Air flow meter to MAP conversion

**amichie** · 02-09-2008, 02:07 PM

Originally Posted by oldcorollas

these kind of MAP sensors (MPX4250 etc) are cheap... well.. about $15-20 from digikey. same as used in Megasquirts
http://www.freescale.com/files/senso...t/MPX4250A.pdf

That is just what I'm looking for. I might build a board with a few of these and plumb it into my intake at a number of points.

**oldcorollas** · 02-09-2008, 02:41 PM

there are also two port differential versions as well

**Duk** · 02-09-2008, 05:05 PM

Originally Posted by oldcorollas

there are also two port differential versions as well

Probably the prefered type, to compensate for changes in atmospheric pressure/altitude.

**brett_celicacoupe** · 02-09-2008, 09:52 PM

you can always use a U-tube manometer to look for pressure differences.

not hard to make either (clear hose + water)

**amichie** · 04-09-2008, 10:04 AM

Originally Posted by brett_celicacoupe

you can always use a U-tube manometer to look for pressure differences.

not hard to make either (clear hose + water)

Agreed. Worth a try.

**oldcorollas** · 04-09-2008, 12:11 PM

agreed as well, with water it is pretty sensitive as well... and cheap...

then again, if you have an ADC and logger lying around, $20-25 for a differential MAP sensor could make life easier.. ie drive and log..

Plonka, atmospheric compensation is usually done at startup with ECU. these single port absolute MAP sensors might be internally vac referenced perhaps? so no need to calibrate sensor based on atmospheric changes... maybe?
Edit: page 4 of the pdf above shows a sealed vacuum reference...

**Duk** · 04-09-2008, 08:51 PM

I believe Haltech provide a mappable and continuous atmospheric pressure compensation based on an individual pressure sensor.
Reading the Megasquirt stuff some time ago, I recall (vaguely) that their atmospheric pressure compensation was only used to potentially move the load cell higher or lower.
IE, it wasn't some fantasticly acurate form of compensation (even they/the person who wrote it said so).

**oldcorollas** · 04-09-2008, 09:11 PM

yah MS1 just checked atm on startup and applied a %... but the whole MS thing is abotu applying % changes to a nominal VE table...

some people were running modified MS1 code with two MAP sensors for continuous atmopsheric pressure adjustment, mostly guys living in the mountains in the US.
I am not sure what the latest MS2 and MS2extra code provide, but if you can code it, it is there

that said.. there is not really much need for atm compensation, unless you are changing altitude drastically. the fix for basic installs of MS1 was, when you start getting to the point where your O2 correction is outside it's range (or load is running a bit lean, since oyu are probably going up a hill), then you pull over, stop, restart, and then atm is corrected for that altitude (give or take whatever.. say +-500m or 1km?)

**amichie** · 07-09-2008, 05:32 PM

I had a play with the FORD amp sensor again today and it works well above atmospheric pressure. I hooked up a little rubber hose and a syringe and so I could apply vacuum or pressure. The frequency seems to change in a linear fashion. I managed to change the frequency from about 160Hz at atmospheric pressure upto about 270Hz with the syringe fully in and about 80Hz with the syringe fully out.

Not bad for a $5 ebay purchase.