Now that we have snow on the ground here all I have to do is putter around with my bird getting it ready to ride. In Oct I installed an MSD 6AL ignition and coil. The 57 Dist. has petronix innards. My question is do I need to connect the vacuum advance with the MSD hooked up? It starts ok and seems to be fine without it so just wondering if I need to connect it.  Thanks

55Birdman  Hickory NC

Birdman-



The MSD box only multiples the spark right? It doesn't have much affect on timing I believe. So for a street driven car or truck the short answer is yes- for steady light throttle driving on the road or freeway, you will get much better fuel economy with the extra 15 to 20 crank degrees of advance provided by the vacuum canister. With a fairly standard 10-15 degrees initial setting plus the 20 to 25 degrees mechanical and another 15 to 20 degrees vacuum advance you will be in the neighborhood of about 50 degrees BTDC on the highway under light throttle. That's a good thing. . Where you will get disagreement is where to connect the vacuum advance to.



Depending on compression ratio, quench, carb jetting, pcv valve adding a controlled vacuum leak, weight of the vehicle, gearing, etc. you will have to experiment to find what your rig likes better- connecting directly to manifold vacuum which will give you lots of advance at idle and cruise which your engine may like a lot, and may ping less, but may cause a high quirky idle. Or connect to "ported" (no vacuum at idle- only when you crack the throttle) which may give some pinging under light to medium load, but may solve a slight bog when gently accelerating off the line that can happen when the vacuum can is under the influence of full manifold vacuum.



I would try both ways of connecting the vacuum advance- you will have to adjust your idle speed and the initial timing to properly accommodate both methods. If you can find an adjustable vacuum canister (they usually have a hexagonal shape to the outer part) you can really dial in the best settings. If your distributor won't accommodate and adjustable canister, you can ask the guy at NAPA to drag his big book of ignition parts out from under the counter and you can compare the specs for the various canisters. They all start pulling at different inches of Hg and all have different total advance specs too. Remember that they are listed in distributor degree- you have to double that to get crank degrees.



The vacuum canister generally only affects closed and light throttle engine response. At WOT you won't see any advance using the direct manifold vacuum way, and I believe with most carbs, that hold true for the ported method as well. I'll have to think about that last comment actually, but it's dinner time and I'm getting a little hungry.



Steve Bittman

My vote is to hook up the vacuum advance if simply for the fuel economy aspect that the additional ignition timing provides under cruising situations.  And be sure to put a set of lighter advance springs in the MSD distributor as it typically comes installed with the stiffest combination.  The various advance curve combinations are in both the enclosed instructions and on the outside of the box and reasonably easy to change.

And I’ll also suggest hooking the distributor up to the ported vacuum connection at the carb as connecting to a direct vacuum source makes for a rough idle, surging at low speed, and detonation at mid throttle without making major adjustments to the carburetor and distributor advance curves.  Direct and ported vacuum hookups at full throttle are both expected be a zero vacuum signal at the distributor.

Lorena, Texas (South of Waco)

I have a 57 distributor. Cant afford the MSD .

55Birdman  Hickory NC

You may already have an adjustable vacuum canister on your '57 distributor, and if you don't they are available, so your options re wide open. It takes a 3/32 allen wrench inserted into the nipple of the canister to adjust the deployment of the arm.

There are at least two styles of vacuum can that you find on these Y distributors. This is the one you can adjust with an allen wrench....on an old crusty one - use some WD-40 down the port to loosen things up. Otherwise you can strip the two or three threads in the thin adjustment nut. 

The older models have a thin hex nut out on the nose of the can - if you remove it, you will find shims that adjust the spring preload. More preload - less vacuum advance.

 
 Steve Metzger       Tucson, Arizona

Mine has shims in it.

55Birdman  Hickory NC

Ooops.  My mistake.  Misread what you had and thought you had the MSD distributor also.  But still the same thoughts regarding the distributor vacuum advance hookup.

Lorena, Texas (South of Waco)