In response to a request. Here are some thoughts.
Timing
TDC = Top Dead Center, or the piston is at the very top part of its travel.
BDC= Bottom Dead Center, or where the piston is at the very bottom of its travel.
The ideal “event” as its called
Event= the point where the spark ignites the fuel and the burn “event” starts
The ideal “event” would be at TDC, so why would you want the ignition cycle to start before TDC? Before we get into that let’s backtrack a little.
+timing and - timing.
Think of it like this; if you start something before DTC it would be +, if you started something after DTC it would be after or -
So, when you see +14 deg this is 14 deg before DTC. You wouldn’t want to do it after TDC because the piston is already traveling down away from the “optimal event” point
To just recap +is before - is after. For the best results we want to start everything before our DTC window comes, this way we will be ready.
Why before?
To better understand, lets get an idea on how fast things are happening in your motor. let’s take a snap shot at 6000rpm. At 6000rpm your piston is coming up to TDC 100x a second. Think about how fast that is. When we are looking time wise, 100x a second is incredibly fast. When you are calculating the ignition window, you’re looking at not seconds, but milliseconds. Very small increments of time.
Ok, so here comes a little sports analogy to help us out. For those of you who know me know how big of a sports nut I am. With baseball getting ready to end, lets use that.
We look at the “optimal event” in baseball, that’s having the batter hit the ball right at his best power point, and for this example that’s directly over the plate. So in baseball DTC is directly over the plate.
When a pitcher throws a 100 mph fast ball, the batter can’t start swinging when the ball is over the plate, even tho in theory the best place to have the “event” or contact, take place is over the plate. If the batter started to swing the bat when the ball was directly over the plate, he couldn’t move the bat fast enough to hit the ball and the “event” window would be missed. So, when the pitcher throws the pitch the batter starts his swing before the ball is over the plate. The faster the ball is thrown, or the faster the motor is spinning, the sooner you need to swing your bat or start the ignition.
You need to start before (+) the “event” happens. So, to further the example lets say that the pitcher is throwing 100 mph and the optimal time for the batter to start swing the bat is when the ball is 15 feet away from the plate. So, here is what happens. The ball is pitched the batter starts his swing when the ball is 15 feet away and the “optimal event” takes place right as it should. The bat hits the ball right over home plate and, home run, the ball gets smashed out of the park.
So, why is it such a big deal then when you see people saying that the ignition timing is too low. Well, now lets look at what happens when the batter swings later then the optimal window. If the batter swings when the ball is at say 12 feet away from the plate, its after the optimal window so, now some of the energy getting transferred in the “event” is lowered and the ball won’t travel as far. This is the same thing that happens in the motor.
As the motor accelerates there is a calculated optimal ignition window for each RPM. Depending on how far in or out of that window you are you will not be able to accelerate faster then you did your last “event” One of the things the Audi computer will do is reduce ignition timing, to slow the acceleration rate of the motor. So, when you see one guy that is hitting the optimal ignition timing of say 22deg (if that is the actual optimal figure) and then you see in a log another guy with 15 deg there is going to be a big difference in acceleration rate of the motor. (this considering same car mods etc.)
Each RPM point depending on load, CFM, fuel, temperature and several other factors, will have it’s own optimal ignition point. Knowing how to calculate that point and knowing where the perfect “event” window is for part throttle, WOT, low load, high load etc etc etc. is part of making safe productive power. At WOT you would be shocked to find out just how many times the car actually misses its “event”. Check out a stock RS4 and tell me how many misfires you have. You want to reduce the amount of missed “event” cycles, so getting this right will effect how the car accelerates. If you don’t get the curve right, the car won’t accelerate as fast as it could. Or if the car is protecting itself the acceleration rate will be greatly reduced.
Hope this helps. This was just the quickest thing I could think of to help those that might not know how this worked. This was far from an in depth explanation.
