Notes on the Centrifugal vs Vacuum Controversy

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.

Rob Boardman says - Sorry John, but your information is just not right. The 009 centrifugal-advance was never designed for the road-going VW - it was used on the industrial VW engines which ran at near constant speeds in compressors and generators. Race cars are running flat out or stopped - mostly at high rpm where the spark advance is "all in" all the time, and the mechanical style 009 distributor works well there too, but the meachanical distributors can NOT sense engine load, only rpm, so they often causes acceleration flat spots, especially when combined with the 34PICT/3 carburetor. These flat spots can minimised by ensuring the accelerator pump is adjusted for it's maximum stroke, installing a richer main and sometimes idle jets, and sometimes 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.

"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.

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 mecahnical advance distribuors were used on hard-working 1200cc busses, which had very low gearing (including reduction hubs), and so the engines were running at high revs even at lowish bus speeds. They also appeared on some industrial engines, which work at more constant speeds (generators and compressors). I've been telling the story to anyone who'd listen. VW obviously is familiar with 009 style distributors, but they didn't use them with any of the bigger engines, or on the 1200 engines fitted to the bugs. 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, and less at high throttle/more at part throttle). 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 inside the cylinder, resulting in 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 so higher flame speed in the cylinder). 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.

The mechanical/centriufugal advance distributors have no regard for the throttle position and the mixture of burned and unburned gases in the cylinder - they move the advance plate ONLY in response to rpm.

"Combustion 101." The importance of that mixing of burned and unburned gases cannot be overstated, and is not well understood by most people. When the spark ignites the fuel/air mixture, the flame front races out across the piston top, heating the "air" in the cylinder and creating a lot of pressure which pushes the piston down to produce power. That flame front takes roughly 2-3 thousandths of a second to complete it's job. It's not an explosion - it's a steady fast burn.

The one thing that alters that flame speed significantly is the presence of contaminants in the fuel/air mixture. Your 1600 VW engine has 400 cc per cylinder. It has 7.5:1 compression ratio, meaning that the head space above each piston is 400/7.5=53cc. So with every exhaust, there is 53cc of burnt gases left in the cylinder head space. With a wide open throttle, the next intake stroke will get around 400cc of fresh mixture mixed into that 53cc of burned gases. But if you are running at part throttle, you might have say 200cc of fresh gases mixed with 53cc of burned gases. That higher level of contamination results in a reduced flame speed across the piston top, so you need to light the fuel a little earlier - more spark advance. Vacuum distributors do this - providing up to 40 degrees of maximum advance with partial throttle openings. But with then throttle wide open at medium speeds, they will reduce the advance to around 30 degrees, because the flame speed is now higher as there is less contamination with 400cc of fresh air/fuel compared to the part throttle situation, and the flame speed increasews. Then as the rpms increase top meet the new rpms, the vacuum distributor will allow more of that 40 degrees advance - higher revs need more advance so the fuel is all burned just as the piston starts it's descent, no when the piston is already part way down. That's Load Sensing. The 009 distributor ignores all that, and must be limited to around 30 degrees max advance so it does not OVER advance in some condtions - which can cause detonation/knocking. That means it's also UNDER advanced in part throttle cruising, which reduces fuel economy. And of course it wont provide that needed extra advance off idle either. More reasons to fit a vacuum distributor than not.

Whew - hope you followed all that!

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 H30/31 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.

Rob says: The VW shop guy has it a bit backwards. The H30/31 carburettor (modern replacement for the smaller carbs) has a low vacuum signal and wont work well with vacuum distributors - they will struggle to pull in the amount of advance needed unless modified, so that carburettor does work "ok" with the 009 ... because it wont work well with vaccum distributors. But the 34PICT/3 carb, and the older 30 and 28 series carbs, were designed to work WITH vacuum distributors, so they work better "with" than "without" vacuum!

There are many many more carburetted engines, from many different manufactures, designed to work with vacuum distributors and there are using mechanical-only advance, for all the readons outlined above.

The most widely used vacuum style VW distributor these days is the SVDA - single vacuum double advance - meaning it has a single vacuum connection to the carburettor, and uses both that vacuum and mechanical advance together, to get the best of both worlds. RPM related advance of up to about 30 degrees maximum, plus additional maximum 10 degrees of vacuum advance when the engine needs it.

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!)

last modified June 2019