Do you tune from boost gauge or MAP sensor readings?

[slowsixtyduece]

By default, I use my ECU’s map sensor to retard my timing under boost. Since I am at altitude (over 5,000 ft asl), there is always a 5 PSIG difference between my Map sensor values (absolute) and my mechanical vacuum/boost gauge (atmospheric) values. For instance, when the Ecu sees +10 psi of boost from the map sensor, my mechanical gauge is registering at only +5psi of boost. So the gauge is always reading 5 psi less than the map sensor.

Should I trust and tune from my map sensor or from my mechanical gauge? I want to properly retard my ignition 1 degree for every 1 psi of boost (pump gas), but it needs to happen when I’m actually under boost. Is the gauge a more accurate representation of what is actually happening - when the engine is actually seeing positive pressure and the ignition needs retarding - or the Map sensor?
Thx


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[Chip Birks]

Does your ecu recognize 101kpa as boost or 84kpa?

[slowsixtyduece]

Does your ecu recognize 101kpa as boost or 84kpa?

Unfortunately I couldn’t tell you since I’m using an older SDS unit. It simply uses a 2 bar GM Map sensor and converts it to a positive or negative number on the LCD readout.


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[Chip Birks]

Does your ecu recognize 101kpa as boost or 84kpa?

Unfortunately I couldn’t tell you since I’m using an older SDS unit. It simply uses a 2 bar GM Map sensor and converts it to a positive or negative number on the LCD readout.


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Interesting. Too bad. I know my setup uses a second map sensor to compare against for altitude correction. No idea what SDS does though.

[bad62bug]

If you get the two readings from the same location they should read the same or real close to the same.

[Bad Bob]

I have SDS as well. I’ve always used the MAP sensor with no problems.

[Chip Birks]

I have SDS as well. I’ve always used the MAP sensor with no problems.

Are you at sea level?

[slowsixtyduece]

Does your ecu recognize 101kpa as boost or 84kpa?

Unfortunately I couldn’t tell you since I’m using an older SDS unit. It simply uses a 2 bar GM Map sensor and converts it to a positive or negative number on the LCD readout.


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Interesting. Too bad. I know my setup uses a second map sensor to compare against for altitude correction. No idea what SDS does though.

Chip, are you able to see the differences between the first and second map sensor? If so, what does it look like? And do you know what your ecu uses as the final map input?

I think I’m effectively trying to manually do what your ecu does automatically.


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[Chip Birks]

My second map isn't connected to the engine. So it just gives a constant reading unless I change altitude, which does happen here in Utah occasionally. My ecu can think a few different ways. The simplest is that it thinks that everything below 100kpa is vacuum and everything above is boost. Or i can tell it to look at the second sensor to know where vacuum ends and boost begins. For me, 84kpa is ambient, anything above that is boost. As soon as the MAP sensor sees anything above the 2nd sensor, it kicks into "boost mode."

[buguy]

Then all hell breaks loose?!

[slowsixtyduece]

I decided to email the manufacturer and here was their response:
“The engine only cares about absolute manifold pressure, gauge pressure isn't relevant so base retard on manifold pressure.”

It still doesn’t quite make sense to me since there are other ECUs, like Chips, that take atmospheric pressure into account to compensate.


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[Bad Bob]

Home 4100’, work 1512’. So, density altitude about 7000’ on the evening drive home on a hot day. No more work though.

[bad62bug]

I decided to email the manufacturer and here was their response:
“The engine only cares about absolute manifold pressure, gauge pressure isn't relevant so base retard on manifold pressure.”

It still doesn’t quite make sense to me since there are other ECUs, like Chips, that take atmospheric pressure into account to compensate.


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He is right
You should tee your lines and hook up a 10 psi source to see if they read the same, just like they should if you car is making 10 psi of boost.

[slowsixtyduece]

I decided to email the manufacturer and here was their response:
“The engine only cares about absolute manifold pressure, gauge pressure isn't relevant so base retard on manifold pressure.”

It still doesn’t quite make sense to me since there are other ECUs, like Chips, that take atmospheric pressure into account to compensate.


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He is right
You should tee your lines and hook up a 10 psi source to see if they read the same, just like they should if you car is making 10 psi of boost.

My gauge line and MAP both share the same line, only difference is that the gauge line runs all the way up to the gauge at the front of the car and the map has a line that’s about 3’ long to it. Both are taking their readings from a shared source - my intake manifold right before the end castings.

The reason they don’t read the same is because they have different “starting points” — one is absolute pressure and the other is atmospheric (relative) pressure. The map sensor’s starting point is a ‘zero reference against a perfect vacuum’ (built into the sensor). Whereas the gauges starting point is a ‘zero reference against ambient air pressure’ - wherever you are, it will read zero.

As an example, if I turn the car on but don’t start it up, my map sensor does not read zero (due to the higher elevation) but my gauge reads 0psi. This would make little to no difference on a N/A car. But for a turbo car, where I reference boost for timing retard, it makes no sense to start retarding timing at +1psi on the map sensor when there is actually no positive pressure in the intake tract yet.

I’m still trying to wrap head around all of this, so maybe I’m completely wrong.




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[Clonebug]

I went from sea level to over 9000 ft and never had any issue with the ecu working out fueling or ignition.
I do have a mapdaddy sensor to read the atmospheric pressure while driving. It is running on Mario's default setting and it seems to work fine.

Granted I never saw much more than 3-4 lbs boost but it ran flawlessly through the whole trip. The MS ECU takes a reading of the MAP right during "key on".
If you have a second Map Sensor it reads it all the time. I added it just "because" I could....since I travel longer distances and variable elevations I thought it would be a good upgrade....it's pretty seamless as far as I can tell.