Engines are, on their best days, within nanoseconds of self-destruction. Like a ballet the dancer performs while juggling flaming chainsaws, sticks of dynamite and live badgers, the whole orchestration can go very wrong very quickly if any single component doesn't do exactly what it's supposed to do and when. This is especially true where ignition is concerned, since timing advance determines when that dynamite goes off and whether or not your badgers eat the chainsaws, or vice versa.
Fuel burns at a certain speed in the cylinder; fuel-richer mixtures burn faster, and fuel-lean mixtures burn more slowly. Ideally, you'd always trigger the fuel burn just after the piston reaches the top of its travel and finish the fuel burn before the piston reaches the bottom. That way, all of the fuel's power would go into pushing the piston downward. And this would be possible if the engine ran only at a single, very low rpm. But the reality is that, while the piston speed changes with rpm, the rate of burn does not. So you need to dial in a bit of advance -- trigger the spark before the piston reaches the top of its travel -- to give the fuel burn a head start on the piston. The faster the engine spins, the more advance you need to ensure that the combustion chamber pressure peaks just after the piston reaches top dead center, or TDC.
The key to getting the best fuel economy out of your timing setting is to use as little timing as possible while getting cylinder pressure to peak at about 15 degrees after TDC. While adding a bit of timing is said to increase fuel economy, adding too much will trigger the charge too early and cause chamber pressure to act as an engine brake. In this sense, the extra advance will cause your engine to try to run backward. Too little advance will peak cylinder pressure too late, resulting in an incomplete burn and power loss. There's an important interaction to consider between advance and air/fuel ratio, since slower-burning lean air/fuel ratios require more advance to ensure all the fuel gets burned. So you've got to strike a balance between an air/fuel ratio that will be lean enough to give you good fuel economy and rich enough not to require excessive ignition advance that will work against the piston.
This is the setting most of us are familiar with. Initial timing is your advance at idle, determined by the position of the distributor relative to the block. Initial timing acts as the base point for the centrifugal and vacuum advance. But initial timing in itself is really responsible only for providing crisp throttle response off idle and efficiency during idling. Once rpm starts to rise, the system of springs and counterweights inside the distributor -- the mechanical or centrifugal advance -- rotates the distributor shaft to add timing advance to keep up with rpm. The distributor's vacuum advance mechanism is also critical for fuel economy; it adds timing advance when engine vacuum is high, at idle, allowing for leaner fuel mixtures when you're not flooring the gas for maximum horsepower.
It stands to reason that if you're tuning for fuel economy, you're probably not going to spend the bulk of your time with the engine at wide-open throttle. So the vacuum advance is going to play a crucial role, because it's always working. Think of your mechanical advance as the one you use for power, the vacuum advance as the one you use for fuel economy. While horsepower junkies often set timing with the vacuum advance disconnected, you need to consider it a vital part of your timing strategy. For fuel-injected engines, you want to set the timing to fit your desired air/fuel ratio -- and not the fuel injector cycling to fit your timing.
While horsepower junkies typically set initial timing with the vacuum advance disconnected, you'll want to leave it connected if you're tuning for maximum fuel economy. This will allow you to set initial timing to maximize cylinder pressure at part-throttle settings, where you're going to spend most of your time. But this approach requires delicate tuning of the mechanical advance mechanism to balance timing advance to the air/fuel ratio you want to use. In this sense, the priority is somewhat backward. Normally, you'd set initial timing, then mechanical, then vacuum advance. To tune for maximum fuel economy, you'll start at the leanest air/fuel ratio your engine can handle, set initial timing with the vacuum advance connected and fine-tune the advance curve with the mechanical system. This approach is difficult, time-consuming, expensive -- when you consider the cost of an air/fuel ratio monitor and distributor parts -- and sure to cost you some horsepower, but it should yield the best economy on most engines.
Tuning a fuel-injected engine for maximum economy is actually quite a bit easier than tuning a carbureted engine, because you already have most of the tools on hand, apart from a computer to tune it with. In this case, you're going to go strictly by the desired air/fuel ratio and map the fuel injection to maintain that ratio at anything under three-quarter throttle. From there, it's just a matter of dialing in the advance curve -- using the least amount of advance possible -- needed to maintain combustion. Watch your knock sensor readings, and add timing as needed to stay out of detonation. If you can't keep the engine out of misfire or detonation, enrich the fuel mixture until you can. Again, this approach is backward in a way, since normally you'd set the ignition first, calibrate the injector cycling for it and then readjust the timing as necessary. But that approach, while good for horsepower, won't allow you to take advantage of that leaner fuel mixture and the potential increase in fuel economy it represents.
