Centrifugal vs Vacuum Controversy - Presented as "Pros" and "Cons"
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 does often cause acceleration flat spots, and these can usually be overcome by ensuring the accelerator pump is adjusted for it's maximum stroke, and you might need a richer main jet in the carby.
I have heard of a couple fixes for a 34PICT and 009. One from this newsgroup which is to pop rivet the hole in the throttle plate and the other is to go with a bigger idle jet.
Dave wrote to Rob -
Regarding the distributor: Have you been following the responses on the Newsgroup to the guy who put in a 009 and thought it was wonderful? The responses are running VERY heavily in favor of the vacuum advance dizzy. I think we did the right thing.
Rob responded -
Yes, I noticed this too. Since I found out what they were originally designed for (hard working 1200 busses, and they also appeared on some of the industrial engines, which work at more constant speeds), I've been telling the story to anyone who'd listen. My main argument is that since VW obviously ARE familiar with 009s, and 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, 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. unacceptable, but in their inimitable style, they fine tuned the vacuum distributors with the various engines.
~~~
Pro Vacuum Advance
Rob wrote -
The 009s are a very common replacement distributor, but have serious limitations when used in road vehicles, since they are centrifugal only, and so they can't "load sense" like the vacuum distributor.
They are also 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, or powering compressors and power generators.
And when rpm is high we need more advance, and when rpm is low we need less advance.
Fortunately, the vacuum distributor (and the combined centrifugal/advance of 74+ distributors) does a reasonable job of following these two conflicting needs. The vacuum port on solex carburettors 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 - 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 "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 remember?). 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.
"Speedy Jim" wrote -
Sounds like somebody substituted the well-known Bosch ?009) distributor. This 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. (Do check the pre-heat temp, though, as well.
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.
See The Wisdom of Bob Hoover on this subject.
Dave wrote, regarding installation of the vacuum advance distributor -
I turned the rotor to #1 and pulled the 009 distributor out. Then I set the points in the new one (double vacuum) and slipped it in place. It's interesting that #1 is on the left side; it was on the right on the 009. I attached the vacuum port on the silver canister to the port on the back (back) of the carburetor with rubber tubing. Then I attached the port on the bottom front (front) of the canister to the vacuum port on the left side of the carburetor--rubber tubing on the ends and a piece of copper tubing bent in the shape of a trap in the middle.
I static timed it to 5 degrees ATDC just to get in the ball park, and fired her up. The car started with no problem. I pulled it out of the garage onto the driveway and proceeded thru the detailed tuning -- dwell angle at 50 degrees, timing at 5 degrees ATDC with the timing light -- then to the idle. No dice. I tried to reduce it below 1200rpm -- wasn't having any. The engine died when I got it down to 1000. I'm absolutely clueless.
(Dave later discovered a severe air leak around the throttle shaft in the bottom of the carburetor into the intake manifold, which was making the fuel mixture very lean and requiring a high idle to keep the engine running. This discovery resolved Dave's nagging problems with hesitation and inability to properly set the idle.)
Then I took the car out for a test drive. It's considerably better, but not perfect. My wife said it seemed to "fweem" better to her, and it does. But it's still a little rocky. The guy at the VW shop claims that the problem is the 34 PICT 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 dizzy 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 dizzy 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 that was causing our '73 SB to hesitate on acceleration. The guy that sold me the new dizzy said, "Be sure to keep the 009--you'll be putting it back on in a week!" I don't think so--the new dizzy 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!)
Rob wrote -
Don't forget, with the 009 distributor you normally need to run a larger main jet, and sometimes a smaller air correction jet (I'm not sure how the progressives are actually set up). This helps make up for the lack of vacuum, and reduces any flat spots in acceleration, at the expense of higher fuel consumption.
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 does often cause acceleration flat spots, and these can usually be overcome by ensuring the accelerator pump is adjusted for it's maximum stroke, and you might need a richer main jet in the carby.
I have heard of a couple fixes for a 34PICT and 009. One from this newsgroup which is to pop rivet the hole in the throttle plate and the other is to go with a bigger idle jet.
Dave wrote to Rob -
Regarding the distributor: Have you been following the responses on the Newsgroup to the guy who put in a 009 and thought it was wonderful? The responses are running VERY heavily in favor of the vacuum advance dizzy. I think we did the right thing.
Rob responded -
Yes, I noticed this too. Since I found out what they were originally designed for (hard working 1200 busses, and they also appeared on some of the industrial engines, which work at more constant speeds), I've been telling the story to anyone who'd listen. My main argument is that since VW obviously ARE familiar with 009s, and 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, 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. unacceptable, but in their inimitable style, they fine tuned the vacuum distributors with the various engines.
~~~
Pro Vacuum Advance
Rob wrote -
The 009s are a very common replacement distributor, but have serious limitations when used in road vehicles, since they are centrifugal only, and so they can't "load sense" like the vacuum distributor.
They are also 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, or powering compressors and power generators.
And when rpm is high we need more advance, and when rpm is low we need less advance.
Fortunately, the vacuum distributor (and the combined centrifugal/advance of 74+ distributors) does a reasonable job of following these two conflicting needs. The vacuum port on solex carburettors 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 - 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 "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 remember?). 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.
"Speedy Jim" wrote -
Sounds like somebody substituted the well-known Bosch ?009) distributor. This 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. (Do check the pre-heat temp, though, as well.
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.
See The Wisdom of Bob Hoover on this subject.
Dave wrote, regarding installation of the vacuum advance distributor -
I turned the rotor to #1 and pulled the 009 distributor out. Then I set the points in the new one (double vacuum) and slipped it in place. It's interesting that #1 is on the left side; it was on the right on the 009. I attached the vacuum port on the silver canister to the port on the back (back) of the carburetor with rubber tubing. Then I attached the port on the bottom front (front) of the canister to the vacuum port on the left side of the carburetor--rubber tubing on the ends and a piece of copper tubing bent in the shape of a trap in the middle.
I static timed it to 5 degrees ATDC just to get in the ball park, and fired her up. The car started with no problem. I pulled it out of the garage onto the driveway and proceeded thru the detailed tuning -- dwell angle at 50 degrees, timing at 5 degrees ATDC with the timing light -- then to the idle. No dice. I tried to reduce it below 1200rpm -- wasn't having any. The engine died when I got it down to 1000. I'm absolutely clueless.
(Dave later discovered a severe air leak around the throttle shaft in the bottom of the carburetor into the intake manifold, which was making the fuel mixture very lean and requiring a high idle to keep the engine running. This discovery resolved Dave's nagging problems with hesitation and inability to properly set the idle.)
Then I took the car out for a test drive. It's considerably better, but not perfect. My wife said it seemed to "fweem" better to her, and it does. But it's still a little rocky. The guy at the VW shop claims that the problem is the 34 PICT 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 dizzy 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 dizzy 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 that was causing our '73 SB to hesitate on acceleration. The guy that sold me the new dizzy said, "Be sure to keep the 009--you'll be putting it back on in a week!" I don't think so--the new dizzy 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!)
Rob wrote -
Don't forget, with the 009 distributor you normally need to run a larger main jet, and sometimes a smaller air correction jet (I'm not sure how the progressives are actually set up). This helps make up for the lack of vacuum, and reduces any flat spots in acceleration, at the expense of higher fuel consumption.
