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timing advance

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tahoe healey

tahoe healey

Yoda
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I was speaking with an MGB owner up here (6500 ft)who advances the timing by 5 to 10 degrees beyond the recomendation amount. He feels it runs better this way. He changes it back when at lower altitiude. I tried 5 degrees more than the 15 BTDC (total of 20). It did seem smoother and maybe quieter but I put it back becouse I cannot justify it to my thinking. I do adjust for richness as I change altitude but why the timing? /ubbthreads/images/graemlins/confused.gif
 
Since air density decreases about 3.4% per 1000 ft elevation increase, the mixture drawn into a cylinder & compressed into the combustion chamber, would be about 20% less dense at your altitude.

A less densely compressed mixture burns more slowly & so needs more timing advance to burn at it's optimum time in the engine power stroke.

For the same reason, a compression ratio of 9 to 1 at sea level would be equivalent to 7.2 to 1 at 6000 ft elevation.

I think you already know that optimum fuel mixture would also be 20% leaner at 6000 ft. than at sea level.

I wouldn't push it too far, but 5 degrees more advance should help & maybe 10 would be ok. Just don't forget to set the timing back when going to lower altitude.
D
 
Dave is correct. On the Alpine 500 up in Tahoe I set 5 degrees advanced to reduce backfire (mostly noticable when holding back via compression) on the long downhill runs. Once back down I changed back. If everything is still working the adjustment is available on the distributor advance knurled nut.
 
Thanks very much. Now I can sleep well knowing it is safe to play with that up here. They have special high altitude baking instructions but they don't work and no one uses them up here. Maybe cars should come with them.
 
Just keep track of where you were when you started adjusting (both altitude and adjustments) so you can get back to the starting point. As I get older and my part-timers gets worse I tend to forget things. You know like, did I adjust that clockwise or counter clockwise. heheheheh
 
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Not many cars need baking. Maybe high altitude is different?
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---- /ubbthreads/images/graemlins/iagree.gif Most of them cook their own goose.---Keoke- /ubbthreads/images/graemlins/grin.gif
 
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