Bypassing Vacuum Advance

[commplexone]

I have noticed that on alot of the performance Z's i have seen recently that the vacuum advance from the distributor to the balance tube has been plugged and not used at all. What are the benefits of this and why do some people do it?

[Arne]

Let's make certain we are talking about the same thing. Because the vacuum line for the advance on the distributor was not routed to the balance tube, but to a small fitting on the front carb body. US spec 240Zs had two fittings on the balance tube for emissions gear. These parts have been removed on many (most) cars by now, and those fittings on the balance tube need to be plugged in that case.

There are some who have removed their vacuum advance, typically those who have triple carbs and the like. But then they don't even have balance tubes, so I don't think that's what you have been seeing.

I attached a picture of my car's engine, which is complete with all the emissions gear. I noted the vacuum advance hose with a green arrow. It's hard to see where it connects because it's hidden by the carb, but I pointed it out with an orange arrow.

I also attached a more typical picture of my other car's engine. It's not as clear but does show the lack of smog gear and the plugged nipples on the balance tube.

Hope this helps.

[c david]

the vacum advance tube will normally be removed after the vacum advance function of the distributor is no longer working. very few of them work as the ball bearing come loose. take your tube of a pull air through the tube with the distributor cap off and see if the advance mechanism moves. if you see the ball bearings laying around then you know it's not working. after all the common mods and gearing changes made along with the addition of an msd, i have no need or want of the vacum advance and for my taste, prefer to remove the tube and any other part which is not usefull or working. blocking ift off is good for the prevention of debris getting in said parts.

cd

[Arne]

You know, people always say that the vacuum advance mechanisms are mostly all shot by now, but all three distributors I have now (two single-point 240Z and one 280ZX w/E12-80) still have nicely working vacuum advance units. So maybe they are more durable than everyone assumes.

[jmortensen]

I've only ever seen one E12-80 distributor that still had the plastic cage that holds the balls intact. They're almost always broken, at least the ones I come across.

[Arne]

Don't know if it makes any difference internally, but ZX distributor I have started life with the E12-92 module. So perhaps the later units are more reliable?

[JLPurcell]

We did it in our early performance Z cars to have a certain advance curve. We would remove the vacuum advance have a speed shop set up the advance curve in the distributor to start at 1200 rpm and be all in at 2400 rpm (11 degrees distributor advance). This is done by changing the springs inside the stock distributor. We would set the static advance at 15 degrees giving us a total of 26 degrees at 2400 rpm. I have a couple of Mallory distributors which you can dial in the advance curve you want. Jerry

[Carl Beck]

It really isn't a good idea to remove, or have a non-working, vacuum advance on a street engine.

On a road race engine - intended to spend most of it's time at higher RPM's and geared to do so - low end throttle response is not so much an issue. Not to mention that most of the triple carb. set-ups lack a port for carburetor vacuum anyway. They also run finely tuned timing curves in their distributors... and can live with a lot of initial advance at the crank... they also have accelerator pumps!!

A great performing street engine is an entirely different matter. It routinely rev.'s from 750 RPM to 4000 or 5000 RPM. Sometimes that rev range is accomplished slowly... other times you want it accomplished as quickly as possible.

The ignition timing advance/retard that your engine needs at various RPM's and throttle positions is a pretty specific number if you want to extract it's top performance potential. Spend a lot of money on "special" spark plug wires, a super hot coil and/or MSD... and all is for NOT - if that fat spark is to early or too late - in the combustion process.

I believe it is better to think of the vacuum advance - as being a "timing retard - overridden with a vacuum control signal". This just gives you a broader range over which you can control and adjust the specific timing advance curve that your engine needs in order to preform at it's highest potential at any point in time.

From idle to wide open throttle - the L6 responds best with about 20 degrees of additional advance...

Without the vacuum advance, your distributor (and thus your engine) has to depend on the centrifugal advance weights to spin up and advance the timing... that is a far slower process, than the vacuum advance would have taken... so you get slower acceleration. (and possible bogs or poor throttle response).

The vacuum advance gets it vacuum signal from carburetor vacuum (ported vacuum), when you go to WOT the vacuum signal from the carburetor increases as the engine demands(sucks) more air/fuel through the throttle opening.... to handle this effectively your engine needs more advanced timing until either the centrifugal advance catches up - or the engine RPM (load) levels off...

The reason you can't use manifold vacuum to control a vacuum advance on the distributor is because manifold vacuum "drops" suddenly when you go to WOT - where carburetor (ported) vacuum increases.

Yes - on a street engine you can simply set the static timing at the crank to a more advance point and to an effect, off-set the lack of the vacuum advance - - - but that is not a good trade-off. because your timing on the rest of the timing curve - over a range of engine RPM's and Loads is no longer as flexible nor nearly as well matched to the needs of the engine.

Timing is everything... a street engine with a weak spark and correct timing, will out perform a street engine with a huge fat hot spark delivered to early or too late... That vacuum advance mechanism is a critical control element in the over-all timing curve that a good running street engine needs.. Make sure your's is working correctly. You'll get better initial acceleration, broad range performance and better fuel economy..

FWIW,

Carl B.

[240ZGL]

I've been looking at this very same problem. I'm going triples on my 2.4. The manifold is drilled and tapped at every runner, I think to make use of carb sticks, the mercury vacume tube set up for balancing mulit carbs like on motorcycles. If I were to link these vacume ports together would I get the same result as the vacume setup off a regular carb set up?