terje

Hi.
My ambient pressure is reading wrong numbers. (870) Should be 1000.
I am at sea level.
Is there a barometric senor on the car?

Thanks
Terje

terje

It is my understanding that the barometric/altitude, is calculated from MAF and throttle position sensor.
Anyone know if this is right?

Terje

Felix

Where are you getting the barometric reading from?

terje

I am getting the barometric (ambient pressure) reading from the bosch "hammer", under actual values menu.

I know now that the ambient pressure is calculated from the MAF sensor and the Throttle position sensor. (there isnt any pressure sensor for this reading)

My low ambient pressure was because i have Ruf turbo R camshafts.
(the ruf camshafts produce lower vacuum, so the calculation gets wrong)

My mechanic did some grinding on the throttle body (ruf mod) and now my ambient pressure is OK (around 1020mbar) i live 115meters over sea level.

Terje

Felix

Hmm, your answer surprises me. My understanding of the Motronic ECU on the 993tt is that it knows nothing of the air pressures, either ambient or after the turbos. It measures the mass of the air going into the engine and manages the mass based upon lookup tables for given engines speeds and loads and also takes knock and intake air temperature into account. Air mass management thereby results in what is effectively boost management but the air mass is the measured and managed parameter.

There is a boost pressure sensor on the engine but it only feeds the boost gauge in the dash - that gauge is completely separate to the ECU.

So no idea where the Bosch Hammer is getting an air pressure value. Any chance it has an air pressure sensor built in? That said a value of 1020 is plausible at your altitude but 870 would mean you were rather higher up and/or there was a heck of a storm on the day.

terje

No the ecu do not have a buildt in pressure sensor.
My ruf mechanic told me that the ecu calculates ambient pressure from the flow over the maf sensor in correlation with the position of the throttle position sensor. (the value i read with bosch hammer is a calculated value)

So my low ambient pressure was because the ruf camshafts produce lower vacuum, so the maf calculate lower air flow over the maf, with the same throttle position.

After grinding my throttle body, the air flow over the maf is greather, at the same thottle position, and the ambient pressure rises.

Ambient pressure before grinding 870
Ambient pressure after grinding 1020