When the first Type 2 Bus vehicles came out, they used the 1200cc VW engine and needed reduction hubs so that the tiny engine could push a heavy vehicle (with a top speed of about 85kmh since the reduction hubs meant it was revving it's heart out!). Because the engines in the Type 2 were working at high rpm and high throttle most of their life, the VW version of the centrifugal-advance distributor was used on them too.
Most engines in road vehicles operate at various engine speeds and load conditions, which require a lot of variation in the amount of advance needed for optimum engine performance and economy. Ideally, the amount of advance varies from about 7 degrees to a maximum of about 42 degrees, depending on the engine model and its intended use. So most road vehicles use vacuum sensing, or a combination of vacuum and centrifugal, to get the best timing over a wide range of engine operations - low throttle low rpm, low throttle high rpm, high throttle high rpm and every variation in between.
The early Beetles used single-vacuum distributors (SVSA). Then in 1971 VW introduced the double-advance distributor using both rpm-related and vacuum-related advance. The first US model of this distributor was the '71-'73 double vacuum distributor (produced to meet emissions requirements) called the DVDA. In other parts of the world a single-vacuum dual-advance (SVDA) was used. From 1974 onwards the U.S. also went to the SVDA. The SVDA distributor works like a high quality 009 distributor with added vacuum advance. These distributors are quite expensive to build in comparison with the Bosch 009 distributors (the Bosch equivalent of the VW centrifugal distributor). So the cheap-to-build Bosch 009 distributor became the "one size fits all" replacement distributor, since it IS cheap and it does work moderately well. But precisely because the 009 is a "one-size-fits-all" distributor, it is NOT ideal for most engines, and it can cause problems for some engine/carburettor combinations.
To accelerate an engine smoothly, you need both extra fuel and an extra advance. The accelerator pump provides the extra fuel and the vacuum distributors provide the additional advance needed. But an 009 distributor cannot provide any advance until AFTER the engine rpm starts to increase (the advance starts happening at around 1200-1300 rpm) so, if the carburettor is set to run a little lean (LESS fuel), you get stumbline or hesitation (a "flat spot") which usually means the driver has to blip the throttle a time or two to get the rpm up to the point where the 009 distributor is starting to advance, then "feather" the throttle (and slip the clutch) so the rpm stays high, to avoid that flat spot.
The most common technique to overcome the flat spot associated with the 009 distributor is to replace that "missing" advance with extra fuel - a larger main jet, maximum stroke on the accelerator pump, and in some cases filling in the air-bleed hole in the throttle butterfly. By running the engine richer than normal throughtout its operation, the flat spot is minimised.
The earlier Solex carburettors (28PCI, 28PICT and 30PICT/1 and /2) are set to run a little on the rich side (the VW engine actually likes around 13.8:1 air/fuel ratio, where the ideal is 14.5:1). So since these carburettors are set to run rich anyway, the flat spot is often not noticable, or can be tuned out with minor adjustments.
But in 1970/71 the emissions problem was becoming recognised, and VW changed the jetting on the 30PICT/3 (1970 US only) and the 34PICT/3, and /4 to run the carburettor leaner - closer to the ideal 14.5:1 air/fuel ratio (the California 34PICT/4 used especially lean jetting and a throttle positioner to ensure that the throttle closed slowly after you lifted your foot off the pedal). So with this leaner running carburettor set-up, the 009 distributor flat spot becomes a real issue.
You'll hear most complaints from folks who use the 34PICT/3, 34PICT/4 or the modern equivalent for the smaller carburettors - the Brosol H30/31 (which is almost identical to the 1970 30PICT/3) - all of which normally come with lean jetting.
Another issue with the 009 distributor is the limited maximum advance. The vacuum distributors can run up to a maximum of about 40-42 degrees under the right conditions (light throttle and medium speeds for example) and this helps fuel economy. But if you maintained that much advance with a full throttle and low/medium rpm, the engine would ping/detonate, a problem which can destroy an engine if not corrected. The reason is rather technical, but is mainly related to the amount of residual exhaust gases which are left in the cylinder compared to the incoming charge. There is always SOME residual gas, since there's a head space above the cylinder. At part throttle there is a proportionally larger amount of burned gases in comparison to the fresh stuff (throttled fresh mixture but same head space of burned gasses). At full throttle there's a lower proportion of burned gases because you are letting in MORE fresh mixture for the fixed volume of head space. This "contamination" of the fresh mixture alters the flame speed (the time it takes for the fuel/air mixture to burn) so open-throttle needs less advance (burns faster) and part-throttle needs more advance (burns slower) at any particular rpm.
Incidentally, this is part of the reason why the low compression 1200cc engines need more advance - usually 10BTDC compared to 7.5 BTDC for most 1500/1600cc engines. Low compression means more head space (compared to the cylinder volume), and so there is more contamination of the fresh charge throughout the rpm range, and that means a little more advance is needed so all the fuel is burned just as the piston starts it's descent.
There is an rpm-related issue, too. As the engine rpm increases, the spark needs to occur sooner (more advance) to make sure the maximum pressure on the piston occurs just as it starts its descent.
The vacuum distributor senses throttle position, so if you floor the throttle at low-medium speeds, the distributor is able to "back off" the advance until the engine speed catches up with the new throttle position (slowly allowing the extra advance back in as the rpm/airflow rises). The 009 distributor can't do this, so it HAS to be set to "worst case," which is a maximum advance of between 28 and 32 degrees at 2500-2600 rpm (which is why we set the 009 distributor at 3000rpm - to make sure that all the advance is present).
Since the 009 distributor has been limited to a maximum advance of 28-32 degrees compared to the vacuum distributor's maximum advance of 40-42 degrees, it's actually under-advanced for a lot of driving conditions, and this means worse fuel economy. And because you need to set the carburettor to run rich to reduce the flat spots, fuel economy suffers even more.
Because the 009 distributor is built very cheaply, the total amount of advance varies from one distributor to the next. This means that when setting the 009 distributor, it's important to set it at maximum rpm, not idle, since the maximum advance is much more important for most engine conditions. Always use as much of the maximum advance of 28-32 degrees as the engine can take without pining/detonating, as this will reduce any flat spots just a little, and also help fuel economy just a little. If it still pings at a maximum advance of 28 degrees, always use a higher octane fuel. Never use less than a maximum advance of 28 degrees because it means the engine is seriously under-advanced at higher rpm and will run hotter than it needs to, and fuel economy will suffer as well.
In the heavier-bodied Kombi/Bus and Karmann Ghia vehicles, you might need to limit the maximum advance to the 28-30 degree range. These engines are less likely to tolerate the 32 degree maximum advance, as they are driven with more throttle to accelerate the heavier body.
Once you have determined the maximum advance which works well for your 009 distributor, you can then measure the idle advance (which will probably be between 5 and 10 BTDC but might be outside that range). If you want to, you can then use THAT timing setting for that 009 distributor, set either at idle or statically.
So you can see that the 009 distributor is NOT ideal, but it will work. For best performance under all engine operating conditions, we recommend using a vacuum distributor. Hope all that makes sense. There are Tune-up Articles on our Web site which include info on both vacuum and 009 distributor settings.
Rob & Dave