Distributor Advance

by Steve Denis

As we all seem to be both shaken *and* stirred about the poor 009 I'll jump in to muddle it *all* up. VW engineers operate(ed) under the same parameters as do their counterparts everywhere: make a perfect product, that costs nothing to produce, and sells for all the money in the world. *Or* build and ship *anything* so your "end of the month" (1/4 year) looks good.

Let's look at that distributor.

The V-dub had many different types of distributors and carburetors over the years. With a given carb, manifold, valve size, cam and compression ratio the engine will need a spark right *now*, at any temperature, air pressure, load and fuel type.

If your engine ran at one speed, temperature, and load it would only need a fixed spark timing. A typical Briggs and Stratton has just such a set up. Although on long hills it *seems* that you only have one speed, the engine has to perform under many different types of conditions, hence the advance "curve".

We all understand that the spark has to happen sooner at high speeds to allow the mixture time to burn and reach the peak pressure *just* when we need it. Modern engines do this electronically with computer "maps" and knock sensors. At any given set of conditons the computer signals the coil(s) to fire the plug at the time it thinks it *needs* to. The knock sensor is a feedback device to let the computer know if it got a *wee* bit carried away on the advance. It will drop back just a *smidge* and run the engine right at the edge of detonation.

The older cars had to do this mechanically, so right off the bat the system has a lot of built in margin. Ain't no feedback on a 57 bug...until you blow a hole in a piston.

Total mechanical advance

What this represents is the basic timing setting for the engine at any RPM at wide open throttle under the "worst case" conditions of pressure and temperature (maybe minus a bit for safety). It's all the advance the engine will take at that rpm under wide open throttle.

"Put an 009 in my bug--I'll get more power 'cause it's just like the Porsche distributor!!" Umm..well.....no...

Porsche didn't bother with a vacuum advance. The idea was that you would *always* be at wide open throttle. If not, why did you buy this car??? So the vacuum advance would never come in to play anyway. Ditto for the early transporter. Look at an early splitty. The gas pedal is bent to match the contour of the floor where some cat spent all his time trying to push it into the pavement. So for some engines, VW (Bosch) made mechanical advance distributors.

"Ok" for full power settings, but at lower throttle settings, engines can use some more advance. Reason? The flame front moves slower as the mixture in the cylinder gets less dense. So we can get started earlier and catch the peak pressure just where we want it, even at low outputs. This helps the engine temperature, as the mixture is not burning late, helps the power, as the peak pressure is at the right place in the stroke, and helps the fuel economy as the fuel is being used effectively.

The *full* vacuum advance VW distributors are nothing of the sort. If you put one on a distributor machine and run it up without the vacuum hooked up, it *still* advances! Huh???

It has a crude mechanical advance: the friction of the rubbing block on the cam drags the breaker plate around and changes the position of the points.

"Ok, you, the wiseguy in the back row."

"Ah, yeah man--but if it pulls the points around with the cam, doesn't that *retard* the time?"

"Remind me to give you an A on the next quiz."

Yes, that is true. *BUT* the breaker plate swing is not concentric with the cam and the change in *dwell* advances the time more than it gets retarded! Clever, no? Ford did this for *years* with a vengeance. Look at a mid-sixties Ford with the pivot point out near the edge of the cap!

The vacuum adds advance at anything less than about WOT. It uses what is called "Ported" vacuum. It's blocked off at an idle, but as soon as the thottle opens, it gets full manifold vacuum. There is a built in lag in the vacuum advance that allowed you to change throttle settings quickly and snap back to where you were and the advance would not have a chance to change (much). This makes the engine seem "smoother". You are unable to "suprise" the thing with quick throttle motions. This is what John Muir was saying about having to keep your foot on the gas with "elan".

These "vacuum" distributors worked well for the limited rpm range and the typical bug driving conditions. All was well, not *great*, but hey, it was cheap and it ran well.

As the car got heavier and speeds went up they needed more power *and* economy from the flat 4. They finally gave in and put a mechanical advance with a vacuum advance to get the best of both worlds. A combo unit that acts like a full mechanical under full power (because it *is* a full mechanical) and adds more advance under low throttle settings when the engine needs and wants it. Great, the best (mechanical) system! Then in step the smog regulators. Oh boy. What is good for the engine, fuel economy and power is *not* always good for the air. More power, better economy and smog *too*?! argggggh!

Then VW went to the dual port 1600 and power went up to 60 hp! The bigger manifold has a larger internal volume. Due to all kinds of strange atmospherics in the manifold, the pulse through the carb is "softened" with this "big" manifold. Years before, this simply meant bigger main jets or smaller air correction jets. But now the emisson/economy gremlins were out in force and this was not allowed. And,let's face it, the upright has a *weird* manifold setup anyway. They brought everything right to the edge: jetting, ignition timing, cam and valve timing. And when everything is spot on the car ran pretty well.

Let one thing get "loose" and God couldn't get it to run right. *Blame* the carb, *blame* the distributor. But ask yourself, is the:

When you have checked/adjusted all of this and the car *still* dosen't run correctly, you have a beef. Until then, fussing with different distributors or screwing with the carb is a waste of time. Do what you want/can with what you have. Chances are your *best* bet for the 1600 dual port is a vacuum/mechanical unit.

Oh yeah: the dual diaphragm distributor. It used manifold vacuum to retard the spark so much that at an idle that the heads ran hot enough to burn off the rich mixture required by all engines at that speed. As soon as you crack the throttle, it springs back to the base setting and acts like a single diaphragm unit.

The 72 bus (1700) used a vacuum advance cut-off: no vacuum advance until you hit high gear. It's the little gizmo next to the heater fan and the "extra" back-up light switch on the tranny.


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