dizzy vacuum source: to drill or not to drill?

[simon]

By definition, the strongest vacuum signal is to be found where the venturi tube´s smallest diameter is.

The autolite1100 uses exactly that spot for distributor vacuum source.

On my favourite carb, the CARTER RBS 1bbl, the vacuum signal for distributor advance comes from just above the throttle plate, where vac signal should be lower than in the high-vacuum zone above, where the booster is located.

While looking for the strongest vacuum signal possible for my advance canister, and since I have to drill out a crumbled, pressed-in vacuum hose adapter from the carburettor housing anyway, would it be a good idea to drill a new vacuum path at the smallest venturi diameter, like on the autolite1100?

 

[wsa111]

simon, what year is your distributor? If you had a 1100 carb which has the vacuum control valve. if you are using a 68 and up distributor without the loadamatic system, drill and tap & put a plug in the venturi vacuum hole & just run the distributor off manifold vacuum. thats how i have my 67 1100 done & the vacuum advance works to perfection. With your carter carb just use the ported manifold vacuum to the vacuum advance. If you used venturi vacuum you would bring the vacuum advance in at full throttle & that you do not want. At full throttle all you want is full centrifigul. hope this helps. :idea: william

 

[simon]

Now I´m confusd.

Off to read some more about spark advance...



ok:
on acceleration, spark advance is needed to ignite the fuel/ air mixture before the steadily faster going piston leaves TDC and goes down.
most of that will be accomplished by mechanical advance, which has its greatest effect at full throttle.

Since my vacuum canister has two ports, one for advance and another one for retard, and since I am not forced by law to run any smog equipment but PCV (´69 250ci in early ´65 car) I was planning on leaving the retard-port disconnected and just use the advance port of the canister.
with the 250 being a low-reving torquey axleshaft- snapper, I´m looking for low end performance by all means. whatever happens above 4000 rpm, I don´t really mind.

Now on to the vacuum source:

- manifold vacuum is highest at idle and falls when accelerating.
- venturi vacuum is lowest at idle and rises when accelerating.

common sense tells me I should use venturi vacuum, but then again, I could be totally wrong.

 

[wsa111]

Simon, use the ported vacuum for your application. Venturi vacuum system is only used in the 67 & before applications using the loadamatic advance. For your application bump the initiol advance up slightly, that may help your low end performance. the correct thing to do is have some one check how much centrifigul advance is in your distributor. But you sure don't want vacuum advance added on at full throttle. william

 

[Anonymous]

Rather than try to dirll a port, it sounds like it would be easier to advance the initial advance and change out the springs to provide more/earlier centrifugal advance.
If you drilled a hole, how big of a hole would you drill?
Too small of a hole and the signal is attenuatted and dampened when the small hole is connected to the larger volume of hose and the vacuum cannister. Too big of a hole, and it starts upsetting the flow in the venturi.
The venturi signal is going to be sensitive and change rapidly. Wouldn't this make the timing more erratic? Manifold vac would be more stable.

 

[simon]

First of all - I overrated vacuum advance on a dual advance distributor.

My latest conclusion is that manifold vacuum, providing the strongest signal at idle and decreasing to the amount the trottle plate is opened on acceleration, aids timing advance until the centrifugal advance mechanism really kicks in.

I already have fitted my centrifugal advance mechanism with the lowest tension springs I could find, and in fact, am very happy with what I was teached today on this forum.