Timing advance for altitude

[Surveyorsteve]

Some time back I saw a post that discussed advancing the timing for altitude. Someone said what the degree of of change was for each click on the little gizmo on the distributor. I can't find it now and I'm at 8500 feet. Anyone recall what it is? (Might as well remind me what the little knob gizmo is called while you are at it)

Thanks

 

[Tancruiser]

I’m at 8500 feet also and advanced by 3 degrees so just put a light on it.

 

[Dizzy]

The knob is an 'octane selector.'

 

[Surveyorsteve]

Thanks for the name. As far as the put a light on it, I thought the octane selector was for the purpose of easily changing the timing when you change elevations and not have to get the timing light out when you go into the mountains on a hunting trip. Am I wrong?

 

[Dizzy]

At lower altitudes, the air pressure in the combustion chamber, and the external atmosphere is higher. The process of combustion will be faster with more pressure, so we might slightly advance static timing, or more conveniently thru the octane selector to account for slower combustion.

Spark timing that is too advanced will put excessive wear on an engine, and miss the ideal moment of crank rotation. Ideal timing produces the best situation for both driving torque (not too retarded) and compression braking (not too advanced). Cold air and fuel needs advanced spark timing. Running EGR requires significantly advanced spark timing. Higher octane and ethanol blends require more advanced spark timing. Higher engine compression ratios requires less spark advance, and, there is less compression for a given engine-build at higher altitudes.

The relatively big issue is that there isn't enough air to completely burn fuel at altitude per a given volume of air. So altitude is primarily a matter of opening up the throttle plates in the carb, and adjusting your jet sizes. They call it 'stoichiometric' when your air/fuel ratio is balanced enough to not significantly deposit carbon (and waste fuel) and not to damage your engine by burning up components with excessive oxygen.

 

[Surveyorsteve]

Thank you. That is good info. Understanding the process will certainly help in selection of options. I always wanted to change the jets but now I am moving to sea level so it makes sense to advance it a little for now until I move.