Vacuum-Advance vs.
Mechanical-Advance Distributors

~ The Wisdom of Bob Hoover ~


Dave wrote to Bob Hoover, a well-known VW guru in Southern California -

Bob, I need some input regarding single- vs double-vacuum distributors. It's not real clear to me which of the two distributors (single/double vacuum) is the right one to use with my 34 PICT/3 carburetor.

I called the man at a local VW repair shop; the guy called the warehouse and talked to them about the double-vacuum distributor. They want over $300 for it new! They asked him why he wanted to know; he told them what I had said (passing along your advice) about the 009 and the 34 PICT being incompatible. They said that the double-vacuum distributor was made for use in California ONLY!

This shop has a rebuilt single-vacuum distributor for about $90.

So once again I'm in a quandary. Do I believe a source that has proven itself unreliable and buy the cheaper single-vacuum model and hope for the best? (I'm inclined to, as I really don't want to have to shell out over $300 for a new distributor!)

Any cogent words of wisdom? I'm getting desperate with this lousy hesitation problem; I want to do the right thing, hopefully without breaking the bank!

Bob Hoover responded with the following "Sermonette."


Vacuum-Advance vs.
Mechanical-Advance Distributors

by Robert S. Hoover

(Used with permission.)

A vacuum advance distributor senses changes in the engine's LOAD -- and does so almost instantaneously. Mechanical advance distributors, which sense only changes in rpm, are meant for special purpose applications, such as as drag racing or poorly designed multi-carburetor set-ups.... or moving a loaded Type II with a 25hp engine.

If you try to use a purely mechanical advance distributor in your daily driver you will have to jack-up the output of the accelerator pump, since providing a jolt of raw gas is the only way you can bump up the rpms, which is the only way you can get the advance to kick in. But before the revs can build up you waste a lot of gas. This plays hell with your fuel economy and shortens your engine's useful life at the same time.

People who run mechanical advance distributors usually have no idea how well their Volkswagen can perform.

When you go back to a vacuum advance distributor you usually see a remarkable improvement in both performance and economy. The performance-change being the typical perception of an engine that is now suddenly more responsive than before. The improvment in economy comes about because you're no longer throwing away most of the accelerator pump's output each time you shift gears or pull away from a stop.

The fact John Muir.... and all the tits & ass VW magazines.... say the mechanical advance distributor is the best thing since canned beer is no reason to assume this is true. Muir advocates any number of things that are incorrect, including many that are detrimental to your vehicle.

I've written about the vacuum vs mechanical distributor situation many times but most people still don't get it. Volkswagen -- and all other auto makers -- use vacuum-advance (or vacuum/centrifugal) because for normal driving such distributors do a better job than a purely mechanical-advance device.

Now look at what you've got: A dictated AFTER-TDC static-timing point on an engine whose optimum operating speed demands a firing point of about 30 degrees BEFORE-TDC... and inherent limitations in vacuum-advance distributors which prevent them from covering that range.

So you use a composite distributor, one having BOTH vacuum advance, which gives the required responsiveness, AND centrifugal advance, which gives you the desired range.

The retard side of the canister was added to insure a positive back-down of the firing point when the throttle was closed. Otherwise, the firing point would decay according to the speed of the engine (ie, under the control of the centrifugal-advance system). Even then, it was an interim solution until they could get the bugs out of the fuel injection system. (Mexican bugs are fuel injected. Very nice, reliable system. Illegal in Ami-Rica, of course.)

If you do not see the retard function, don't use it. On a dual chamber canister that means disconnecting the retard-signal line. (It will usually be the lowest pick-off point on the carburetor.) You need to seal the lines to prevent anything getting into the canister or being sucked into the manifold. And of course, re-time the thing.

If you have a single-canister distributor (or a dual-canister distributor with the retard side disabled), set your static timing at about 7.5 degrees BEFORE TDC and drive on. Most vacuum-advance distributors will give you about twenty degrees total advance. Check it with a timing light by revving the engine. Tweak accordingly but don't push it too far -- hot day, low-octane fuel, you'll get detonation even though everything is set to spec. I run about 28 degrees total advance.

(In earlier messages to you and Rob I've not mentioned the initial static timing point because I assumed the principle was understood with regard to emissions and the carburetor/distributor combination.)

If you don't have a good vacuum-advance distributor, go get one. Go to a junkyard and buy half a dozen of the things in beater condition. Remove the vacuum canisters. If you can't find a good vacuum canister, go buy one. (Replacements run about $25, new, Bosch, here in southern California.) Throw the distributors in a can of oil to soak over night, then dismantle them. Be careful to preserve the spacers and washers -- they're hard to find (I often buy a new -009 just to get the spacers!) Match the best fitting shaft-to-housing and reassemble the distributor using a proper number of shims and spacers to give minimum end-play. Go drive the thing.

If you want to make it last forever, or at least longer than the 70,000 or so it normally lasts, get rid of the points. Opening the points represents as asymmetric load on the distributor. Opening the points is the major cause for wear on the shaft and bushings. So get rid of them. Use a Pertronix unit. No load on the distributor shaft. Runs concentrically. Vastly reduces the wear-rate which is already pretty low because the distributor only turns at one-half engine speed. (Don?t use an optically triggered unit. The Pertronix is magnetic. The Hall effect sensor, amplifier and SCR are contained in the pick-up unit.)

Okay, so you're not a mechanic (you should be, if you want to own/operate an orphaned, antique vehicle). A new, vacuum advance distributor costs about $90. Roland Welhelmy bought one a couple of weeks ago from the Old VW Company in Escondido and sent a note about it to the Type 2 mailing list. (OVW = (760) 73-7587). Give them a call, see if they'll send you one. Ditto for the Pertronix unit, while you're at it. (It runs about $60.)

Regarding advance mechanisms: You can make a centrifugal system advance as far as you want and at whatever rate you want. Friction and loss of spring tension is the only problem. But with a vacuum-advance system, the thing is only linear across a fairly narrow range, although marvelously responsive within that range. Most cars use composite or combination systems -- a vacuum-advance to initiate acceleration and a mechanical-advance to continue that advance to whatever limits -- and at whatever rate -- the engineers decided upon.

But one thing you CAN'T do is stick a purely mechanical system into a VW and expect it to be responsive to the throttle. First, you must bring about a change in rpm, usually by feathering the throttle -- pumping a little more gas -- THEN the advance kicks in -- and will advance as far as you want it to, depending on what you've done to the bob-weights and springs. This is perfect for the dragstrip, where you jack it up to three grand before you even pop the clutch, and thereafter go flat-out through only two gears, at most. The 009 is great for that. Otherwise it's a piece of crap.

Personally, I think you should be driving a Toyota. Early air-cooled Volkswagens require about ten times the annual preventative maintenance of a modern vehicle and it must be SKILLED maintenance. If you gotta pay someone to do it, your VW is going to cost you more than a luxury car. And if you don't do the maintenance, you're going to have a crappy ride. And if you try to do it yourself and screw things up -- a Toyota seems like a good idea to me.

- Bob Hoover


And some follow-on material from Bob -

  • The 34PICT's will work with ANY vacuum-advance distributor.
  • The double-canister distributors are used on engines having a very low static-timing point to insure complete combustion at idle speeds. (The alternative would have been using a different cam.)
  • The VW cam, valves, intake & exhaust volume dictate a particular advance-point for proper operation, which is up around 30 degrees BEFORE TDC and which generally is all-in by about 2500 rpm. Above that, the same factors serve to limit the engine's output, a very sophisticated form of rev-limiting.
  • A vacuum-advance distributor will NOT give you that much advance in a linear fashion.


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