Manifold Absolute Pressure Vs. 'Boost' & More
Posted by Dave Veal on
Hey everyone! Dave here with a knowledge drop about Boost Pressure Vs Manifold Absolute Pressure, some unit conversions, and atmospheric data!
Many of our customers have seen their "boost gauge" not match what is displayed on their AccessPort or in their Datalogs and have been concerned about seeing 38+psi" on the display.
Today we're going to discuss why that is, and why you shouldn't worry about it.
Your vehicle ECU uses absolute pressure for just about everything related to boost control, while the Cobb AccessPort makes it easier for the end user to understand without doing any math. The key difference here is, that the Cobb AccessPort is displaying what we call "Gauge" pressure or "Manifold Relative Pressure" and the VW Performance Monitor displays Absolute.
But what does that mean? is the VW reading wrong? is my AP Wrong?
Well, the answer is neither! Both gauges are correct, they're just displaying different types of pressure.
So what is the difference?Absolute pressure includes the pressure of the atmosphere and 'boost', while Relative pressure is the pressure 'relative' to atmospheric pressure which is just known as "boost". I know, thats a lot to get your head around so I'm going to default to some experts in Turbocharger technology, Garrett Motion.
Gauge Pressure (in units of psig, the g refers to “gauge”) measures the pressure above atmospheric, so a gauge pressure reading at atmospheric conditions will read zero. Boost gauges measure the manifold pressure relative to atmospheric pressure, and thus are measuring Gauge Pressure
So how do I figure out what my Performance Monitor screen really means? and why is it different?To figure out what your actual "Boost" or "Positive" pressure is, you simply have to subtract your atmospheric pressure.
For this we're going to convert units from Pounds per Square inch (PSI) into what the ECU uses natively, which is Millibar (mbar) and do some simple math.
*for an approximate result, multiply the pressure value by 68.94 to turn Psi into Millibar
1 Bar = 1000 Millibar = 14.5038 Psi (14.5 for short)
1 Atmosphere = 1013 mbar = 14.6923 Psi (14.7 for short)
First, lets get some background knowledge about what Atmospheric Pressure is.
The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.
At sea level atmospheric pressure according to National Geographic is 1013 mbar, however it commonly gets rounded down to 1000 mbar. Depending on where you live, and what the weather is that day this value can change at any time.
If you're up high in the rocky mountains of Colorado you may see atmospheric pressure of only 800mbar or sometimes less.
This atmospheric pressure data can be found on various weather websites, but also, it's recorded by your ECU with a sensor to take all the guess work out!
Manifold Relative Pressure / Boost
To determine the Manifold Relative Pressure, also known as boost, we would take the value displayed as Absolute pressure and subtract the Atmosphere to make it "relative."
So lets say we show 30psi (2058.43 mbar) on our VW Performance Monitor Gauge, and for all intents and purposes, we're going to say that we're at Sea level and have an atmospheric pressure of 14.7 (1013mbar).
While our ABSOLUTE pressure is 30psi, once we substract 14.7 from 30, we're left with 15.3psi of Boost or 1045.43mbar
It's pretty obvious now how the gauge shown on the VW Performance monitor can show some scary high numbers, however knowing that 30psi on the gauge is only 15 or so psi of actual boost pressure it’s not that scary after all!
What should peak boost be on my EQT Tune?
For staged tunes, your peak boost will be around 27-28 target depending on elevation, as the RPM's increase it tapers off to 16-20psi by redline depending on the turbo and other atmospheric conditions as discussed above.
Boost Pressure will vary on Custom tunes depending on your hardware so see the note below for your targets.
EQT boost control as well as many others operates on a "Closed loop" system where the ECU is constantly checking the Requested boost vs the Actual and will apply more or less wastegate duty cycle (N75) to the system in order to reach its requested level. The amount of authority the wastegate has for boost control is electronically limited and precisely control by your EQT Tune.
NOTE: If you're ever curious as to what your live boost request is you can set up a monitor on your AccessPort for the requested boost. Actual boost may vary in name, it may simply be called "boost" or manifold relative pressure.
My boost is off of the target, What should I do?
If your vehicle is off of target boost by a fairly large margin you may want to verify that the Charge Pressure Actuator / Wastegate has the correct voltage and is within the allowed calibration settings. Click here for the FAQ Post
If the voltage is within specifications then you'll want to inspect the vehicle for a boost leak either by eye, ear, or with a boost leak or smoke tester.
Does EQT change boost targets depending on Atmospheric pressure?
Yes, we absolutely do! At higher elevation where the atmospheric pressure is lower we cannot run as much boost safely without the risk of overspeeding the turbocharger.
EQT has specifically calibrated our tunes to look at the atmospheric data from the vehicles sensors and adjust these boost targets to make sure your car and turbo has a long life and performs regardless of where you live, from the shores of California to the snow capped peaks of the rocky mountains
I really hope this helped answer some questions and explain why you shouldn't be too concerned with "38psi" on your VW Performance Monitor.
Remember, "38psi" at sea level is only 23.3psi of boost!, or at elevation 26.4psi (calculated using 800mbar - Typical atmospheric pressure of Colorado Springs, CO)
Thanks for reading!
- Dave At EQT
Bonus: If you thought millibar, bar or kpa was a weird unit of measurement? Have you ever seen what Mitsubishi did in the Evo's? Kg/Cm² was the unit of measurement on the factory boost gauge.
1 kg/cm² = 14.22psi
1 kg/cm² = 980.6mbar
So this gauge had a max reading of 21.3psi
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