Distributor question

[tcabece427]

Hopefully I can answer your question. First off you need to understand that gasoline "burns", it does not explode (not nitromethane). That means it takes a few milliseconds for the gasoline to reach full burn. This is the reason most engines are set to fire a few degrees Before Top Dead Center (BTDC), typically 4 - 6 degress. When you set you initial timing the engine is turning over at idle RPM fairly slow (and warmed up). With mechanical advance what you set it at is what you set it at. However when you're using vacuum advance then you attach it, after initially setting your timing. It advances it a little more typically from 4 - 6 to something like 8 - 10.
Another thing about gasoline is the different grades. Higher octane gasoline not only has more power but burns faster. Requiring less timing advance. Now today's gasoline there isn't a lot of difference in octane, usually 85 - 92. Vacuum advance works better for lower octane gasoline. Vacuum advance also "buffers" the advance based on load. If you open the throttle wide open your vacuum drops to 0 therefore your advance drops to the initial 4 - 6 degress BTDC. But when your engine is turning over faster than idle you actually need more degrees BTDC because you piston is moving faster but gasoline still burns at the same rate. Mechanical advances more (up to a point based on springs and weights) based on RPM.
From a strictly street driving and this means typical commuter, running to the grocery store driving. Vacuum advance makes more sense from a fuel economy and warranty perspective. But Cobra's and specifically 427's are not about fuel economy or warranty. Their more about fun driving and occasionally getting your foot into it. Certainly for all out racing Mechanical advancers are a requirement. Additionally with high lift overlapping CAMs these typically affect vacuum. Using a Mechanical advance probably makes sense based on your CAM more than race driving vs. commuter driving.