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# Question: What is the mass flow at an air pressure of 14.7 psi?

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Discussion and questions related to the course EFI Tuning Fundamentals

Ok, what am I missing here? I don' see how I can calculate mass flow with just knowing air pressure of 14.7 psi. MAF=VxD. How can you determine volume from psi?

Anthony,

I suspect your question may be based on something specific you've seen in course material. Can you link us to a video and time mark to review what you're asking about?

Sorry, I should have been more specific. This is a question in the Test Your Knowledge section of How the ECU Works. Seems like a terrible question lol.

Right before that Andre was talking about standard conditions that assumed 1 Bar of atmospheric pressure or 14.7 psi...

But when an ECU makes air mass calculations it makes adjustment based on actual air pressure in the area

For instance Mitsubishi Evo MAF has inbuilt baro and temperature sensors for making necessary adjustments. Pins 52, 64, 65...

Right, but how are you supposed to calculate the answer with only knowing air pressure of 14.7?

What is the mass flow at an air pressure of 14.7 psi?

A. 107 kg/min

B. 10.7 lb/min

C. 1.07 lb/hour

D. 10.7 lb/sec

Ah gotcha. Could you share the whole question? Without context I'm unclear how to help clear up confusion.

My post above is literally the whole question as it reads.

You know the volume of your engine, you know VE, you know RPM, you know ambient temperature and pressure- that's all known and all you need to calculate air mass flow.

That one wasn't for calculation thought but rather for remembering the information from the course and thinking. Two units of measure (lbs/sec and lbs/hour) aren't correct at all. So we have only two left - lbs/min and kg/min. Both can be correct. But the thing is if that mass air flow was calculated in kg/min the air pressure would have been measured in Kg/cm2. This is not the case since you were given pressure in psi. So correct answer is 10.7 lbs/min.

The question would have been better phrased as 'Which value could be a valid Mass Flow at 14.7 psi?"

Yes, that's a bit confusing indeed...

Thank you Shota and David! I actually did guess the correct answer based on the units like Shota said. I still thought I was supposed to calculate the answer lol. That question was worded terribly. I don't feel as dumb now...sorta :)

This question was a follow up to the question right before it.

The answer to the prior question is the volume flow.

To get mass flow you multiply volume flow by density.

Multiplying that volume flow from the prior question, by the density value taught in the course for use at standard pressure (14.7 psi mentioned in this question), gives you the correct answer to this question.

Out of context for the folks here who weren't taking the test at the moment, it does seem like volume flow info is lacking, and I can understand why Shota tried to narrow it down based on units, but that's not necessary. All info to do the calculation has been given, but part of the info was in the prior question.

I've proposed an update to the question to make this more obvious. Thank you for your feedback.

Mike, I'd also like to add that the order in which the questions are given are randomized. They are in a different order each time you take the test. This particular question can be the very first question, which gives no context.

Ah thanks. I don't recall if it was random back when I took that course or not.

It's the weekend in NZ so I'll check in with the crew Monday to see what they think.

Yep, it came first randomly without any question prior. That's why i was so sure it wasn't up for calculation...

We usually reply within 12hrs (often sooner)

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