Moderators: JeffC, rdoherty, stieg, brentp
MartinM wrote:A quick Google on "ignition timing theory" brings up these two that seem to describe it well:
http://www.giant.net.au/users/wight/Ign ... ground.htm
http://www.giant.net.au/users/wight/Ign ... Timing.htm
...indeed your link (last row but one) describes why advance increases with rpm.
I would say that your second map is incorrect - the advance is increasing as load increases (%TPS increases or absolute MAP KPa increases) where as it should decrease.
To remember "which way up" a map is I always recall the phrase "vacuum advance" - when you have a vacuum (because the throttle closes and the engine has to suck against the throttle butterfly thus causing a vacuum), the ignition advances.
So more vacuum = less KPa = nearer the top rows of the map = more advance
could you elaborate more on what you mean? e.g. lack of definition in timing for both high & low load conditions?alexander wrote:...that most users here dont seem to have enough differential between high load advance, and low load.
Twincharger wrote:Gas takes time to burn. That's the answer.
At low speed 10 degrees before tdc allows the flame front to travel from the plug and get a large portion of the air/fuel burning as the piston starts to go down.
If the engine spins faster, you need to start the flame burning earlier.
If you start too early, the piston has a hard time going up, and wastes power.
The flame front travels at different speeds depending on cylinder pressure, so intake runner length, exhaust header diameter + length, compression ratio, cam timing, overlap and duration all have an effect on cylinder pressure at specific RPM, so therefore affect optimum timing advance. The distance the flame front has to travel affects timing too, so a small bore engine is different than an oversquare, larger bore engine. So it can get very, very complicated.
Your maps look like base maps that haven't been fine-tuned to a specific car, but in general, low RPM likes 10-15 degrees, high RPM likes to start about 20 degrees before that. High cylinder pressure like when your cams are at their peak torque, headers and intake are resonating may require retarding timing a few degrees compared to off-resonance RPM. Also forced induction turbo, supercharged should remove about 1 degree of timing every 2 psi of boost, because the flame travels faster thru the highly compressed air/fuel mixture.
Just to throw a couple more variables in... The temperature of the air/fuel mixture can exceed the flash point of the fuel and make all the fuel ignite at once - detonate. You want to start the flame front burning before that happens. The octane of the fuel affects its flash point.
But, you were asking about mechanical and vacuum advance...
ok, imagine your nice ignition maps reduced to 4 cells. And a hypothetical distributor would work like this...
The mechanical advance (m) will advance the timing 30 degrees all at once around 3000 rpm.
The vacuum advance (v) will retard the timing 25 degrees at high manifold vacuum.
Imagine your distributor static timing set to 10 degrees
----------------------under 3000-------------------over 3000
-----------------------------------------------------------------------------
throttle closed---- 10+v+m=10+25+0=35--- : --- 10+v+m=10+25+30=55
-----------------------------------------------------------------------------
throttle open----- 10+v+m=10+0+0=10 --- : --- 10+v+m=10+0+30=40
-----------------------------------------------------------------------------
It's much cruder, but pretty easy to adjust. The amount of vacuum advance is sometimes adjustable by a stop screw in the diaphragm, and the mechanical advance rpm can be adjusted by replacing some springs, and the amount of mechanical advance can be adjusted by filing some parts.
I'm sure there are many web pages on the subject that can explain it better than me...
hi. the point of vacuum advance is to advance the ignition under lean (ie light load) running conditions. that is because the flame travels more slowly through the lean mixture, requiring ignition to occur earlier, so it (in theory) is completed by the same point in the cycle. my impression is that there is something in the vicinity of 10deg difference between full load and light load conditions (at any given engine speed) in an oem ignition curve. i (think) i have noticed that most maps loaded on this site have much less than that. if so, it is most likely causing less than optimal performance at light load (ie small part throttle), which in turn is likely to lead to less than maxiumum fuel economy at easy highway travel speeds. i am not adamant about that: it is just a general impression as it warrants some closer attention when setting up a map.dr.occa wrote:could you elaborate more on what you mean? e.g. lack of definition in timing for both high & low load conditions?alexander wrote:...that most users here dont seem to have enough differential between high load advance, and low load.
