The Ford Barn

Old182 · 04-01-2015, 05:06 PM

FWIW: I run a vacuum gauge in my coupe and at idle it reads about 15"Hg with the spark fully retarded and about 17"Hg lever down (it dithers a bit). There are a few hills around here that my car doesn't like and it tends to barf coolant out the radiator cap near the top of one of them at full throttle and makes a mess. With the vacuum gauge, I can run slightly less than full throttle (about 2"Hg) and keep the coolant where it's supposed to be. Keep in mind that just because your gas pedal is not touching the floor doesn't mean you're not at full throttle - but the vacuum gauge tells you where the throttle is. Also, if you know your vacuum and you have some idea of the barometer, you can easily calculate manifold pressure, an indication of how hard you're running your engine. Barometer + the vacuum reading = manifold pressure, e.g. 30.0 Baro + -10.0 Vac = 20.0 MP. With all due respect to the folks on this forum who are much smarter than I, I was taught in 1960 to start the engine with the lever up and pull it down when the engine fired and this engine has been run that way ever since with no problems. Model As are very robust and forgiving machines. But I do run the lever up a little at high vacuum readings (low loads) and slow speeds. I like having vacuum data while driving but I don't know why anyone would time a Model A engine any other way than Henry's and I couldn't tune an engine with a vacuum gauge if my life depended on it.

04-01-2015, 05:06 PM	#13
Old182 Senior Member Join Date: May 2010 Location: Newburgh, NY Posts: 222	Re: Timing and vacuum FWIW: I run a vacuum gauge in my coupe and at idle it reads about 15"Hg with the spark fully retarded and about 17"Hg lever down (it dithers a bit). There are a few hills around here that my car doesn't like and it tends to barf coolant out the radiator cap near the top of one of them at full throttle and makes a mess. With the vacuum gauge, I can run slightly less than full throttle (about 2"Hg) and keep the coolant where it's supposed to be. Keep in mind that just because your gas pedal is not touching the floor doesn't mean you're not at full throttle - but the vacuum gauge tells you where the throttle is. Also, if you know your vacuum and you have some idea of the barometer, you can easily calculate manifold pressure, an indication of how hard you're running your engine. Barometer + the vacuum reading = manifold pressure, e.g. 30.0 Baro + -10.0 Vac = 20.0 MP. With all due respect to the folks on this forum who are much smarter than I, I was taught in 1960 to start the engine with the lever up and pull it down when the engine fired and this engine has been run that way ever since with no problems. Model As are very robust and forgiving machines. But I do run the lever up a little at high vacuum readings (low loads) and slow speeds. I like having vacuum data while driving but I don't know why anyone would time a Model A engine any other way than Henry's and I couldn't tune an engine with a vacuum gauge if my life depended on it.