since I came onto the VW scene I've been hearing people going
on and on about the 009 centrifugal-advance distributor. Where
did this distributor come from? Was it a stock part for certain
VW's? What difference will it make on my 1969 1300sp engine?
And what's the "flat spot" all about?
responded with a great treatise -
History of the 009
built a lot of industrial engines (called Type 122 for the 1200cc
and Type 126 for the 1600cc engines), as well as those engines built
for the Beetle itself. Industrial engines powering generators,
compressors and such run at near constant speed, so only a simple
distributor was needed. For this application, a centrifugal-advance
distributor was developed by VW and later by Bosch. This type
of distributor increases the amount of advance as the engine
speed rises, but can not sense the throttle position (engine
load). This is fine for engines operating at constant speeds like te industruial engines,
or at high power and high rpm (for VW racing engines for example).
the first Type 2 (Kombi etc.) vehicles came out (about 1954),
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!).
the engines in those ealry Type 2s 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 (but just try getting one off
the line - they needed a lot of revs and clutch slip to avoid
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.
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.
early Beetles used single-vacuum single-advance distributors (SVSA)
(no mechanical advance). This practice continued up to and including 1970. 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 (around 30 degrees max advance)
with added vacuum advance (another 10 degrees for a total of around 40 degerees).
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
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), and you dont have any added advnace,
you get stumbling or hesitation (a
"flat spot") which usually means the driver has to goose 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.
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.
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).
Since these carburettors are set to run rich anyway, the
flat spot is often not noticable, or can be tuned out with minor
in 1970/71 the emissions problem was becoming recognised, and
VW changed the jetting on the 30PICT/3 (1970 US cars 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.
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.
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 piston. 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 proportionally fresher mixture needs less advance
because it burns faster, and part-throttle needs more advance because it burns slower,
at any particular rpm.
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 proportionally 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.
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
vacuum distributor senses throttle position (load), so if you floor
the throttle at low-medium speeds, the vacuum 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).
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.
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.
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, so detonation is more likely at higher advance settings.
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.
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.
all that makes sense.
are Comprehensive Tune-up Articles on our Web site
which include info on both vacuum and 009 distributor settings.
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