High Altitude Timing - JeepForum.com
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post #1 of 2 Old 10-08-2008, 09:39 AM Thread Starter
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1972 FSJ Wagoneer 
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High Altitude Timing

I live at 8,000 ft and just purchased a 72 Wagoneer that lived at 3,000 ft. I was wondering what the general rule for high altitude timing was? I thought that for every 1,000 ft above sea level you need to go 1 degree BTDC. So for me that would be 13 BTDC, yes or no (5 degrees plus an additional 8 degrees for the altitude)?
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post #2 of 2 Old 10-09-2008, 07:29 PM
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2002 TJ Wrangler 
Join Date: Sep 2005
Location: Somewhere, MI
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This may sound a little nutty at first but bear with me...

Forget specs - get yourself a vacuum gauge and tap into a manifold vacuum signal at idle. With the distributor loose and vacuum gauge in hand, advance the timing until you obtain the maximum vacuum signal - there will a range of motion that you can continue turning the distributor where the vacuum reading will stay the same, then begin to fall off. Retard the timing to the point where you first achieved that maximum vacuum reading - i.e. you want to be advanced, but not too advanced, if you know what I mean. Lock the distributor down, reset the idle speed (it probably will be idling faster now) and you are done.

The theory behind this method of timing is this: An engine is basically a vacuum pump - by setting the timing to produce maximum vacuum, you are in essence maximizing the efficiency of the engine. This method also compensates for altitude as vacuum and engine efficiency is affected by barometric pressure. I timed my old 1976 F-150 this way while living in Colorado and the engine ran smoother, got better mileage and had more power to boot. The 70's were a tough time emissions-wise and the manufacturers often specified timing that was set deliberately retarded to keep emissions and combustion pressure/temperature in check. The irony is that an engine producing maximum vacuum and therefore operating as efficiently as it possibly can (given constraints such as engine wear, general state of tune, carburetor issues, etc) generally runs cleaner. My old truck could pass emissions standards for vehicles 10+ years newer - it ran that clean!

Good luck!

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