Thursday, November 23, 2006

VW - Altitude Adjustment

Due to the lower air density at higher altitudes your engine may run slightly rich and therefore cooler. The general rule of thumb is that you may advance your timing one degree for each thousand feet of altitude. And with that stated let me offer some personal advice. If you do not have a degree wheel, and are not running electronic ignition the chances are that your timing will wander a bit even under ideal circumstances, thanks to the plug gap, wear in the distributor shaft, rubbing block and so forth. If you intend to operate above 5,000 feet elevation for a prolonged period of time (i.e., a move to Denver, etc) then by all means, re-time your engine. Otherwise, leave it alone, unless it gives tells you in an obvious way that it would prefer a different setting, in which case crank in a degree or two . . . whatever the engine likes . . . to return its operation to harmony with your driving habits, load carried, type of gas and, for all I know, the color of your hair and eyes. The point is, the settings in the manuals refer to new engines operated under a set of fixed conditions. Reality is seldom so kind, yet the engine is always willing to let you know when it is happiest.

The symptoms of altitude sickness usually resemble retarded timing; lack of power, poor economy, possibly a bit of black smoke or start-up or when attempting to accelerate. However, if you are running a hot CDI or other all-electronic ignition module, the symptoms may be partially masked by the cleaner burn and longer spark duration provided by most CDI’s ( which are easily retro-fitted if you’re not presently using one. Coupled with platinum plugs, the combo virtually eliminates tune-ups for air-cooled veedubs.)

You may put aside any fears of high-altitude (i.e., low-pressure) vapor locking (as opposed to high temp vapor locks). Since about 1980 all automobile fuel sold in the United States has a vapor pressure that allows it to be used safely to about 12,000 at 100 degrees Fahrenheit (an unlikely combination).

For the past twelve years the Experimental Aircraft Association has been using mogas in aircraft, reporting the results to the FAA and the EAA members. Several certified aircraft engines are now permitted to use mogas, or ‘tractor’ gas as pilots tend to call it. Works fine, even in old Volkswagens; no problems with low-pressure vapor-locking.

(Ed. Note: This is NOT true if the gasoline contains alcohol. Not only does alcohol reduce the energy content of your fuel, it promotes vapor lock and carburetor icing. R.S.Hoover, 2006)

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