Manifold pressure
Thread Starter
TECH Veteran
iTrader: (53)
Joined: Feb 2004
Posts: 4,433
Likes: 0
From: Monticello, Kentucky
Manifold pressure Anyone know right off hand what our cars' manifold pressure is in KPA at idle? Normal driving/highway speeds? WOT? Just wanna get an idea before I hook it up to the log this weekend.
TECH Apprentice
Joined: Feb 2005
Posts: 384
Likes: 0
I'm more used to inches of mercury ("Hg) as opposed to kilopascals (kPa), but you can easily convert between the two. Mercury readings are about 29.53% of equivalent kPa readings.
Readings will change depending on the car. A stock car will show more 'vacuum' than a car with a massive lopey camshaft.
With the car off, the MAP sensor reads the barometric pressure within the manifold. This will change depending on your altitude. Here in Kansas City, my barometric readings are typically around 29.5"Hg. [29.5 / .2953] = 99.9 kPa
With the car running, again, it really depends on your setup. On a stock car, the idle MAP readings might be around 10"Hg (33.86kPa). A highly modified car might see 20"Hg (67.73kPa) at idle.
'Vacuum' is determined by subtracting the inital MAP reading (BAR) with the engine off -- with whatever the engine is seeing at idle. For instance, if I had a BAR reading of 29.5"Hg with the car off, and 15"Hg with the engine running at idle, then my car is theoretically producing 14.5"Hg vacuum [=29.5 - 15]. Or, ~49.1kPa. You could double-check this with a mechnical vacuum gauge hooked up to one of the vacuum ports on the intake.
As you approach WOT from idle, the MAP readings will slowly increase -- until it hits that original BAR reading that you got with the car off. For instance, even if you had 8"Hg at idle (high vacuum), that number should steadily increase back up to 29.5"Hg when you go WOT (no vacuum) -- at least here in Kansas City.
That help? I probably screwed up the math somewhere...
Readings will change depending on the car. A stock car will show more 'vacuum' than a car with a massive lopey camshaft.
With the car off, the MAP sensor reads the barometric pressure within the manifold. This will change depending on your altitude. Here in Kansas City, my barometric readings are typically around 29.5"Hg. [29.5 / .2953] = 99.9 kPa
With the car running, again, it really depends on your setup. On a stock car, the idle MAP readings might be around 10"Hg (33.86kPa). A highly modified car might see 20"Hg (67.73kPa) at idle.
'Vacuum' is determined by subtracting the inital MAP reading (BAR) with the engine off -- with whatever the engine is seeing at idle. For instance, if I had a BAR reading of 29.5"Hg with the car off, and 15"Hg with the engine running at idle, then my car is theoretically producing 14.5"Hg vacuum [=29.5 - 15]. Or, ~49.1kPa. You could double-check this with a mechnical vacuum gauge hooked up to one of the vacuum ports on the intake.
As you approach WOT from idle, the MAP readings will slowly increase -- until it hits that original BAR reading that you got with the car off. For instance, even if you had 8"Hg at idle (high vacuum), that number should steadily increase back up to 29.5"Hg when you go WOT (no vacuum) -- at least here in Kansas City.
That help? I probably screwed up the math somewhere...
Last edited by Alex94TAGT; Apr 27, 2005 at 04:31 PM.
TECH Apprentice
Joined: Feb 2005
Posts: 384
Likes: 0
For clarification, if you're wondering how I came up with that conversion:
I looked up that 760 mm Hg = 101.325 kPa.
[760 mmHg / 25.4] = 29.921 inches Hg
[29.921"Hg / 101.325kPa] = ~.2952998 x 100% = 29.53%
Thus, "Hg readings are roughly 29.53% that of kPa readings. You can divide "Hg by .2953 to convert to kPa, Or you can multiply kPa by .2953 to convert to "Hg.
I looked up that 760 mm Hg = 101.325 kPa.
[760 mmHg / 25.4] = 29.921 inches Hg
[29.921"Hg / 101.325kPa] = ~.2952998 x 100% = 29.53%
Thus, "Hg readings are roughly 29.53% that of kPa readings. You can divide "Hg by .2953 to convert to kPa, Or you can multiply kPa by .2953 to convert to "Hg.
Last edited by Alex94TAGT; Apr 27, 2005 at 04:25 PM.
