Distributor Advance

[ew1usnr]

My 260 V-8 with an automatic is specified to have a base advance of 8 degrees.
A mechanical engineer buddy of mine at work says that he has his 1967 Fairlane with a 302 and an automatic set at 14 degrees advance.

I tried my new timing light last week and found that the engine was set to 10 of advance.
When I advanced the timing to 14 and the idle speed increased from 450 to 550 RPM. That seemed to indicate that the engine was running more efficiently, so I left the timing at 14.
I then deliberately drove the car in stop and go traffic while the air temperature was 91 degrees and the engine did not seem to run any hotter than it had previously. The temperature gauge stayed at mid-range and there was no problem. The engine did not ping when it was accelerated from 0 to 65 mph or when the car was driven on the Interstate at 80 mph. The car starts instantly, idles smoothly and quietly, and there did not seem to be any negatives from having advanced the timing. The car felt like it had more power while zooming up the interstate on-ramp, but that might have just been my imagination. I will see if advancing the timing has improved the gas mileage. That would be nice.

Then I wondered how much total advance the engine was getting at higher speeds.

Background: The previous owner had "hot rodded" my 260 with an aluminum intake manifold, a four-barrel carburetor, and some other stuff.
I put everything back to stock when the engine was rebuilt four years ago. I assumed that the old distributor was good, so I re-installed it on the rebuilt motor.

The stock distributor is supposed to have 14 degrees of mechanical advance maximum.

Stock Distributor.jpg

Present: I bought a set of vacuum caps ($2.99) so that I could seal off the vacuum advance to find how much mechanical advance the distributor was providing:

Dorman AutoGrade Vinyl Color Vacuum Cap Assortment .99.jpg

Yesterday evening I sealed off the vacuum advance and had my neighbor rev the engine while I looked at it with a timing light.

At 550 RPM idle the advance was 14.
At 1000 RPM the advance was 16.
At 2000 RPM the advance was 32.
At 2500 RPM the advance was 36.

36 - 14 initial = 22 degrees of mechanical advance. That is greater than the 14 degrees maximum that the stock distributor is supposed to have.
I think that the previous owner did something with this distributor. I found this quote that may explain it: "Stock Ford distributors have two slots. There is a pin that sits in one of the slots to limit travel. To change slots you simply remove the armature assembly turn it 180 degrees and reinstall it so the pin is in the other slot. The slots are numbered as to how many distributor degrees it will pull in, double it to find the amount of crank degrees. So if you want 24° mechanical advance you need to find an armature assembly with a slot marked 12L." From "Distributor advance": http://www.gofastforless.com/ignition/advance.htm The previous owner may have done this to double the mechanical advance from 12 to 24.

Not that this is necessarily a bad thing. In "How to Set Your Timing for Peak Performance", See: http://outintheshop.com/faq/Lars%20timing%20.pdf the author says ""Small block Chevys (and most other performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) and is set to 36 - 38 degrees at 2,500 - 2,800 rpm. The 36-degree 2500 rpm advance curve is optimum for performance (but retard the timing if there is pinging).”

My engine is spot on with this advice and it has 36 degrees advance at 2500 RPM. My little 260 is "tuned for peak performance" with the same timing as a vintage Corvette. It does not ping or run hot, it idles steady, starts easy, and runs great. Yaaay!!

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