By Chris Troudt
There is a big difference between adjusting the timing and altering the
advance profile. Adjusting the timing at the plate affects the timing throughout the entire RPM range.
This is commonly known as "initial advance" or "base timing". The advance "curve" or "slope" determines how quickly the
timing advances with respect to RPM, at what RPM maximum advance is reached, and sometimes how the advance behaves
under heavy loads. If you have too much advance at a given point, and you retard the timing at the ignition plate, you
might have too little advance at some other point. You need to have the ability to adjust both the initial timing and the
advance profile independently in order to optimize the timing for a particular engine.
See if this hypothetical example helps explain it better: (Keep in mind this example is not specific to HD engines.
It is also extremely simplistic to avoid dragging in lots of unecessary details which will only cloud the point I'm
trying to make. Modern electronic ignitions are *much* more sophisticated than the hypothetical ignition system
presented in this example, and are therefore able to deal very well with the shortcomings apparent in this example.)
Let's say we've got an engine that runs best with 30 degrees total advance at 3000 RPM. Further, let's say this engine is
running high compression and pings badly at 1500 RPM under a load. We determine the timing profile using a dial-back light
and discover that we're advanced 15 degrees at 1500 RPM. After a few tests, we find the pinging at 1500 RPM
can be completely eliminated by backing off the timing 2 degrees at the ignition
plate. The problem is, now the timing is retarded 2 degrees throughout the entire RPM range. That means maximum advance
is only 28 degrees at 3000 RPM, hence the top-end performance is degraded slightly. Clearly, we need to be able to alter
the timing slope to avoid this sort of dilemma.
For the sake of simplicity in this example, let's assume that the timing slope in the first situation was linear and added 1
degree of advance per 100 RPM. (Electronic ignition modules are not restricted to linear slopes, nor do all timing slopes
typically begin advancing immediately from zero RPM, but let's just avoid that complexity here.) What if we set the ignition
plate back where it was, and alter the advance slope to add 1 degree of advance per 115 RPM? (A less "steep" advance slope.)
With older mechanical advance units we would do this using lighter weights, heavier springs, or some combination. With
modern ignition modules, we simply tell the computer to do it.
Now at 1500 RPM we'll have 13 degrees advance like the engine wants. OK, at 3000 RPM we'll only have 26 degrees advance,
*but* we are not yet at maximum advance, which is determined by the ignition plate setting to be 30 degrees. We still have
4 degrees to go, and by 3450 RPM we've rolled them all on. Sure, this is still a tiny compromise, but it eliminates our
pinging problem, and it gives us our back our optimum top end performance a mere 450 RPM later. You cannot achieve
the same result by only adjusting the ignition plate.
Keep in mind this compromise is really only an artifact of this trivial example. Trust me, the modern ignition systems
readily account for this type of situation. They are able to generate non-linear advance curves that would solve the
above problem by providing exactly 13 degrees at 1500 RPM and exactly 30 degrees at 3000 RPM.
There are programmable modules that allow you to adjust the timing advance at *every* RPM via a PC cabled to the module.
Sure, it's nothing more scientific that a look-up table, but you sure can't do that with springs and weights. These
same modules can record RPM profiles and play them back on your PC so that you actually have a built-in
pseudo-dyno to help you dial in the tuning. Additionally, some modules have the ability to stage the rev-limiter for drag racing
applications, and often have multiple retard points to handle things like nitrous oxide or forced induction.
Sure, this is probably overkill for the typical street bike, but all the same it demonstrates the possibilities. It might
even seem to justify your remark about "paying for capability you can't use." Nevertheless, you should consider this. There
are countless products on the market that are "configurable" or "programmable." Typically you only use one "configuration"
at any given time. Does that make all of the other available configurations a waste of money? I say no. For the guy making
frozen margaritas in his wife's multi-speed blender, all but the high speed button are useless. I suspect his wife is glad
to have the slower speed button for chopping tomatoes. Does a blender really need 400 different speeds and a dual range
transmission? Perhaps not, but...
You have to understand the manufacturer's perspective. One product that can be configured to work with a variety of
applications is cheaper to produce than hundreds of products designed specifically for one of many applications. Those 3
extra advance curves on that Dyna 2K module that you don't use on your bike and consider "wasted" actually "saved" you money.
Later on down the road, when you decide to shave 0.075" off your heads during a rebuild, you'll be glad you can simply
flip a switch and change from curve 2 to curve 3 on the module you already have, rather than having to go buy a new module.
Regarding king86's claim that every bike winds up using the same curve after dyno tuning, I suspect that his entire sample
set had heads decked 0.050" and S&S-E carbs. ;-) Sure, most of the Dyna 2K modules out there are probably set to curve 2
because *most* of the bikes out there are generally running the typical stage-2 mods (cam, carb, compression). Does
that make curves 1, 3, and 4 wasted? Not for the guys that use curve 1 for their stage-1 mods (dynajet CV, mufflers).
Not for the guys like me that can only get their motor to run on curve 3 without pinging. One ignition module that
supports many possible states of engine modifications... It really is a good thing, IMHO of course.