Supercharger and BARO
06-27-2006, 10:47 AM
#1
Supercharger and BARO To my knowledge the PCM checks the atmosferic pressure at start up (BARO), this using the MAP.
At WOT (I'm not sure if at a certain load or a certain TPS) the BARO value is also updated.
With the supercharger I noticed that the BARO value is updated at partial throttle too, as soon as the intake pressure is higher that the pressure at start up ( I saw that already at 40% TPS), it was still cell 15 and definetly not in PE.
The max pressure that my stock map can see is 104 kPa and as soon I hit that pressure the BARO will have that value.
A normal value is 95 kPa, 104 kPa is about 10% higher!
So now: how important is BARO?
How does this value affect fuel?
Is there a way to let the PCM check for it only at start up?
I started to look into this topic as I noticed a sudden change in LTFT after a WOT: from -2 to -10.
Is that a BARO related issue?
When I turn off the car and let it cool down LTFT goes back to the normal values
Thanks - Stefano
At WOT (I'm not sure if at a certain load or a certain TPS) the BARO value is also updated.
With the supercharger I noticed that the BARO value is updated at partial throttle too, as soon as the intake pressure is higher that the pressure at start up ( I saw that already at 40% TPS), it was still cell 15 and definetly not in PE.
The max pressure that my stock map can see is 104 kPa and as soon I hit that pressure the BARO will have that value.
A normal value is 95 kPa, 104 kPa is about 10% higher!
So now: how important is BARO?
How does this value affect fuel?
Is there a way to let the PCM check for it only at start up?
I started to look into this topic as I noticed a sudden change in LTFT after a WOT: from -2 to -10.
Is that a BARO related issue?
When I turn off the car and let it cool down LTFT goes back to the normal values
Thanks - Stefano
06-27-2006, 01:21 PM
#2
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You're a smart cookie Stefano. If the computer is updated to a higher baro than what you actually have it will try to fuel for higher density air. Since that's too rich your LTFT are recalibrating for actual conditions.
You could try this, and I don't know if it will actually work. Place a vacuum T inline with the map sensor. On the open end of the T place a one-way vacuum check valve opriented so that if you suck on the line going to the T there will be no flow and the check valve is closed. Place nothing on the otherside of the valve.
When you get into any boost the checkvalve will open and not allow the map sensor to see full pressure. What I'm not sure about though is if the check valve would flow enough to keep to sensor from seeing any boost at all, which I doubt it really will. I think that it will see some....but it might work and if it does it won't cost much. You could always place a restrictor in the vacuum line leading to map sensor to ensure the valve will flow more than the line.
Worst case if you're using a 1 bar tune you could wire in another map sensor with a relay on a WOT switch so that the computer wouldn't see higher than actual underhood map pressure. This will still be higher than in manifold pressure by a touch.
You could try this, and I don't know if it will actually work. Place a vacuum T inline with the map sensor. On the open end of the T place a one-way vacuum check valve opriented so that if you suck on the line going to the T there will be no flow and the check valve is closed. Place nothing on the otherside of the valve.
When you get into any boost the checkvalve will open and not allow the map sensor to see full pressure. What I'm not sure about though is if the check valve would flow enough to keep to sensor from seeing any boost at all, which I doubt it really will. I think that it will see some....but it might work and if it does it won't cost much. You could always place a restrictor in the vacuum line leading to map sensor to ensure the valve will flow more than the line.
Worst case if you're using a 1 bar tune you could wire in another map sensor with a relay on a WOT switch so that the computer wouldn't see higher than actual underhood map pressure. This will still be higher than in manifold pressure by a touch.
06-27-2006, 04:06 PM
#3
Smart cookie, haha! 
Ok, I understand what you mean: you are suggesting to insulate the MAP and let it see barometric pressure only at start up. Mechanically.
Well: a WOT switch doesn't help because pressure gets higher already at half throttle.
I like the other solution, I just don't know how easy is to remove the MAP sensor from behind the engine (it's an F-body). Sure, once I have it I can install it somewhere in the engine bay and connect it to the manifold. But how get it? Isn't it necessary to remove the engine, or at least the manifold?
Something I tried today:
BARO at start up = 95.6 kPa; LTFT at idle = -1.6 (after 10 miles, engine warmed up)
A little boost...
BARO = 103.4 kPa; LTFT at idle = -4.7
Turn off the engine, crank it up again... the BARO remains the same!
There is no update! I don't know if it needs to cool down or if it waits a certain time.
After crank it up, wait a couple of minutes LTFT at idle was -9.4
103.4kPa is 108.1% of 95.6kPa - at the same time LTFT decrease 7.8%... about the same increase!
Is everyone with a 1 bar MAP having the same issue?
Ok, I understand what you mean: you are suggesting to insulate the MAP and let it see barometric pressure only at start up. Mechanically.
Well: a WOT switch doesn't help because pressure gets higher already at half throttle.
I like the other solution, I just don't know how easy is to remove the MAP sensor from behind the engine (it's an F-body). Sure, once I have it I can install it somewhere in the engine bay and connect it to the manifold. But how get it? Isn't it necessary to remove the engine, or at least the manifold?
Something I tried today:
BARO at start up = 95.6 kPa; LTFT at idle = -1.6 (after 10 miles, engine warmed up)
A little boost...
BARO = 103.4 kPa; LTFT at idle = -4.7
Turn off the engine, crank it up again... the BARO remains the same!
There is no update! I don't know if it needs to cool down or if it waits a certain time.
After crank it up, wait a couple of minutes LTFT at idle was -9.4
103.4kPa is 108.1% of 95.6kPa - at the same time LTFT decrease 7.8%... about the same increase!
Is everyone with a 1 bar MAP having the same issue?
06-27-2006, 05:05 PM
#5
OK I can buy another MAP and let mine there as a plug.
It would be interesting to lock a maximal value of about 95 kPa, maybe with an interface that cuts higher values (voltages).
But this is all prototyping... I just can't imagina that all those FI cars with stock MAP are having this kind of issues. Isn't there a way to program the PCM handle that?
It would be interesting to lock a maximal value of about 95 kPa, maybe with an interface that cuts higher values (voltages).
But this is all prototyping... I just can't imagina that all those FI cars with stock MAP are having this kind of issues. Isn't there a way to program the PCM handle that?
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06-28-2006, 01:01 AM
#8
Originally Posted by kbracing96
I used EFILive to set my max Baro update from 4000 rpm (stock) to 2300 rpm, so it would never update when I was in boost. Seam to work for me
Thank you! that's a good information
