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**Gregski** · 03-08-2020, 07:20 PM

now before we go any further please allow me to rant about Absolute Pressure since I had the hardest time switching from thinking in terms of vacuum to thinking in terms of pressure, one reason being that when I think of pressure I think of PSI as in I pump up my car tires to 35 PSI and off I go and not in Pascals, (I'm looking at you Blaise)

so if you are like me and are more confused than a blind goat on astro turf in terms of Pascals here's how I think of them

Imagine God is pushing down on your intake manifold, and the hardest she can push is 100 kPa or one atmosphere (technically that's how hard she's pushing on one square inch, but we're not even going to go there... ha ha)

so with your engine not running the pressure inside your intake manifold is equal to its surroundings. It's 100 kPa all around, on your truck seat, on your head, in the bed of your truck etc. that's just how hard the atmosphere is pressing down on us at all times, at sea level anyways.

so what happens when you start your engine, well in the old days thinking in terms of vacuum there was suction and perhaps we had 15 InHg read Inches of Mercury (Hg = Mercury, WTH?... "It comes from hydrargyrum, a Latinized form of the Greek word ὑδράργυρος" ah yes of course... Not! ha ha)

but now that all the Tuners think in Pressure not vacuum now what happens is...

At Idle - imagine when we fire up our engines a trap door opens underneath your intake manifold and now some of that 100 kPa being bushed down on the intake manifold falls down inside the engine, so we are essentially subtracting or taking away them Pascals so 100 kPa minus what ever fell through the trap door usually about 65 kPa or two thirds falls through... leaving us with only 35 kPa still in the intake at idle that is with the throttle blade almost all the way closed preventing more air from coming in

At WOT - now when we gun it and are in Wide Open Throttle WOT the throttle blade swings open allowing more of that air being pressed down on the intake manifold if you will to enter the intake, essentially returning the pressure inside it to almost nearly 100 kPa but not quite as the trap door is still open under it since the engine is running

confused? great! you're welcome

03-08-2020, 07:20 PM	#362
Gregski Post Whore Join Date: Aug 2014 Location: Sacramento, CA Posts: 10,809	Re: HP Tuners Tutorials - Absolute Pressure now before we go any further please allow me to rant about Absolute Pressure since I had the hardest time switching from thinking in terms of vacuum to thinking in terms of pressure, one reason being that when I think of pressure I think of PSI as in I pump up my car tires to 35 PSI and off I go and not in Pascals, (I'm looking at you Blaise) so if you are like me and are more confused than a blind goat on astro turf in terms of Pascals here's how I think of them Imagine God is pushing down on your intake manifold, and the hardest she can push is 100 kPa or one atmosphere (technically that's how hard she's pushing on one square inch, but we're not even going to go there... ha ha) so with your engine not running the pressure inside your intake manifold is equal to its surroundings. It's 100 kPa all around, on your truck seat, on your head, in the bed of your truck etc. that's just how hard the atmosphere is pressing down on us at all times, at sea level anyways. so what happens when you start your engine, well in the old days thinking in terms of vacuum there was suction and perhaps we had 15 InHg read Inches of Mercury (Hg = Mercury, WTH?... "It comes from hydrargyrum, a Latinized form of the Greek word ὑδράργυρος" ah yes of course... Not! ha ha) but now that all the Tuners think in Pressure not vacuum now what happens is... At Idle - imagine when we fire up our engines a trap door opens underneath your intake manifold and now some of that 100 kPa being bushed down on the intake manifold falls down inside the engine, so we are essentially subtracting or taking away them Pascals so 100 kPa minus what ever fell through the trap door usually about 65 kPa or two thirds falls through... leaving us with only 35 kPa still in the intake at idle that is with the throttle blade almost all the way closed preventing more air from coming in At WOT - now when we gun it and are in Wide Open Throttle WOT the throttle blade swings open allowing more of that air being pressed down on the intake manifold if you will to enter the intake, essentially returning the pressure inside it to almost nearly 100 kPa but not quite as the trap door is still open under it since the engine is running confused? great! you're welcome Attached Images Last edited by Gregski; 03-09-2020 at 12:49 PM.