Vacuum advance with triple carbs

[WarrenZ]

Hello

I am very close to bolting a set of 40mm Delortos on a Cannon manifold on to my L24 and I am not sure what I should do about the vacuum advance from the distributor.

From my reading and looking at pictures there appears to be a mixed approach. Some have drilled the manifold and connected it there but my reading says this is not the best solution. Others have not connected the vacuum advance at all.

I am running the standard distributor with Pertronix ignition.

If I am going to drill and tap the manifold I would like to do it before I bolt them on so I am interested in opinions on what way I should go.

Warren

[sblake01]

You'll pretty much need to eliminate the vacuum advance. Even if you drill the manifold and connect it, it won't work right. I've never done this on a L6 but on L4s with Webers or Mikunis, I used a fixed breaker plate and eliminated the vac. advance mechanism.

[d240zx2]

Ditto Stephen's comments. My triple Mikunis have no provision for vacuum advance, only for brake boost. Centrfugal advance is all you need. I, too, run the Pertronix ignition and the combination of that and Mikuni carbs is awesome.

Frank

[jmortensen]

You'll pretty much need to eliminate the vacuum advance. Even if you drill the manifold and connect it, it won't work right. I've never done this on a L6 but on L4s with Webers or Mikunis, I used a fixed breaker plate and eliminated the vac. advance mechanism.

Good advice. The ZX distributor has a nice advance curve for use with triples.

[ozconnection]

My understanding of triple whatevers is that they are the gun performance setup for track work on engines like ours. For an engine that rarely sees less than 4k rpm, then vacuum advance is a waste of time.

When the car is used as a street car or a dual purpose machine, then the boundaries become less clear IMO.

Perform your own experiment. Do the work needed to run a vacuum line to your distributor. Try both setups for yourself...on the street. Tell us which one works the best. ie response economy etc. On the track, have the advance disconnected and plugged.

Tell us how you get on.

[sblake01]

Problem is that those carbs (or any triples) pull so much vacuum that, with those and the brake booster, there isn't enough vacuum to move the advance mechanism no matter where you tap the vacuum from. BTDT. Best thing to do is a custom curve on the centrifugal advance or remove it all together and use a fixed breaker plate. Just plugging the advance pot does nothing on a stock distributor. The breaker plate will still move on it's own when the distruburor turns and not in a way that works well.

[Nissanman]

Some have drilled the manifold and connected it there but my reading says this is not the best solution.

It sure isn't:disappoin

The distributor requires a PORTED connection for the vacuum advance.

That means a connection that is physically opened and closed by the operation of the throttle butterfly.

A manifold connection is NOT suitable, and will give totally erroneous signals to the advance mechanism.

Unless there is the necessary connection available on the side draughts then it can't be done.

[ozconnection]

There are two ways of connecting your vacuum advance mechanism up. One is ported and the other is full manifold vacuum. With full manifold vacuum, the amount of advance is proportional to the amount of vacuum it sees. So, doddling around with a closed throttle creates a high vaccum in the manifold, which pulls hard on the advance diaphragm on the dizzy advancing the timing. The engine should be able to tollerate this because of the decrease in dynamic volumetric efficiency with a closed throttle. Now, as the throttle is opened, more mixture is allowed into the engine, VE goes up and advance decreases because the vacuum drops...get it? Sure the mechanical advance still works, adding timing up to a point around 3500rpm (this varies with engine to engine). So at wide open throttle, vacuum is low and over 3.5K rpm the mechanical advance is all in and the vacuum advance mechanism contributes nothing to the advance curve.

At a very light load, most engines can tollerate advance of up to 45 degrees BTDC without problems. The higher the octane of the fuel used, the better this works. High octane fuel burns relatively slowly. Race cars that have 11:1 or higher compression often have the luxury of running very high octane gasoline or other additives which helps to maintain their advance curves. For the street however, octane is lower, compression should be a little lower and cams are often very big, which 'bleeds' the engine of its low rpm efficiency due to valve overlap. Often the dizzy is regraphed for such a setup, making the springs lighter so the advance comes in quicker than stock. A street type engine is more tolerant to an advanced ignition at low rpm due to the way in which it was constructed.

Cheers.