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Good Input for a Altitude compensation sensor WRXLink107 and Waterspray


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Evening! (I know the 107 has a onboard BAP.. but that's my issue....)

Is there an advised An to use for an engine bay Altitude compensation sensor? I'm Thinking of using An Volt 8 as Its next to the "standard" MAP sensor Pin. (So repurpose the MAF input to keep inputs together.) 

The issue is being a rallycar with a roof vent you get an artificially High pressure in the cabin of the car and its known to cause issues...... I can always allow the ECU case to take "unpressurised" air but that could get annoying pressure proofing the case!

Probably sounds pedantic, but Its better than getting a slight leanout and burning the valves!

And I presume reuse the Aircon Switch input/Relay out to drive the Waterspray is fine?

Unless there is a way to put a "cap " on the onboard BAP so it sees the air pressure outside?


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Im with rrob here.  Before going overboard and wasting a valuable input I suggest you log the internal BAP to see if there is any speed influence. I highly doubt a roof vent will do F all.  Even with highly developed venturi airboxes from Lemans cars etc you will be lucky to achieve 3kpa at 250kmh from aerodynamic pressure.  When that is acting on the turbo intake and multiplied by the pressure ratio it is quite a valuable increase to have. 

However in your case it is not acting on the turbo intake, it is only acting on the ecu case.  So if we fantasize that you do achieve the same 3kpa of aerodynamic pressure in the cabin/ecu case, and you are running only 1 bar boost at max speed, the only thing that 3kpa change in BAP is going to do is move the cell lookup on the fuel table axis by 1.5%.  Based on the VE curve of a typical rally engine that will end up causing a 0% change in fuel mixture.

You are probably better to use that input for monitoring something that has a bigger influence such as fuel pressure.



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It’s actually a well noted issue (Ralliart even had an advisory on it) 

ive seen the numbers, you will be surprised on a well sealed car like a Group N car with the back vents sealed what the pressure change is.

But anyway, the question was more about if there was an advised output (which there appears not to be) so I’ll figure it out tonight.





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The important thing to understand is even if the pressure change is much bigger than I suggest, a change in BAP is only going to move the look up on the MGP axis - BAP has no direct effect on the fuel equation.  The only thing you are doing is moving the cross hairs on the fuel table and if you look at any rally car fuel table it is pretty near flat.  

For I/C spray you can use any spare digital input for the switch and any injector/ignition or aux output for the relay.  The AC ones would do fine. There are also spare aux outputs and DI on the expansion connector. 

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