• Sign Up! To view all forums and unlock additional cool features

    Welcome to the #1 Fiesta ST Forum and Fiesta ST community dedicated to Fiesta ST owners and enthusiasts. Register for an account, it's free and it's easy, so don't hesitate to join the Fiesta ST Forum today!


MAP vs Boost Pressure?

twolf

Active member
Messages
607
Likes
266
Location
Canton
#1
So I've got a few questions... Just been messing around trying to learn more etc.

So the MAP sensor on the manifold. It's obviously post throttle, which at idle should be in vacuum. Right? However it shows like 5 psi on the Accessport. Looking at Cobb's site, I see the Boost Pressure monitor is "MAP minus barometric pressure" which is 14.7 psi... okay, so to me this doesn't make sense. If I were to put a hole in the manifold and stick my finger over the hole, I would feel it sucking my finger in.

I've run into this weird (at least in my head) way of reading pressure/vacuum in other places as well, can anyone give me a simple explanation as to WHY it is measured like this?

Actually, I think I just figured it out myself. Anything BELOW 14.7 psi is technically "vacuum" and anything ABOVE is boost. So if my MAP is actually sitting at "5" psi, it's 10 PSI below the ambient atmospheric pressure. Doh. Then I guess that leads me down another path... how does the car figure out barometric pressure? I see a monitor, but where is the sensor?

Also I see a sensor on the cold side intercooler piping. What is it? Charge air temp sensor?

Last question: will intercooler/piping EVER see even a slight amount of vacuum? I know a leak in the IC piping won't result in a vacuum leak, or anything like that, because it's on the wrong side of the throttle body. And I know the vacuum inside of the intake manifold is caused by the restriction of the closed throttle plate, but on the turbo side of the throttle body, it would have to be under a very slight, nearly un-measurable level of vacuum unless the turbo provides no restriction (or is actually moving enough air) at idle.

Or am I wrong? Sorry, total newb with only basic level of understanding trying to learn a little more. :p
 


Messages
331
Likes
144
Location
Gloucester, VA
#2
So I've got a few questions... Just been messing around trying to learn more etc.

... how does the car figure out barometric pressure? I see a monitor, but where is the sensor?

... Also I see a sensor on the cold side intercooler piping. What is it? Charge air temp sensor?

...will intercooler/piping EVER see even a slight amount of vacuum?


- barometric pressure sensor is inside the ECU

- MAP sensor for throttle inlet pressure (TIP) and temp (CAT)

- yes it sees relative vacuum at idle and low throttle/load openings
 


Messages
108
Likes
25
Location
Manassas
#3
No wonder why i have no vacuum on my boost guage. Running off the boomba sound symposer delete.
 


OP
twolf

twolf

Active member
Messages
607
Likes
266
Location
Canton
Thread Starter #4
lol yeah, wrong side of throttle body. You'll have to T into a vacuum line that's post throttle body or get a MAP spacer.
 


Messages
210
Likes
59
Location
Metro Detroit
#6
I try to keep all my datalogs for MAP in kPa or bar, that way you know that anything below roughly 100 kPa or 1.0 bar is "vacuum" and above that is "boost". Of course that changes with the weather, but it's close enough for casual glances.
 


Similar threads



Top