Reply to "spark timing"

The mark makes it easier to visualize & adjust the ignition timing for static/initial timing of 6 degrees BTDC when you're using a timing light to set the distributor. That was the timing for the Australian 351C, and may be the timing De Tomaso used for the 351W engines too. Those were the engines installed in the GT5-S. 6 degrees BTDC should be used with a distributor set-up to use the advance plate marked 15L.

The ignition settings of the factory engines have nothing to do with the engines compression ratio or the camshaft. Its a setting based on tailpipe emissions. All 351 Clevelands can use more initial advance than what the factory specified. I've set-up 1973-1974 Q code motors for 20° initial advance and more. The 351C LIKES lots of advance at idle. The real limitation is the effort required to crank-over the engine when it is hot. The higher the compression ratio, the harder it is to crank the engine when it is hot. I've set engines for optimum advance at idle only to find afterwards they wouldn't crank-start. DOH

As the engine's static advance is increased you have to close the carburetor butterflies more to lower the idle speed back to the proper rpm. This lessens the engine's propensity to run-on after the key is turned off. The intake manifold vacuum increases, thus the fuel/air mixture at idle becomes richer. Next you have to screw-in the idle jets to optimize them again. The idle improves. The engine's low rpm drivability, torque and throttle response improve. The engine feels peppier.

The best/classic ignition calibration for a 351C having OEM iron heads & flat top pistons is:

16° to 18° initial advance
20° centrifugal advance, all in by about 2800 rpm.
This equals 36° to 38° total advance
10° vacuum advance, connected to ported vacuum.
Advance at idle shall be 16° to 18° due to the initial advance setting, there is no vacuum advance at idle with ported vacuum.

This requires setting-up the Ford distributor to use the 10L centrifugal advance plate.

In general, even a high compression engine can be cranked-over when it is hot if the initial ignition advance is limited to 16° to 18° BTDC. That's why 16° to 18° initial plus 20° centrifugal is the "classic Ford" timing curve.

At the bottom of the page is a pictorial aid for timing the ignition based on which advance plate is being employed in the distributor. All of these settings provide 36° advance at WOT (WFO). Three of them provide 16° advance at idle.

If you have questions I'll be glad to try and answer them.