enginehistory.org

[bhazel]

Hello all,
I would like to understand manifold pressures a bit better. I am familiar with automotive manifold pressures, which are always measured in pounds per square inch. Aviation engines list pressures in inches of mercury.
Does anyone know how the 2 relate?
Thanks a million,
Ben

[kmccutcheon]

[bhazel]

OK.
If 29.92 inches of mercury (inHg) = 14.7 psi, then if I measured the manifold pressure on that motor 150 inHg in psi, I would see about 5 times atmospheric, or 73.5psi???
I am an auto shop teacher, and I often relate current auto technology ie 4 valves per cylinder, dual cams, fuel injection, turbochargers etc to the older motors where they were actually new stuff.
They all understand boost numbers in psi, and I'd love to be able to show them boost numbers that they can relate to from what I've read about Reno racers and other 60 year old motors.
Thanks,
Ben

[kchristensen]

I'm a shop teacher myself. I am, at present, teaching kids in Grades 6-7-8. I'm engendering interest and conversation with them this year by building a 1/2 scale Axial prop for a Fokker DR-1 and a replica Eindecker wing for the wall.

When I taught high school and explained that all IC engines operate by the same physics and that Honda didn't invent variable valve timing, dual overhead cams, pentroof combustion chambers, etc. I'm met with slack jaws. I go further to point out all those things were first invented around WW1, the amazement is palpable. I also liked pointing out that multi-stage compressors were commonplace in the 30s and 40s, when single stage forced induction isn't that common on cars, even today.

Cheez,

F=MA

[tmcdaid]

An acceptable approximation in many cases is that 1 psi is approximately equal to 2" Hg and (the old math teacher in me shudders). While Kim's numbers above are much more accurate, very roughly, standard sea level atmospheric pressure is about 15 psi or 30" Hg. So, remembering that in automotive use, "boost" general means manifold gauge pressure, or pressure in excess of atmospheric pressure while normal aircraft usage is to refer to absolute manifold pressure (MAP), you can do the 2:1 conversion, remembering to add or subtract atmospheric pressure as required. Examples:

My F-150 Lightning will develop 8 pounds (more correctly, psi) boost, which would be 16" boost or 46" (16 + 30) MAP.

In Kim's example of an aircraft engine developing 4" pressure at idle, that's about 2 pounds absolute pressure, or 15-2=13 psi of vacuum.

In Kim's racing engine example, 150" manifold pressure in a racing aircraft engine is about 75 psi absolute pressure, or 60 (75-15) psi boost.

I typed that pretty quickly. I think I did it all correctly, and hopefully that gives a feel for the comparison. You can do some easy (or lesss easy looking if you use more accurate numbers) formulas, but I have always preferred to understand the relationship and go through that way. If someone would like, I'll spend a few minutes and figure out a few formulas (there would really be one equation with some algebraic variations on it).

Cheers!
_________________
Tim McDaid

[szielinski]

Perhaps www.onlineconversion.com/pressure.htm can assist for the web-enabled young 'uns.

[jjuutinen]

I have a follow up question. Some charts and folks claim that the unit used by Germans , i.e. ata refers to "technical atmosspeher", i.e. 1 ata=1 bar. However, one Finnish guy who know aero engines, says that the ata refers to normal atmosphere, i.e. 1 ata=1.01325 bar. I´d agree with the latter. Anyone better informed?