Aww, I'm disappointed.
Do you realize that the air/fuel mixture burns at different rates, depending on the density of the incoming charge?
Do you also realize that, as the crankshaft comes up to TDC, you actually have to ignite the air/fuel mixture
before the piston arrives at TDC?
See, that air/fuel mixture will burn at a certain, steady rate. As you increase the density of the air/fuel mixture, the burn takes longer to complete, and the slower the flame front. This is why you get so much torque - the air/fuel mixture burns longer
Think for a moment on that.
A normal combustion cycle compresses the air/fuel mixture to a certain degree, the spark comes and *pop* you have combustion. As that combustion is taking place, the force pushes against the piston, forcing the piston down the cylinder.
Well, as the piston goes down, the amount of room that explosion has gets larger, so the amount of pressure on the piston goes down dramatically. When you increase the density of the air/fuel mixture, there is more of it, so it takes longer to burn - when normal combustion has run out of fuel to put pressure on the piston, the forced induction combustion still has fuel burning, gases expanding, and pressure on the piston
All that said, let's talk about
timing for a bit. Everyone hears that 'adding extra timing increases power!'
Is this true? Yes and no. Depends on how optimized your timing is, from the factory
IIRC, optimum mechanical force from the piston to the crankshaft occurs 20 degrees AFTER TDC (which means you want the highest cylinder pressure at this point)
That said, you want to ignite the mixture at the point that gives you the highest pressure around 20 degrees ATDC. Keep in mind, when we say we are 'advancing' timing, we are actually firing the plugs further before TDC!
So does advancing the ignition help power? If before you advanced the timing, you were achieving the highest pressure after 20º ATDC, then yes - you should see some small torque gaines.
If, on the other hand, you were already firing @ 20º ATDC, advancing would give you almost nothing in return! You can actually end up with maximum pressure anywhere from 1-19º timing, and be seeing cylinder pressures that are no higher than @ 20º ATDC - meaning you are actually NOT getting any more torque to the crankshaft, and only increasing the wear on the engine (namely, the crank bearings)
Now, taking all of that in consideration, what timing differences would we see between say.... 4000rpm and 7000rpm?
Well, the piston is certainly moving faster @ 7000rpm, right? If we have the same incoming air/fuel pressurization (be it NA or Forced), the fuel is going to burn at the same rate. But hey - the piston is going to hit TDC quicker spinning 7000rpm, than spinning 4000rpm, right? This actually means you want to
advance your timing a bit in the upper rpm reaches. You don't want to go overboard with this. You actually don't want to go overboard with timing at all - remember, you want to hit that 20º ATDC mark - no need to add more timing than that!
It's a concept I've heard called (and I call) MTBT - Minimum Timing for Best Torque. If you stick with MTBT, you'll have a perfectly safe, very powerful timing curve.
Hopefully, you will see the difference in density from off boost to say, 20psi of boost requires a very different timing curve.
