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EFI Tuning Fundamentals

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If you dont have a measure tool to can find the real cfm in this case 572 cfm, why i can do for this? In your experience what is the porcentaje of loss at sea level, for example 698cfm can lose 20 percent of this due the mechanical restrictions and barometric pressure, im my case i live at 2600 meters above the sea level, that means a lot of VE lose how i can measure more accurately the real cfm with no tools, or that is impossible?

Try forgetting about working in CFM, but rather in mass - or percentage. I think most people here will already have had enough of my ranting why VE is a BS way of calculating anything other than the actual vs potential filling of the cylinder.

100CFM volume of air at your altitude will have only 74.8% (or so) of the mass of 100CFM of air at sea level. But the volumetric efficiency will be about the same, as that's a function of the engine cylinder filling.

If I understand your question correctly, you want to compare the mass of air entering the engine at your elevation compared to sea level, possibly so you can get an idea of what fuelling is going to be required? You may not need to worry about that if the engine management uses a manifold pressure sensor, as that is what the engine will be seeing.

You can use a simple pressure guage that reads ABSOLUTE pressure to find the relative density where you are - these measure from zero, or vacuum, whereas most pressure gauges measure 'guage', which is comparing it to the outside air pressure. You can also fing charts on-line, like this - which give the change with altitude.

Hi understood the answer thank you, in the video andre sayed they test the engine in a dyno and the real cfm was 572 the actual airflow, theorical was 698, if i want to apply the formula how i can measure the real cfm with no tools?


I don't believe I've seen the video, but there are at least two possible reasons he said that.

It could be the measured airflow into the engine compared to the theoretical for the engine capacity and manifold pressure and temperature - the former will always be lower.

The second could be that his dyno' is in Queenstown, with an elevation of ~310m, and correcting air density and temperature to the standard values meant that the measured airflow was the equivalent of the lower volume at standard values, which would be denser. Numbers don't quite agree, though, unless it was very warm!

However, it doesn't really matter, unless optimising the engine's breathing/efficiency, as it is normally the air mass, and the oxygen content, that matters.

Off hand, cant really think of a way to measure the air volume, although some vehicles/ECUs use that as part of the air-fuel mapping process, so may have that available as a readout.

You could work back from the fuel used and the lambda to estimate the mass of air required to oxidise the fuel, then correct for temperature and density where you are - that graph posted earlier would help with that.