The Stovebolt Site Forums The Shop Area Engine Shop Vacuum and altitude

I just checked the vacuum on my rebuilt 261 and fiddled with the timing, and got only a mas of 15 inches. I thought it was due to the 3/4 cam or dual carbs, or leaky gaskets or bad valves.

However, after checking all that I talked to a friend and he said that engines produce less vacuum at altitude. Where I live is about 5,500 feet above sea level. My friend said its about 1 inch per 1,000 feet. He said 15 inches is good here and it would be 20-21 at sea level.

Has anyone else heard this?

NM51,

Yes, two "rules of thumb" might affect your vacuum: ". . engines with performance camshafts tend to read lower vacuum. Readings are also lower at higher altitudes; the rule of thumb is approximately 1 inch of mercury for every 1,000 feet of altitude gain."

Tim

heres a way to look at it.. vacuum equals no air.. altitude equals less air... sooooo ya go up to about a bazillion feet altitude, no air, hence a vacuum..... altitude also affects boiling point.. at albqqqqq altitude of 5500 ya say,, id guess the boiling point of water is about 200,,, oh.,,,ps... ya also need to change your timing with alititude.. run more advanced with more alititude if i remember right....
i never time with a light or to specs anway... just run it as far advanced as ya can without pinging...or starting problems when hot...