'Tis a black art for sure for the average guy.. I've been playing around with my little herald that runs manifold vacuum because of the SU carbs, total for that motor should be no more than about 30 degrees, the flathead V8 , 24 ( without vacuum ) all so different ..
I was hoping Ted might give us a clue as to ' best' timing. I recently read that ' racers' of the y blocks timed for about 38 degrees and all in at 3600 rpm.. But maybe that when you had good fuel and you could actually read sparkplugs??



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

The auto manufacturers have been struggling with the ignition advance curve for 120 years now and are just now getting a solid handle on it.  It’s tough for a single advance curve to cover all the different drivers and driving conditions even when the engines remain the same.

Full throttle timing attributes differ from cruise timing attributes.  This has to to do with the air/fuel mixture being in the neighborhood of 12.5:1 at full throttle and 14.5-14.7:1 under cruise conditions.  There is no standard ignition advance curve that can be applied to all situations as carburetion, intake manifolds, compression ratio, and camming all play into this.  The vacuum advance chamber does allow for the increase in timing under cruise situations and still allow the timing to backup under full throttle by lieu of the vacuum signal to the distributor dropping off when the manifold vacuum drops off.  But here are some basic guide lines.  As fuel mixtures get leaner, then additional ignition advance is required in the lower rpm bands.  The lower the compression ratio, then more total ignition timing is also required than what would be necessary on a higher compression engine.

As the camshaft lobe centerline is decreased, then additional initial ignition timing at low rpms helps with the idle characteristics.  This is simply due to inefficiencies taking place in the combustion chamber and the additional initial ignition advance simply helps to start the combustion burn earlier to help combat those ineffeciencies.  This in turn requires a shorter advance curve so that there is not too much ignition advance at full throttle and in the higher rpm ranges.  Because a street engine is operated under a variety of conditions, the combination of mechanical and vacuum advance helps to cover these.  Moderate acceleration has the vacuum advance dropping off which in turn results in less ignition advance seen by the engine.  Once the engine rpms are held in steady state (cruise), the air/fuel mixture goes leaner which is where the vacuum advance helps by adding additional degrees of ignition advance on top of what is already being supplied by the mechanical advance portion ofthe distributor.  Leaner mixtures are more difficult to start the burn cycle which is why the ignition must be advanced in which to compensate.

On the race engines, total timing is the single attribute I aim for and I simply want an initial timing that allows the engine to startup without ‘ignition locking’.  Cruising ignition timing attributes are not required and are therefore not taken into account.  On my roadster, total timing is set at 36° BTDC with the MSD electronics automatically retarding the timing 20° when the starter is engaged.  Once the starter button is released, timing is at 36° both at idle and at speed.  I do have the capability with the MSD to put some additional retard into the system with a switch if desired.  This would allow a 40-42° ignition advance off the line and if the switch is put on the shifter, then a 4 or 6° ignition retard could be made when the car goes to high gear. While I did that with my FE powered cars, I’m currently not doing that with the Y powered roadster but the capability is there if desired for a harder launch with the additional advance (42°) and still maintain a strong mph with the original timing (36°).

With all that being said, 10-12° intial and 36-38° total at 3500 rpms works well for many street Y’s.  As the camshafts get wilder, then 18-20° initial at idle may work better but this requires that the curve within the distributor be shortened so that the total advance without the vacuum advance is not excessive.  As a general rule, don’t exceed 10° of additional timing with the vacuum advance activated.

Lorena, Texas (South of Waco)

Thanks for that overview Ted.  Do you know why the 10 degree limit for vacuum advance is the rule of thumb?

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

Since you have such bad gas mileage and your vacuum reading are so low. Can you review a few things first?

