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Tonight I hooked up a vacuum gauge to the manifold on my stock '57 312 Fairlane and cruised around at various speeds from 30-70 mph, uphill and downhill, both with and without the vacuum advance hooked up. I wanted to see what difference the timing change would make before I started trying to recurve the distributor.
The distributor is stock and the mechanical curve is close to what the shop manual specifies but the vacuum advance is a bit more aggressive. According to what I have gleaned from this site it's far too much advance. The motor runs well but the fuel economy is pretty sorry (best average so far is 12.5 mpg) and the temperature creeps up to the upper limit of the "normal" bar at speeds of 60 mph and up. Of course, the mpg and temp issues could be caused by bad alignment due to a worn-out front suspension and/or faults in the cooling system, but seeing as the stock advance curve isn't optimized for today's fuel, I thought I'd start there.
What I found was that, in general, disconnecting the vacuum advance resulted in slightly lower vacuum numbers at low speeds (20-40 mph) and slightly higher vacuum readings at highway speeds (55-70 mph). Regardless of speed or presence/absence of vacuum advance, maintaining speed up a moderate grade put the vacuum around 10-13". Accelerating up a grade at highway speeds brought it under 10". Cruising vacuum at my best guess of a level-ground scenario was about 17-18" at most speeds, dropping to more like 15-17.5" at 60-70 mph. Downhills of course brought vacuum over 20" at any speed, if steep enough.
When driving, that needle moves around a lot more than I expected. It was hard to pin down a change with/without the vacuum advance. It seemed to me that, without the advance on the highway, I saw an overall average of about 16.5"Hg as opposed to an average of about 15"Hg with vac adv. At low speeds, average without vac adv was about 16" and with vac adv was closer to 17.5".
So, if manifold vacuum is a good measure of engine efficiency, it seems that the degree of variation in efficiency to be had by making significant changes to the total advance curve is not very large. Now, it's true that my test scenarios were both "extremes"; I'm sure there was too much advance in most instances with the vac adv hooked up and probably less advance than was desirable with it disconnected. Still, even with the optimum curve I wouldn't expect to see a blatantly obvious increase on the numbers I mentioned here. Improvement would be by small degrees moment-by-moment, but I suppose they add up.
For those who've driven with a gauge, do those readings sound consistent with your experience?
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