Fuel Question
 

Does anyone know the mix ratio for octane? How much 114 octane fuel do I need to add to 93 octane to get 100 octane?

I don't know what you have in the engine,but I do remember when 100 octane was available at the pump and boy, did we sell lots of valve jobs.

Well, I have the original 390, 10.5:1 compression ratio, 335 HP in it which called for 100 octane way back. Now it's bored 0.030" over and has a 10.76:1 compression ratio. It's running really well right now on 110 octane, but all the "above" 93 octane I can get is unleaded, and the 110 octane where I live is $6.80 per gallon. I've installed harden seats on the exhaust side to run unleaded fuel, but would like to "ween" it off the 110.

Octane Rating
 

Octane Rating is kind of a nebulous thing, based on a perceived threshold at which "ping", or self-induced pre-ignition begins to occur. So, if a given engine has the same tendency, with a given fuel, to begin to "ping" under given operating conditions, as it does when fed pure octane as fuel, the fuel in question is given an "octane rating" of 100, the standard number associated with octane as fuel.

For a given fuel, if an engine begins to ping under the controlled conditions as it does when "fed" a fuel mix containing 85% octane, the given fuel is rated at "85 Octane".

I believe the "burnt valve" concern for older engines using unleaded fuel was attributed to the loss of "lubricating effect" which tetra-ethyl lead provided as it "plated out" on the valve and seat surfaces along with carbon particles. The actual Octane Rating was relatively inconsequential, as "ping", when it becomes detonation, becomes destructive to pistons as well as valves, due to excessively high temperatures generated, regardless of octane level. Generally, the higher the octane rating, the SLOWER the burn-rate of the fuel, contrary to the gut-feel that "super" must burn like hell! Just the opposite. Ethanol, for example, when added to gasoline, improves it's octane rating, as ethanol burns more slowly than gasoline. Ethanol introduces problems with carburetors, however; it don't vaporize very good!

My '66 Mercury with 410 engine (a little-used spin-off of 390) ran pretty well on unleaded, 87 octane, didn't ping audibly, and accumulated many thousands of trouble-free miles. I do not recall it's compression ratio, sorry. Bear in mind, however, that today's electronic controls are allowing use of high compression ratios pretty successfully.
Edit: Reason for this is we didn't used-to have available to us, a means of continuously varying spark advance, thereby allowing highest possible advance for ALL driving conditions, while stopping short of pre-ignition, as we have today.


Still think it's fun? imp :(

These old FE engines really required one great thing when being rebuilt or overhauled to stop this problem: and that is hardend valves (stainless steel) and good seats. Once these were installed (usually during any valve job done, but they're still out there), you could drop the timing down a bit (which kills power), but the engine can't detonate for long without piston damage.
I WISH we had some of the 'old gas' still around for these cars...

Agreed! Wholeheartedly! imp