignition
- Ethanol has a VERY slow burn rate, but the octane rating is also much, much higher than petrol and therefore is even less prone to preignition. Additionally, E100 requires ~35% more fueling then conventional petrol.

Air Fuel Mixture (atomizing/tumble/physical mixture of air and fuel) - This very much contributes to how effective and efficient your engine will be. If the air and fuel are mixed poorly, you can end up with fuel on the walls of the cylinder, but also pre-ignition, as the uneven mixture of air and fuel contributes to hot spots and a large collection of fuel in the cylinder is more prone to pre-ignition. VANOS helps us with this, though.

Air Fuel Ratio (AFR) - A leaner mixture is more prone to pre-ignition, where a richer mixture has a much lower flame front velocity. I won't touch on it too much here (since this is dedicated to ignition) but a richer mixture needs more advanced ignition, which sounds great! We all think more ignition = more powa, right? Not always. As I will touch on in the fueling area, the N55 really likes to be as lean as possible with as much ignition as possible (at that AFR) to make the most power. Remember how flame front velocity and AFR are related? If we can burn the fuel faster without pre-ignition, we make more power. There are some minor deviations from this thinking (like very long high loading, where the heat in the valvetrain and spark plug can contribute to pre-ignition when not rich enough) but this concept is a great starting point.

Engine Load - When under a higher load, more air and fuel will be mixed together. The more air and fuel mixed together, the higher the cylinder pressure. With more to compress, on the compression stroke, there is more heat involved in the compression stroke. That added heat (this is adiabatic [very fast - not related to engine or coolant heat]) means that the mixture is, again, more prone to pre-ignition. As such, under higher loads, we must retard ignition.

VANOS - VANOS directly affects the "dynamic" compression ratio of the engine. To sum it up fast and dirty, the compression ratio of the engine does not change (the difference in max cylinder volume vs min cylinder volume) but the amount of time that the cylinder is FULLY CLOSED AND COMPRESSING does change. We do this with overlap (where the intake and exhaust valve are both partially open, with the exhaust closing and intake opening) or relative retard of the intake cam (meaning the piston is on the way up and the intake valve is still open) - This seems counter intuitive until you realize that air is a fluid and carries inertia; the air will still continue to fill the cylinder (to a certain extent) while the piston is beyond bottom dead center, especially on a turbocharged application. BUT to sum it up super quickly, the more overlap, the less compression, which means the more ignition advance you may be able to run. The less overlap, the higher the compression. As such, I don't want to hear any more of this shit about how VANOS doesn't impact knock or whatever. GTFO. I'll cover overlap and "blow through" more in the fueling section of this thread.

If it hasn't already become clear in covering the above topics, more boost also influences how much timing you can run. More boost = more heat.

Additionally, coming from a port injection world, you may see that ignition timing SEEMS much less aggressive (more retarded) when in fact, that's pretty much a DI engine in a nutshell for you. Better mixture, more precise injection timing, and the higher compression that most Turbo Direct Inject motors are blessed with means that we just can't accept as much timing.

Lastly, engine speed has a LOT to do with timing. With shit moving faster, we need to ignite sooner. If we assume flame front velocity is constant (protip - it's not) and we only increase engine (piston) speed, then we need to understand that where we may have had to wait X time for the mixture to fully ignite, we don't get that much time at higher engine speed. As such, we must advance the timing to get our mixture to produce energy INTO the piston on the power stroke at the right time.

Hopefully all of this makes sense. I feel like I took a lot of time to explain that, but the BMW stock ignition maps are pretty good! Most of us will not be radically changing them unless it's for E85, Methanol (which both cools the intake charge AND increases resistance to pre-ignition) or radically changing VANOS tables.

I have changed my VANOS tables pretty radically and as such, must run much less timing in places that I've decreased overlap. I also can get away with advancing timing in places with greater overlap and/or lower load.

DOUBLE LASTLY, injection timing can have a good bit to do with ignition timing as well. There's a point of diminishing returns, but under low load we can inject later and also run more advanced ignition. This means better fuel economy...

So generally speaking
More advanced timing = more power (up to MBT, the point in which more timing does NOT increase power output)
Less advanced timing = less power, also less "knock" BUT can result in an increased EGT (since some of that energy might not be used for power stroke, the energy is output in heat out the exhaust valve).

I'll add more later as well as some tables, but you may also refer to the N55 calculator in post #1 for some clarification.