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Ignition - vacuum advance

5721 Views 7 Replies 7 Participants Last post by  Gordon Raymond
Why did alfa use vacuum advance on some engines and not on every engine? Are there any differences between the Bosch distributors except for the vacuum thing? Is it OK to just disconnect the pipe? 2000 engine with carbs.

Thank you
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Mechanical vs Vacuum advance distributors.

Here is a very simplified answer. The mechanical system, no vacuum, counts on weights, springs, and dist rotational speed for advance. VERY generally, the advance curve is achieved by springs and weights, and does not take into account engine load factors. The vacuum system also contains a mechanical advance mechanism, but is governed by a vacuum diaphragm. This diaphragm IS affected by engine load, and variable vacuum caused by butterfly position. For street use, the vacuum system will yield an engine that pulls more evenly, throughout it's normal running range. The mechanical system is better on a performance engine where the ignition advance is over at somewhat lower engine speeds.
The Bosch vacuum system, can be altered into a mechanical distributor, by removing the vacuum system, making an aluminum cover plate for the hole in the dist body, and then changing the weights and springs on a dist machine to match those of the mechanical dist. This is time consuming and not inexpensive. You need a source of parts, access to a good dist machine, and a mechanical unit to replicate. It can be done, but it is NOT as simple as disconnecting and plugging the vacuum line. THere is a lot more to this, but unless you are really interested in busy work, you are much better off locating a mechanical dist, even if you still need to alter it's curve. Hope this helps. :DGordon Raymond, a no bling subscriber:(. I JUST contribute $!:cool:
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Steven, if your car is a road car I believe it is more desirable to have a vacuum system on the distributor so I personally would not disconnect. Gordon has explained well and I would add that the vacuum side is merely an additional control system to adapt the advance curve to the current engine environment. A non vacuum distributor is set on engine revs only whereas the vacuum advance adapts to engine load and provides protection against over advance and tends to give a slightly meatier torque curve at lower or low speed under load eg hill climbing revs. Pinking is much less too, with variable quality petrol (gas).
Consider it an additional insurance policy.
As to whether a car was equipt with a vacuum or mechanical unit I think it depended on the flame path characteristics of each version, and where it was being sold.

Richard J

`65 Giulia Ti, `69 GT Junior, `74 GTV 2000, `76 Alfetta GTV, `77 Alfetta GTV, `84 GTV6

Why did alfa use vacuum advance on some engines and not on every engine? Are there any differences between the Bosch distributors except for the vacuum thing? Is it OK to just disconnect the pipe? 2000 engine with carbs.

Thank you
The only Alfas I know that were fitted with a vacuum advance were the ones with a single downdraft carb - 750/101 Normale Giuliettas, 101 Giulia Spiders and Sprints, and Giulia TI sedans. Anything with double carbs or Spica injection had a mechanical distributor. Some later european model must have had vacuum, as Highgate Alfa in UK is clearing some NOS Bosch vacuum distributors. Maybe some 80's euro Alfettas and Giuliettas.

Vacuum advance will provide more advance at 60 mph, as the low throttle opening will allow a high vacuum level, while the 3000 rpm engine speed would not allow mechanical advance to build up to the maximum (at least with Alfa's original advance curves). This will promote higher fuel economy.

Alfa probably felt buyers of dual carb models would not run them at 60 mph, as the trend in Europe in the 60's was to run everything flat out on the highways.
Yes, I'm not aware of any twin-carb 105 car with vacuum advance. Does your GTV have it?

I don't think so either. The Alfetta series had this on some cars with Bosch system. To get a good engine running even better, clean the interior and weights in the distributor and oil it with some very liquid thin oil. This plus a new cap will do it.
Does anyone have the correct Part number for the Bosch unit without vacuum advance that will work with Steven's engine? How about someone with a parts book (mine's not new enough) looking up the number? Then Steven can hunt one up and his problem's solved. Otherwise, we are all just speculating here on what might work.
Gordon Raymond
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