Just wanted to post a final follow-up.  I am finished tuning my advance curve and am pretty pleased with the results.

I bought the Mr. Gasket kit 925d.  It contained two identical light springs, a bit lighter than the primary spring I was using to begin with.  I ended up using one of these new springs in the primary position and the original secondary spring.  Any other combo brought all the advance in too early, resulting in spark knock.

I have my vacuum advance limited to 6 degrees.  Even that much is enough to produce some irregularity revving the engine in park in the 950-1400 rpm range, but it is better than it used to be and I notice no negative side effects driving.  The car is quite responsive even without the vacuum advance, and WOT does not incur any spark knock.  It's still not what I'd call a fast car, but it's faster than it was, which is nice when you need it.  I don't know how fuel economy has been affected yet.

Anyway, I'm gonna take it as it is!  Thanks once more for all the help and wisdom along the way.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

Inside my distributor I have 18L and 21L slots.  I cut the end off of a small hose and fit it snugly over the limiting tab in the 18L slot, and based on my test results it gives 28* of crank advance. With the springs that came with the distributor, the primary to secondary spring transition appears to occur around 1500 rpm, perhaps a touch earlier, at 22* mechanical + initial.  The combined weight of the springs brings total advance of 38* (crank) at around 3600 rpm.

With the distributor set as is, vacuum advance disconnected, it gets unhappy around 1500 rpm but clears up as engine speed increases.

When attempting to set the mechanical side, will road tests with vacuum advance plugged show me whether I'm getting closer to where I want?  I mean, should I make the car run and drive the best I can with the vacuum advance capped off first, then deal with the vacuum side when that's done?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

STEVE, again your knowledge of all things electrical was explained so well in this quote, very interesting and informative. Best regards bill.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

Here's another question that I haven't found any info on the internet about:  at what point (engine speed and advance) should the mechanical curve switch from relying solely on the low-speed spring to both the low and high-speed spring? 

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

I believe you will find that the optimum mechanical advance limit for a pretty much unmodifed y-block is going to be around the 36°-38° mentioned earlier. More efficient higher compression engines will use less - less efficient engines might use more. This value applies to an engine under load - high throttle opening, low to no vacuum - passing uphill etc. That is the maximum we are talking about - the upper advance limit when the engine is heavily loaded. Even a "gas economy" rig has occasions where full power has to be available - without injuring the mechanical parts. 

The "early" advance curves we have described are used to take advantage of the additional torque available - low in the rpm band - when the spark advance is optimized toward building maximum cylinder pressure. It takes about one millisecond to light the fire - as the revs go up the spark initiation point has to get further and further ahead - but you are also moving toward less and less compression of the mixture (away from top dead center on the front end, the piston is rising up the bore). A limit is reached where "enough is enough" - and you can't get any more torque advantage out of the quicker spark curve. The old "Y" cylinder head combustion chamber design is not the best - flame travel isn't like a GM "fastburn" by any means - its 50 year old technology.

This low end torque advantage - you will feel it in the seat of your pants - and it could very well increase your fuel economy because the engine will accelerate the vehicle without as much throttle opening. Thunderbirds like mine are pre-programed from the factory to take off in second gear of the Ford-O-Matic (low is basically a manual selection in ordinary driving). Your two speed might have a better low than I've got - in second. One of these T-birds in poor tune stumbles away from a traffic light trying to transition from idle under heavy load - it's tuned right "or else". When I changed from the "Load-O-Matic" to a '59 distributor (using a manifold vacuum source) - it was like taking 500 pounds out of the trunk. Uses way less fuel too.

Vacuum advance has the effect of leaning out the engine when vacuum is high - low load, slight throttle opening - no call for power. Even in this state - there is a limit to how effective additional advance might be. If the engine has been fitted with a more optimal mechanical advance curve (a higher number of degrees at a slower speed) then less vacuum advance is permissible - to keep the total within bounds. This leaning effect gives better economy - and when the throttle opens a little or a lot you are going back down toward the mechanical limit. About the only way you can get the maximum total spark advance in most vehicles - everything summed (initial+centrifugal+vacuum) - is a high speed downhill in the mountains - with the throttle closed - using the engine as brake torque. You aren't making power in that case - you are using it.

Hope this helps out a bit with the "logic" of the systems as they are applied to our street engines.

 
 Steve Metzger       Tucson, Arizona

I have been reading up on advance curve setting, both in this forum and elsewhere.  Most of the info is for maximum performance, though I am more interested in economy. 

Anyway, I was wondering, why do we want all of our mechanical advance to come in by 3500 rpm?  If one desired to take the engine on up to 4 or 5k rpm, wouldn't the optimum spark time at those speeds be earlier than that of 3500 rpm?

Is there a maximum limit to advance after which more is detrimental, even as engine speeds continue to increase?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

The advance canister shown below (disassembled) has two items that can be adjusted.

After you unscrew the hex cover from the can -there are shims that preload the spring. Adding or removing shims changes the advance travel position versus vacuum applied. If the spring preload gets up to a point where the vacuum can no longer overcome the spring sufficiently to reach the mechanical stop - then it would also have the effect of limiting the total vacuum advance available.

The fiber tube is factory sized to limit the travel of the unit at a particular amount. By installing a longer tube for instance, the total travel would be limited. Both of these items are subject to experimentation.

To run "hard lines" on my outfit, I used brake tubing formed to fit on the manifold - roughly run like original. Its attached to the carb and the distributor with rubber tubing at both ends. At the distributor end - I inserted an electrical "butt splice" (red plastic cover) - and crimped it until the system behaved the way I wanted it to. Others have used a solid piece of brass/plastic/steel and drilled holes to get it to behave properly - try a 1/16 hole and see what that does for you. When mine was working to suit me - I pushed the rubber over the butt splice enough to make it invisible.

 
 Steve Metzger       Tucson, Arizona

Looks like I have the spacer type vac advance, so how do these adjust??

Greenbird.. CAn you give us the rundown on the orifice ...? How what and where...



"Came too close to dying to stop living now!"

Actually the vacuum advance is important for low end performance and fuel economy.You would only run full centrifugal in a race car OR a street vehicle with extremely radical cam.

  

Its a 59 dizzy. off the top of my head a V8 normally runs a total of 36* total mechanical advance, so I should be able to run it without the vac advance ok anyway.. HAd to do that to my Cuda as MOPAR vac advance are crap.. goes ok.. Look forward to more on the topic and sorting it out.



"Came too close to dying to stop living now!"