Notes on the Centrifugal vs Vacuum Controversy

Presented as "Pros" and "Cons"

See also The Wisdom of Bob Hoover on this subject.

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Pro Centrifugal Advance

John Muir* wrote -

Just what kind of distributor do you have? Volkswagen has used three main types through the years: mechanical (centrifugal) advance, semi-mechanical advance and vacuum advance. The first two types have been the same all the way, but the vacuum advance distributor has gone through several modifications and has been standard equipment on all models for many years. I hate them! They are another sop to American buyers who refuse to learn to shift a car with a little coordination. This is just a personal beef, so forgive me. I use a straight mechanical advance distributor, called the Porsche Type, which advances the distributor on rpm as the engine speeds up. Most VW race cars and beach buggies use the same. It gives a very good power curve on the VW engine.

The vacuum distributor advances the spark based on the vacuum in the carburetor (as opposed to rpm) and they will never make good since the principle is wrong. That, however, is what they give you with the car. I saw a demonstration 1969 accessory car on a VW showroom floor with a speed set-up on the engine, and guess what? Right. It has a mechanical advance distributor on it and at o only $30.00 extra, so when you get bread ahead, buy one for your jewel. You have to block up the hole (or holes) in the carburetor so it doesn't leak vacuum. Vacuum leaks burn valves, if you didn't know. Now that's out of my system.

*How to Keep Your Volkswagen Alive -- A Manual of Step-by-Step Procedures for the Compleat Idiot, 1976 Edition, page 90.

Rob Boardman says - The 009 centrifugal-advance does often cause acceleration flat spots, especially when combined with the 34PICT/3 carburetor. But these can usually be overcome by ensuring the accelerator pump is adjusted for it's maximum stroke, installing a richer main and idle jets and a smaller air correction jet in the carburetor, and even plugging the hole in the throttle plate (e.g., with a pop rivet). This helps make up for the lack of vacuum, and reduces any flat spots in acceleration -- but, at the expense of higher fuel consumption.

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Pro Vacuum Advance

See The Wisdom of Bob Hoover on this subject.

"Speedy Jim" wrote - The Bosch 009 distributor is well-known and widely used. This distributor has centrifugal advance only. Some people swear by it. (Others swear at it!) Lacking vacuum advance, it is slower to respond to load changes or throttle opening and thus the hesitation.

My personal preference is for the stock (double vacuum) distributor, but they are hard to find in good condition and expensive to buy new. Swap meets are a good source. Be aware, though, that there were a dozen or more models, all look alike.

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A Treatise by Rob Boardman on the
Advantages of Vacuum Distributors

The 009 distributors are a very common replacement distributor, but they have serious limitations when used in road vehicles. Since they are centrifugal advance only, they can't "load sense" like vacuum distributors. When rpm is high we need more advance, and when rpm is low we need less advance.

With a little research I learned that they were originally designed for hard-working 1200cc busses. They also appeared on some industrial engines, which work at more constant speeds. I've been telling the story to anyone who'd listen. VW obviously is familiar with 009 distributors, but they didn't use them with any of the bigger engines. They obviously felt that the engines worked better with vacuum distributors.

There were only ever two kinds of centrifugal-only distributors -- the 009 and the VW equivalent (which I think was a 050). But they tried lots of variations in the vacuum distributors, so the inference is that they found the 009 etc. distributors unacceptable. But in their inimitable style, they fine-tuned the vacuum distributors with the various engines.

Today the 009 centrifugal advance distributor is viewed as a "one size fits all" distributor, which means that they are not "ideal" for any particular engine type. They do work quite well in applications where the VW engine is operating at high rpm and high power -- for example, racing VW engines, powering compressors and power generators.

Fortunately, the vacuum distributor (and the combined centrifugal/advance of '74+ distributors) does a reasonable job of following these two conflicting needs (more advance at high rpm less at low rpm). The vacuum port on Solex carburetors is placed just UNDER the main venturi, close to where the throttle plate passes as it opens. So imagine the engine idling. The throttle plate is nearly closed, so there is a low airspeed through the main venturi, so not much vacuum there or just under it (above the throttle plate). The vacuum port sees very little vacuum, and the idle advance setting prevails (7.5-10BTDC on most models).

Now open the throttle a little, so the edge of the throttle plate passes by the vacuum port. This creates a mini-venturi with very high air speed, which creates a lot of vacuum, so you get a shot of advance to help speed up the engine (this effect is entirely missing with the Bosch 009 distributor, which is what causes the notorious "009 flat spot"). Since in a part-throttle condition you still have a high proportion of burned gases for a low flame speed, this high advance also meets the advance condition needed to deal with that too.

Now open the throttle right up. The throttle plate moves away from the vacuum port (no mini venturi) and so the MAIN venturi is providing the vacuum effect, but since the airspeed hasn't yet increased much yet (engine hasn't yet increased rpm), the vacuum signal is lower than part throttle, so the advance is reduced a little. Perfect for a fresher mixture (lower proportion of burned gases with an open throttle). Now the engine rpm starts to catch up with the open throttle, so the airspeed through the main venturi increases, vacuum increases, and the advance increases progressively, which is just what you want for the increasing rpm, since the crankshaft is rotating in less time so you need more advance to get that fresh charge completely burned at the right moment.

So the vacuum distributors allow for high advance at high rpm/open throttle, where rpm is the dominant factor; and also allows high advance at part throttle/medium rpm, where the proportions of burnt/fresh mixture is the predominant consideration.

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Dave wrote - The guy at the VW shop claims that my "stumbling" problem is due to the 34 PICT/3 carburetor (they put it on the engine when they rebuilt it!). He says I should go to either the 30 or 31 -- the car will never run right with the 34. The best combination (he claims) is the 009 distributor and the 31 carburetor. I'm getting too much advice -- I don't know who to believe! Actually, yes I do. I lean heavily to your advice (Rob) and that of Bob Hoover, and I will continue down the vacuum-advance distributor path.

Dave wrote to Bob Hoover - Thanks for the great "sermon" on vacuum vs centrifugal distributors. It finally tipped me over the edge -- yesterday I bought and installed a rebuilt vacuum advance distributor, replacing the 009 distributor that was causing our '73 SB to hesitate on acceleration. The guy that sold me the new distributor said, "Be sure to keep the 009 -- you'll be putting it back on in a week!" I don't think so -- the new distributor has virtually eliminated the hesitation problem that has been plaguing us from day one. (There sure is a widely divided opinion on centrifugal vs vacuum advance distributors!)

 

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