Timing is everything - or is it?

[Dave Van]

c-2 1966 300 hp. I had to change the tach drive distributer shaft and the new one came with a different shaped top where the original weights would not work properly. So I found an old GM dist. and took the weights of of that and it worked just fine. Or so I thought. I ran the engine, set the initial timing at 8 degrees. With the vac adv disconnected I took readings as follows: 1000 rpm - 8 degrees, 1500 rpm - 16 degrees, 2000 rpm - 25 degrees, 2500 rpm - 31 degrees. That is where it topped out. I then plugged in the vac adv and it jumped to 50 degrees @ 2500 rpm. Is this too much advance? I know the performance books say 38 degrees max but they don't usually want any vac adv. What I got out of the manual is: Ingition timing = 6 degrees. 0 degrees @ 900 rpm, 15 degrees @ 1500 rpm, 26 degrees @ 4100 rpm.(which means 6 + 26 = 32 degrees) Vac adv = 15 degrees max.(Which now means 32 degrees + 15 degrees = 47 degrees)???? Where do I go from here?

[John H.]

Re: Timing is everything - or is it?

Dave -

Sounds fine to me - shows 23 degrees in the distributor, which is about average, and you want it to top out between 2500-2800 (which it does); I'd crank the distributor a skosh to get 34 degrees topped-out (3 more degrees) and see how it runs. "Total timing" is initial + centrifugal; vacuum doesn't count for total timing, just adds 15 degrees or so at high vacuum (light load) for improved throttle response, idle cooling, and cruise fuel economy. If you had a timing light hooked up where you could see the balancer cruising down the highway, you'd see about 50 degrees advance - perfectly normal.

Lean mixtures (idle and cruise) take longer to burn, so they need more advance to "light the fire" earlier so maximum cylinder pressure occurs just past TDC for maximum efficiency; when you accelerate, mixture goes rich instantly (accelerator pump and power valve), and rich mixtures burn faster, so as the vacuum drops with increased load, the vacuum advance drops out, retarding the spark from its cruise position, to keep the point of maximum cylinder pressure in the same place just after TDC. Centrifugal advance is only rpm-sensitive; vacuum advance adds sensitivity to engine load conditions, which is more important for a street-driven car.

[Duke W.]

Re: Timing is everything - or is it?

For every engine operating condition expressed as manifold vacuum and RPM there is an ignition timing that yields peak thermal efficiency. Maybe you've seen an isometric view of a "timing map" with timing - the vertical axis plotted against load - expressed as manifold vacuum and RPM. It's a three dimensional contour - looks like a mountain.

At light load the mixture is lean and relatively low density. As a result, flame propagation speed is slower, so more ignition timing is needed for peak efficiency. Idle condition flame propagation speed is slow due to the low density and exhaust gas dilution. That's why engines have vacuum advance. The 36-38 optimal advance for a SB is for WOT operation, which means vacuum advance is zero.

Timing maps changed a lot in the early emission era because peak efficiency does not correspond to lowest emissions, so timing maps were changed in favor of emissions and fuel ecomony went down.

The mid-sixties engine's timing maps were about ideal and about as good as you can get with the simple centrifugal and vacuum systems used back then. Nowadays, electronically controlled engines adjust the timing 50-100 times per second based on speed and load from sensor data including knock sensors.

My '63 L-76 has '64 L-76 distributor specs which brings in all 24 degrees of centrifugal at 2350. Combine this with 12-14 degrees of initial and 16 degrees of vacuum and it runs 52-54 degrees of lead at freeway cruising speeds. With the 3.08 axle it gets 22 MPG, tops out at over 150, and never runs hot.

From your measurements it sounds like someone has installed some light centrifugal springs and a vacuum can with more advance. As long as it doesn't ping at WOT or buck or stumble at part throttle, it's probably okay.

Duke

Duke

[Dave Van]

Re: Timing is everything - or is it?

To Duke and John,
Thank you very much for you response to my timing questions. I printed both for my brother to read, he is stuck on 38Degrees max and he had me questioning my own best judgement. On the other hand my brother-in-law told me that Duke would know the answer to my question. (He's always right!) Thanks again.
Respectfully
Dave Van