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Re: Intake, Software and Does it really matter? (archive)

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Posted by Shawn Fogg on April 27, 2000 at 15:15:47:

In Reply to: Re: Intake, Software and Does it really matter? posted by M3 on April 26, 2000 at 15:54:03:

"Im confused now... is there another chip specifically if you have just the HFM

Yes. You gotta understand how the HFM works. It basically measures the air mass passing through it. It does this by measuring the resistance of a heated film near the center of the meter. It 'tells' the DME what the air mass is basically by using a 0-5v signal to the DME.

Using totally fabricated numbers as an example...

say the HFM internally measures 10 lbs/minute of airflow. It tells the DME this by sending a 2v signal which the DME knows means this equals 10 lbs/minute. Now the airflow is 20 lbs/minute so it sends a 2 v signal which the DME understands what the airflow is. The HFM and DME have the same voltage to air mass 'table' so everthing works.

Now you put on a bigger HFM that has the capacity to measure (and flow) more air. But it still needs to work with the 0-5v spec. Now when the DME is seeing a 1v signal you may actually be flowing 12 lbs/minute of air but the DME is basing its injector opening time thinking that the airflow is 10 lbs/minute and it injects the wrong amount of fuel that in this case would make you run lean.

Sooo.... what the HFM 'chip' does is recalibrate the voltage to air mass 'table' inside the DME. With it in place the DME knows a 1v signal = 12 lbs/minute of air.

Hope this helps,


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