Howdy folks,

Since the XEDE uses the stock MAF, can I log how much air is flowing through the intake?

Why you ask? -> I'm trying to figure out the VE of my engine...

l8r)

 

It is better to datalog mass airflow. This would be a reading in lb/min or kg/s or some other mass per unit time.

A cubic foot of air could contain 100 air molecules, 1000 air molecules or 50mol of air molecules. So CFM is basically useless to an engine computer.

What you really want to know is how many air molecules are passing through your MAF per unit time. That is mass airflow, and it is what a MAF measures.

You could convert a lb/min rating into CFM if you make some basic assumptions about what mass of air fits in a cubic foot box at sea level (or wherever you are). But that isn't terribly useful for the calculations you want to do.

Doesn't the XEDE datalog a "calculated boost" number derived from the MAF and how much air a 2.0L engine can theoretically swallow in a revolution? If so, you can compare the calculated boost with the real boost in order to calculate VE. So run a real MAP sensor into your Xede and datalog the actual manifold pressure, compare it to the expected, plug them into excel and you will come out with VE (given the right calculation ). I figured it out once before. It was pretty easy to do. But it makes the assumption that there is no valve overlap.

 

Good point about the mass airflow, I hadn't thought about that.

Would it be beneficial at all to measure airflow between the IC and the TB instead of at the intake? I figure that is a more accurate representation of how much air is actually being pushed into the engine, no?

l8r)

 

Mass airflow at all points in the intake system is identical. You don't magically add or subtract air molecules. All air molecules that go in the air filter go into the engine.

 

Ludikraut, the stock MAF will be accurate for flows in the ballpark of stock. Once you start pushing a whole lot more air through the MAF with mods, the mass flow values will become questionable. At some point the output of the MAF becomes garbage in terms of being able to use it as an accurate mass flow measurement device.

 

Do you have any data on an Evo to back up this claim?

 

At what point is that? No doubt it does happen, but I bet it is a lot higher than you would expect.

This sort of thing can be checked, too, by using a wideband O2 to check the air to fuel ratio vs the calculated air to fuel ratio. If they are mismatched, there is an air metering issue (or your fuel pressure is off or injectors suck, but those can be checked independently). With 2G DSM MAFs, they can usually measure around 50lb/min before needing to be hacked for more bypass airflow. Stock 2G DSM airflow is probably around 25lb/min. Once you do hack the MAF up, you can re-adjust the MAF compensation table in the ECU to match. And this can be checked using the wideband and calculated A:F ratio comparison I mentioned above. There are other methods to compare against, too.

Evo MAFs are good for even more airflow than the 2G DSM MAF.

 

Hmm, I suppose if the stock MAF does end up giving inaccurate readings, Smoggy will find out soon with his 2.3L GT35R setup. In my case I will also be pushing more air than stock with a 3071 on a built block. I'll see about doing some testing as soon as I get my car back from AMS.

l8r)

 

Exactly.

...and/or a fuel flow calculation issue (for the calculated a/f); which is also likely considering that the pulsewidth/duty cycle spat out by the ecu is a rough guesstimation (not based on injector feedback), the four injectors have variance, the fuel distribution in the rail is at best imperfect (and likely gets worse as, you guessed it, flow rates increase dramatically beyond the design point), and do you really trust your $25 fuel pressure gauge? Don't forget, small errors in the fuel flow calculation result in a larger error in calculated a/f.

That said, if the injectors were at 100% duty, you flowbenched each injector, had a gnarly (labgrade) fp gauge, and were convinced of perfect rail distribution, you could get close swag of fuel flow rate.

Then again if this technique was accurate enough to calculate fuel delivery, we wouldn't need oxygen sensors to trim fuel, and no one in the OEM/motorsports world would need fuel flowmeters.

Ludikraut, you could use the stock MAF to determine directionally whether or not a mod made an improvement, but the actual numbers it spits out will be essentially useless if you are, say, planning to use them for compressor map purposes or some other calcuation.

 

This method IS accurate enough to calculate fuel delivery. That is why our cars operate in open-loop mode and don't blow up when you floor the throttle.

And closed-loop mode is used because closed loop is always more desirable if you can do it with reasonably cheap sensors and with great reliability. You have much more control with a closed loop controller. Using a closed loop for high load applications just isn't feasible with a 4 wire O2 sensor.

You can calculate VE or even compressor flow plots on a compressor map using a MAF. It works. You just have to understand that it isn't going to be perfect. Often times, though, it is good enough for what you need or want to see. Have you even tried?

 

...so instead the last resort is to use open loop and live with its inherent inaccuracies in fuel delivery.

Having no sensor at all would be cheaper, more reliable, and more desirable than needing one.

Shape, have you ever correlated "calculated" a/f to a/f as measured with lab-grade instrumentation?

There's no argument that if the MAF's output went up significantly (repeatably), then that's directionally goodness. But how do you have any confidence that the number it spits out is accurate in the context of using it as a measurement device for determining compressor match points?

(I get a kick out of it when chaps say "My stock MAF says I went from 40.27 lb/min to 40.31 on that pass!! On my compressor map that means my compressor efficiency dropped from 67% to 65! Wow!! I need a new turbo!" )

 

Nope. I deal with what I have available to me in my datalogs. And I correlate and calibrate the readings as best I can. I don't need "perfect" when "good enough" works so damn well.

The only reason I did this:


was to satisfy some curiosity. Nothing more. I don't actually use that plot for tuning the car. So it doesn't really matter.

Calculating VE would probably be a bit more useful for before/after comparisons for mods.