Hey guys so I have been playing with timing since i have installed this new MSD 8383 in my 292. I have been experimenting with the initial timing from 12 to 16. Seems like so many different guys like different initial timing. I am also running full manifold vacuum advance. One thing i dont have my head wrapped around is how fast the distributor advance should be coming in. The attached msd charts show the different springs and their rates. I have the blue stop bushing installed which limits the dizzy advance at 21º . I also have the 2 blue springs installed. At 16º initial that puts me at 37º total. but with the vacuum advance i am not sure when it is dropping off and curious if its overlapping in the dizzy advance. If I should be getting all the advance in before 3000rpm then why on the msd chart does the different springs continue to advance way past 3000?

What is your engine combination?.. If it is another post I could look back. Your camshaft will have a lot to do with what type advance curve you should run.. Hooking your vacuum advance to manifold vacuum kind of skews the situation. You are basically adding probably 20 degrees or more at idle because the vacuum pot will be feeding the engine full advance. Your carb should regulate when the vacuum advance kicks in at cruise,not under power or at idle. Normally on a stock to modest build engines like 12-14 initial advance... No vacuum hooked up  and a total of 36 to 38, No vacuum hooked up..3000 rpm is a rule of thumb for when it is all in, but anywhere from 28 to 36 is fine.  You will never tell the difference in the seat of your pants.  ..

My engine is a 59 292 bored .060 with headers, manual T98. The isky cam is 301444 E-4, also Mummert intake.

RB thank you for the reply. When you say "Your carb should regulate when the vacuum advance kicks in at cruise, not under power or at idle."  
Would you mind elaborating on that a bit? After a couple of years of learning i am finally wrapping my head around total advance. Its the vacuum advance that i am still curious about.

I think He means using the ported Vacuum hookup on the Carb rather than full time Manifold Vacuum. The Position of the Throttle Plates regulates the Vacuum available to Distributor Advance... 

  

Plumb your vacuum gauge into the cabin so you can see it when driving. Backing up here a bit, might first try disconnecting and plugging the vacuum advance and taking a test drive or two. This is the way the mechanical or centrifugal advance is tuned anyway, it's always adjusted first.

Full time manifold to the vacuum advance is sometimes very useful for performance applications (big cams) driven on the street, as they often won't idle very well, and need a higher RPM etc. With lots of ignition advance the throttle plates might be able to be dialed back.

But if you like it there regardless of your application, it won't hurt anything. It's a very misunderstood subject. There isn't any load on the engine at idle. So 30° or more at idle wouldn't be unusual with it connected there. Keep in mind the ignition timing will revert to whatever the base initial timing is, as soon as you accelerate.

Vacuum advance is a load dependent mechanism. Mechanical advance is RPM based.

So a manifold connection isn't really going to affect the ignition advance when cruising on flat ground or, at wide open throttle or anything (much) in between etc. Just at idle. Engine will run a lot cooler at idle though, and in stop and go driving, this can be important depending where you live. It will not pass smog however, if that is a factor.

It can sometimes be difficult to get a steady idle RPM with a manifold connection to the vacuum advance. If engine idle manifold vacuum is low, or less than the # for full advance of the canister it may "flutter" and this interferes with the timing, which changes the idle RPM, etc.

In fact, if you read the old manuals from the 1940s, this is why the "ported" distributor vacuum advance connection was invented - that is, above the throttle plates - so as to ensure a steady idle.

Sometimes you'll hear it is an emissions or smog feature - it is true that retarding the ignition timing at idle reduces NOX, but it predates smog considerations by decades. I'm not sure when vacuum advance was invented, but the spark port was probably invented at the same time. Some makes used a manifold connection, some did not.

I tried several different combinations with the springs and advance bushings. To be honest I never really noticed much difference in performance. My engine has a decent sized cam in it (236@50) and I did notice it likes full manifold vacuum on the advance as opposed to ported. I'm now running a custom built distributor and the builder recommended full manifold vacuum also. My suggestion is to try both and see what works the best.

Durham Missouri

For most Y builds and using the MSD #8383 ready to run distributor, using the lite silver and blue springs in conjunction with the blue bushing proves to be the ideal combination for both performance and economy.  I use ported vacuum on most Y builds which eliminates issues with inconsistent ignition timing at idle speeds.

Lorena, Texas (South of Waco)

Awwsome thanks Ted I will try those springs. What do you run your initial timing at on a mild cam and headers? Also I just put on a 600cfm summit carb and I really think my engine likes it better than the 500 avs2 eddy. 

Because the idle speed when using any kind of aftermarket camshaft can vary, the initial timing is not going to be a specific value.  I'll recommended setting the timing with the engine at ~3000 rpms or where all the mechanical advance is in with the vacuum advance disconnected.  Setting the total mechanical ignition timing so it’s at 38° is a good starting point.  This may require you putting a 38° mark on your damper.   Once you set the total timing, the initial timing at idle is expected to fall in the 12-15° range with the vacuum disconnected.  If the carburetor is right and the vacuum is hooked up to a ported vacuum source and not a direct vacuum source, the ignition  timing at idle should be the same regardless if the vacuum is hooked up or not.

Lorena, Texas (South of Waco)