[Diy_efi] Re: Q regarding placement of a pressure sensor

Shannen Durphey shannen at grolen.com
Tue Jul 16 00:36:58 GMT 2002



Bob Moon wrote:
> 
> Thanks for the info.
> 
> I understand that MAP and MAF are completely
> different, but I am not attempting to just replace the
> MAF with a MAP and call it a day.  I'm attempting to
> map the MAF signal to the voltage output of the MAP so
> I can use a uC to simulate the output of the MAF
> (square pulse wave, rather than a voltage).  That way,
> my ECU will still think it has a MAP.
> 
> Since I am only interested in the amount of air
> flowing through the engine at that point in time, my
> question is this: for a given airflow count from a
> MAF, is there a given vac in the intake?  
No.  Think about WOT on an NA engine.  As long as the intake path provides
little restriction, there's 0" vacuum in the manifold.  That's 0" at 500
rpm, 0" at 2500 rpm, 0" at 5000rpm...

In this scenario, what additional information would allow you to make an
airflow map?

> It seems to
> me that a given airflow will always have the same
> pressure signal, regardless of what's happening
> downstream of the turbo.

Nope.  Think about it this way, again using an NA car.  If the throttle is
closed while the engine is running, what is the vacuum reading?  maybe 13"
at idle?  Maybe 15" at 1500 rpm?  Maybe 17" at 2500 rpm?  But the
throttle's closed, so how much difference can there be in airflow?

> 
> I know this is all possible thanks to the existence of
> various conversion kits (TRE's MASC and the VPC, for
> examples of two voltage-to-frequency converters).  I
> just don't know for sure where in the airstream I
> should place the MAP tap to get the most reliable and
> consistent readings.

MAP sensor is best connected post throttle plate.
Shannen

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