Pete,The rear axle is 3.10:1.  At 60 mph, my tach reads right around 2500 rpm.  I will have to try the maximum advance thing tomorrow but right now at 9 degrees initial and about 550 rpm vacuum is 19.75".  My gauge is connected to true manifold vacuum (the line that goes to the vacuum booster on the fuel pump from the intake).  Also, when I coast downhill vacuum spikes up to about 24-25".  I don't think I've ever seen it go above 25.  The 16-18" on downhills I mentioned was for downhills shallow enough that it's still necessary to keep on the gas to maintain speed, like the sort you experience on the interstate in the piedmont, not the mountains.  There is quite a bit of engine braking, or perhaps drag of another kind; the car certainly decelerates a lot faster than my '93 Buick Century when you let off the gas.  I have always attributed that just to the 3.10 axle and 1:1 top gear.  Now I'm wondering if it's something else.  Compression is good, all cyls around 160 psi.  Not enough in the trunk to matter!  The tires are 225/70R14 on the stock rims though.  Tread is wider than the rim itself.  Perhaps that's a factor.

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

56 Roger,
Yes, I checked it when I had the heads off and more than once since, they're correct within 2-3 degrees.Pete,
I did as you asked about what timing at idle with the vac adv disconnected gave the highest vacuum.  Maximum vacuum at idling speed was about 21.5" and it occurred at about 28 degrees BTDC.  There was an obvious increase of vacuum and engine speed as I moved the timing up from 9 to about 22; from 22 to 28 the change was much less noticeable but still present.  Beyond 28 the vacuum was still strong but started to be a little less steady.

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

Just to be sure, I checked my timing marks again.  Stuck my pinky in the #1 spark plug hole and the marks are definitely still accurate.I did as you said and drove around with the initial set to 25 degrees.  I left the vacuum advance disconnected because the engine didn't like it.  Wow, what a difference!  Not only were the manifold vacuum readings obviously and consistently higher than before, but the engine felt much more lively, moreso than I would've guessed.  I floored it a couple times and there was a little spark knock when the secondaries opened but it felt like it was just past the threshold of knocking vs. not knocking.  Too early to tell whether fuel economy has improved but I expect it will as the average vacuum reading is up by at least 2 points.  Still appears to be moving through the gas pretty quickly though.So, I'm curious as to why my engine is wanting so much advance.  Given good, even compression, good ignition components set correctly, a stock, correctly jetted carburetor, no performance mods, etc, it just doesn't make sense to me.  True, the Fel-Pro permatorque head gaskets will have dropped the compression ratio a little.  I just have never heard of a stock, street-type engine needing that much advance.  Any ideas?

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

Are you using a dial back timing light?  I have found some of these to have the wrong scaling on the knob which in turn gives erroneous readings.  Beyond that, what kind of total timing numbers are you getting with the vacuum advance disconnected?  If the distributor has a very short ignition curve, then the engine would like a higher initial timing setting for overall driving.  Poor quality of fuel would also be another consideration.

Lorena, Texas (South of Waco)

Did I miss what sort of carby you had?  I don't think you have anything going on abnormal.. But you have a 57 , auto, its a big car, how much does one of those weight?



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

Ted, I'm not using a dial-back light.  I measured the scale on the damper and marked out an extended scale up to 50 degrees.  As for total timing, the most I specifically observed was 35 degrees mechanical.  That was with 9 initial.  I don't know the rpm I saw that at because the tach I was using to map advance points maxed out at 2000 rpm.  Once both springs came into play around 1200 rpm the advance rate was very flat, so I expect there was more advance left in the distributor, but how much exactly I don't know as I didn't want to overrev the engine.  In taking the initial up to 25, that would bring my total advance to 51.  It is an original distributor, not the service-replacement later style with the triangular shaft.  I seem to recall reading something about these dists. having a pretty high limit as far as advance range.  Poor quality fuel is a possibility.  All the pumps in central NC "may contain up to 10% ethanol."  I prefer to use Shell or Exxon though I will fill up with others if convenience dictates.
Roger, no offense taken, sometimes with things like this it's hard to be satisfied unless one runs the tests oneself.  Especially when the results are wonky.  I think that rather than sending the unit off to be calibrated on a machine, I'm just going to calibrate it on the vehicle.  Apparently my engine has "special needs," doubt I could anticipate them well enough to get the curve right without road testing.Talkwrench, it's a 1957 Carter AFB with vacuum secondaries.  Correct model number for 1957 312 Fairlane.  I even verified all the jets and rods and such with a '57 Carter data sheet.  Car should weigh about 3500 lbs according to specs I've found.  Nothing too extreme. 

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