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Old 10-24-2002, 04:00 PM   #1
ScoobieSnaX
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Default A word about boost readings (UTEC/Link/etc)

I notied a few posts about the discrepency between aftermarket boost guages and the factory MAP sensor so I decided to share my experience here...which may be incorrect as well. Please feel free to correct me

I used to have a Link ECU and now own a UTEC (stg4). I've noticed that every boost reading from the MAP sensor appears to be off by anywhere from 1.6-2.0psi. This is because I'm taking a reading at 2375ft above sealevel (when I'm in Edmonton, Alberta, Canada). If I view the dashboard of my UTEC with the car shut off, the MAP reading is -1.6psi. Theoretically if I were to drive my car in LA, the MAP sensor would read 0psi with the car shut off. If I were to drive to Jasper, Alberta where the altitude is quite high, I'd get something like -2.6psi. This is all due to the pressure differences associated with the change in altitude. The factory MAP sensor in the WRX is tuned for sealevel as a baseline.

So what does this all mean?

Well, at sealevel the air pressure is ~14.7psi or 1bar. This will also change slightly depending on air temperature and weather conditions. You will have more pressure in the air on a cloudy day vs. sunny, warm day.

...and how does this relate to boost?

First off, your boost guage will always read 0psi at any altitude (remember this). Lets say in Edmonton my boost reading from the MAP sensor is -2.0 (which would be ~12.7 absolute air pressure) and I set my boost controller to deliver 18psi. I'm really only getting 16psi because my mainfold already has -2.0psi to begin with.

THIS IS KEY!!!!: To achieve the same manifold pressure as someone running 18psi at sealevel, I would have to run 20psi in Edmonton. You have to run the turbo extra hard to achieve the same results. Since the IHI turbos are known to blow up around 20+ psi, you wouldn't be able to compete with someone in LA running 20psi already....

The Link and UTEC read the MAP from the car's sensor. I've seen several posts about people wondering about the accuracy of the "UTEC MAP" reading. The UTEC isn't generating this value......its just reading it from the car. If you were to look at the factory ECU's reading it would be the same


Clear as mud?

l8r,

ScoobieSnaX
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Old 10-24-2002, 04:39 PM   #2
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ok ... not sound like a know it all ... but isn't this kind of the obvious???

Your above sea level ... your air is thinner ... whats the issue? *shrug*

sorry ... just never mind me ...

it just seems to me that there isn't too much to argue about ...
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Old 10-24-2002, 05:39 PM   #3
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Sounds like the real problem is that some people are trying to directly determine "boost" (which should be defined as the increase in pressure above ambient) from the MAP sensor which only reports absolute pressure at any given point in time. I would have thought that the UTEC would sample the MAP when the engine is not running in order to determine ambient pressure. I can see how this might cause issues if you drove from sea level into the mountains, but as long as you are aware of the phenomenon then everything is OK.
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Old 10-24-2002, 05:49 PM   #4
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Jenison - He's not arguging, he's stating a fact.
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Old 10-24-2002, 10:45 PM   #5
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Why don't they use temperature and pressure compensated sensors that take ourside pressure and temperature (well maybe not temperature in automotive application it gets pretty hot under the hood) and balance the output accordingly ?
Search www.digi-key.com .
I know motorola makes one that's usually used by enthusiasts as a sensor for digital boost guage.

[Edit] bad link, can't link search results ondigi-key.
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Old 10-25-2002, 08:53 AM   #6
il96
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Default Found it !

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Old 10-25-2002, 09:44 AM   #7
8Complex

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Well, you want to use absolute sensors as it really is a big difference between 10psi at sea level (where atm. is still 14.7psi) and 10psi at Denver level (where atm. is closer to 12psi).

Just by numbers (these aren't totally correct because of other factors involved) at 10:1 compression at a total of 24.7psi (atm. + 10psi at sea level), you get 247psi in-cylinder pressure. At Denver level it would be 22psi, so you'd end up with 220psi in-cylinder pressure. Thats like the difference between running an NA motor at 10:1 and 8.9:1 compression.
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Old 10-25-2002, 12:47 PM   #8
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Quote:
Originally posted by JenisonWRX
ok ... not sound like a know it all ... but isn't this kind of the obvious???

Your above sea level ... your air is thinner ... whats the issue? *shrug*

sorry ... just never mind me ...

it just seems to me that there isn't too much to argue about ...
I agree. The concept of air being thinner when warmer and at higher altitude is one that everyone on this board should know of. However, being able to relate that concept to why your MAP sensor is reading -1.6psi without the car turned on is where the challenge lies.

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