help me understand my vaccum advance

[ZSaint]

I understand this "advance" system like this. You have two systems: mechanical and vacuum. For racing purposes, you do not need the vacuum advance. You time the engine at total advance (36`-38`) depending on your compression ratio. This advance is by weights with different springs attached. (soft springs-early advance... stiff springs late advance) The vacuum advance is controlled by a diaphram) It will run all the way up to 40`-50` advance when the car is NOT under any load (cruising down the freeway at 70). A street car will get better mileage with the higher advance. If you get too much advance your engine will ping/knock. You can modify the amount of travel in the distributor's mechanical advance by welding a stop in the slot. Or, you can file the slot longer to get more advance. You can also get different vacuum advance units. The value is stamped in the rod (X2) say 15 which will give you 30 of advance... 17:34, 20:40, 25:50, etc. I am guessing you just need to experiment with these items to get maximum performance. A dyno is your best test! Most people just stick a dizzy and run it. A lot of performance is lost or gained here.

[datfreak]

I--------It will run all the way up to 40`-50` advance when the car is NOT under any load (cruising down the freeway at 70). ---.

after reading zsaint post i'm more confused-- that is how I thought it worked at the start--low throttle = high advance..

Maybe the datsuns just like it the other way?

Anyways-- Im running it as bob said(and the way it was setup originally) and it runs fine. thanks bob

[Carl Beck]

Hi Datfreak:

I like to think of the vacuum advance as a "load compensation advance" (as in the How To Modify book) - - that is employed at lower RPM's to advance the timing as more fuel is feed to the engine. when a demand or load is placed on it (you start to press on the gas pedal)... As the engine comes up to speed, the mechanical advance takes over to advance the timing as the vacuum advance backs off (it backs off because you aren't holding the throttle wide open once the desired speed is reached.)

What you really have to be concerned with is not only "Advance" that can be dialed in by the distributor and Total Advance numbers (distributor plus initial advance at crank) but the "Timing Curve" - that determines at what RPM the advance is brought in.

Swapping distributors from one model car to another, can result in a real performance loss - if the timing curve isn't the one needed by the car/engine receiving the distributor. (and in most cases the timing curves from the factory are intended to meet emissions standards, not to provide the best performance).

So you need to put the replacement distributor in the hands of a capable automotive ignition shop - who will put it on a Distributor Machine - to measure/record the entire timing curve in the distributor... ie how much advance takes place at each incremental increase in engine RPM. Then make adjustments to it - to dial in the timing curve you need for your engine.

While 36 to 38 degrees of total advance have been mentioned and the L series engines can stand them - every dyno tuned result I've seen so far would suggest that anything over 34/35 degrees of total advance results in a loss of power in the upper RPM range.

Initial timing is set at the crank, by twisting the distributor. Most street L series engines do well at about 12 to 14 degrees initial or static advance (BTDC) at 650/750 RPM (no vacuum advance hooked up).

In terms of total advance (initial plus mechanical advance in the distributor) then - you want to have the distributor mechanical advance start to come in around 1000/1200 RPM and advance 10 to 11 distributor degrees total by 2500/3000 RPM. (10 to 11 degrees in the distributor is equal to 20 to 22 degrees at the engine) So 14 degrees initial + 20 degrees from the distributor mechanical advance = 34 degrees of total advance.

A good automotive ignition shop will have various spring strengths and weights to use to dial in the timing curve on your distributor. They will also have the equipment necessary to measure the changes they are making...until they find the right combination of springs and weights.

Don't take any of the numbers used for discussion here - to be in any way absolute... just a starting point. You'll have to try them - and depending on the throttle response and ping/knock etc - you may have to adjust the timing curves for your engine accordingly.

On top of all that - we get back to the vacuum advance. Because it too works against a spring tension - it is possible to adjust the amount of advance it adds at different vacuum levels - by adjusting the strength of the spring. But here again, work with a good automotive ignition shop...

FWIW,

Carl B.