MAP bar to PSI?

[AVOturboworld]

As posted in the other thread:

The MAP sensor is reading your altitude-dependent air pressure + the pressure from the turbo. At sea level, that was be 1 bar - 14.7psi. But if you are in Denver, CO, the atmospheric pressure is around 12.1 psi. So your boost on top of that, if it's 18.85 psi at the MAP sensor, is about +6.85psi. That's what it's reading at the manifold, and not necessarily what it would read with a boost gauge attached. A boost gauge is generally going to be a bit more accurate.

This is why cars have MAP sensors, of course, so they can compensate for the changes in altitude. One of the reasons it was important to test in Utah was the fact we were at 5000-6000 feet for so long (and at speed! Gotta love an 80mph speed limit).

Now, here's the fun part - what you read at the manifold and what you read at the turbo are going to be two different boost levels. What the wastegate reads is at the turbocharger - it's plumbed directly off the compressor outlet. What your MAP sensor reads is at the intake manifold. Between your turbocharger and your intake manifold, there will be a pressure drop. A big part of that is the intercooler. A fun fact we recorded on the Subaru Legacy/08+ WRX was the fact that the stock intercooler had about a 3psi pressure lost, most likely due to backpressure. Our upgraded intercooler dropped that to only a 1psi pressure loss. So if the wastegate was set to 15psi, at the engine it'd be seeing 12psi with the stock intercooler, or 14psi with the upgraded intercooler.

Also factor in that boost gauges, especially the electronic ones, can be +/- 1psi as well. All of this is why our basemaps won't make the most power possible - we have to factor in all the possible variables (that are always going to be there). And it's why you will usually pick up a lot of power with a dyno tune.

[gdrider77]

Originally Posted by AVOturboworld

As posted in the other thread:
The MAP sensor is reading your altitude-dependent air pressure + the pressure from the turbo. At sea level, that was be 1 bar - 14.7psi. But if you are in Denver, CO, the atmospheric pressure is around 12.1 psi. So your boost on top of that, if it's 18.85 psi at the MAP sensor, is about +6.85psi.

Also factor in that boost gauges, especially the electronic ones, can be +/- 1psi as well. All of this is why our basemaps won't make the most power possible - we have to factor in all the possible variables (that are always going to be there). And it's why you will usually pick up a lot of power with a dyno tune.

Paul.

Thanks for all the info.

So you say the 18.58 at map is about 6.85psi. My boostgauge reads this at about 10psi....and according to my tuner this is really about 12psi...ugh, you can see the reason im confused.

Thing is, my tuner said that my gauge will probably be about -2psi off due to altitude. So if its reading 10psi, its really putting out about 12psi, etc.. Any thoughts on that? And in the end, does it really matter, as long as the tune is good and the boost is adjusted correctly?

I am headed to my tuner in about an hour and hope to get this all fine tuned, and wrapped!

[AVOturboworld]

In the end, it doesn't matter that much as long as the tune is done for the current boost, and the power figures come back at what you are expecting. I always default to the boost gauge in regards to measured boost. It's mainly a data point to measure if we are near where we are supposed to be, or if we are too far over or under.

[gdrider77]

Originally Posted by AVOturboworld

In the end, it doesn't matter that much as long as the tune is done for the current boost, and the power figures come back at what you are expecting. I always default to the boost gauge in regards to measured boost. It's mainly a data point to measure if we are near where we are supposed to be, or if we are too far over or under.

Sounds good. Thing that was scaring me at first, was the map reading. Once converted it was showing as like 18-24 psi....haha, i was thinking i was going to blow my engine.

Thanks for the new actuator and oil breather bolt. They both worked perfectly!

[King Tut]

Originally Posted by gdrider77

Thing is, my tuner said that my gauge will probably be about -2psi off due to altitude. So if its reading 10psi, its really putting out about 12psi, etc.. Any thoughts on that? And in the end, does it really matter, as long as the tune is good and the boost is adjusted correctly?

I am headed to my tuner in about an hour and hope to get this all fine tuned, and wrapped!

Boost is just a number. It doesn't matter what that number is as long as your have the fueling and ignition timing tuned properly for that amount.

[FReSh]

Originally Posted by AVOturboworld

As posted in the other thread:

Now, here's the fun part - what you read at the manifold and what you read at the turbo are going to be two different boost levels. What the wastegate reads is at the turbocharger - it's plumbed directly off the compressor outlet. What your MAP sensor reads is at the intake manifold. Between your turbocharger and your intake manifold, there will be a pressure drop. A big part of that is the intercooler. A fun fact we recorded on the Subaru Legacy/08+ WRX was the fact that the stock intercooler had about a 3psi pressure lost, most likely due to backpressure. Our upgraded intercooler dropped that to only a 1psi pressure loss. So if the wastegate was set to 15psi, at the engine it'd be seeing 12psi with the stock intercooler, or 14psi with the upgraded intercooler.

This pressure drop (and the difference between those two intercoolers) is most likely dominated by frictional and form losses in the heat exchanger (intercooler). The frictional losses for each of the intercoolers is probably about the same(and reasonably small), so I'm assuming the major difference is in the form losses (the physical obstructions to the flow). I think that's what you mean by "back pressure," but I figured I'd interject some science since I study this stuff lol.

[gdrider77]

Ok all, so worked with my tuner, and as many of you guessed. The map setting was wrong. he had it set for 4bar, as he thought i said that. He changed it to 3bar and all seems good in the readings. Boost gauge and map are much closer now. +/- .5 if that.

[King Tut]

Originally Posted by gdrider77

Ok all, so worked with my tuner, and as many of you guessed. The map setting was wrong. he had it set for 4bar, as he thought i said that. He changed it to 3bar and all seems good in the readings. Boost gauge and map are much closer now. +/- .5 if that.

That makes sense. My AEM Failsafe and EcuTek with 4 BAR map sensor are also within +/- .5 PSI.