Vacuum at idle question

crjackson

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So I got a new BT OBDII scanner dongle that works well with my iPhone.

With this plugged in, I have a Vac/Boost gauge dashboard on my phone. I'm wondering about 2 things.

1) Is reading vac/boost like this reasonably accurate?

2) Is 10Hg of idle vacuum normal for a Roush Charged Coyote?

Thanks ...
 

crjackson

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So, answering my own question partially...

Using the OBDII port to read the factory installed MAP sensor isn’t accurate for Vac/Boost readings.

The reading is an inferred number which works for relative pressure changes, but not accurate enough for precise readings.

I still don’t know about #2 but I’ll hopefully know this by tomorrow.
 

Riddick

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2) Is 10Hg of idle vacuum normal for a Roush Charged Coyote?

That seems a little low, I dont have a boost gauge on mine so I cant say for certain. However, based off previous experience I would think these cars should pull between 18-22 at idle.
 

Riddick

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2) Is 10Hg of idle vacuum normal for a Roush Charged Coyote?

That seems a little low, I dont have a boost gauge on mine so I cant say for certain. However, based off previous experience I would think these cars should pull between 18-22 at idle.
 

01yellercobra

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Is it actually reading Hg or is it reading negative PSI? I know the MAP sensors on the Cobras only read PSI so in vacuum a gauge like the Aeroforce will show a negative PSI. In my case it was -10. I found somewhere that Hg was basically double negative PSI. So just multiply the -10 by 2 and get -20" which is what it should be.
 

crjackson

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Is it actually reading Hg or is it reading negative PSI? I know the MAP sensors on the Cobras only read PSI so in vacuum a gauge like the Aeroforce will show a negative PSI. In my case it was -10. I found somewhere that Hg was basically double negative PSI. So just multiply the -10 by 2 and get -20" which is what it should be.

Thanks for that. I think you nailed it down for me. I don't have the car in front of me but the dashboard on this thing shows negative -PSI.

Mine was reading -10 psi.
 

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