What determines mass air flow?
05-22-2005, 06:45 PM
#1
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What determines mass air flow?
When looking at the g/s figures from MAF readings, what is the determinant (or what are the determinants) of the flow readings?
Is flow something that is in some way "requested" by the ECU, or is the flow simply a function of the flow capacity of the system itself (i.e. straight pipes = greater flow potential than a car with a restrictive cat-back system).
Any input much appreciated...
Is flow something that is in some way "requested" by the ECU, or is the flow simply a function of the flow capacity of the system itself (i.e. straight pipes = greater flow potential than a car with a restrictive cat-back system).
Any input much appreciated...
05-22-2005, 10:44 PM
#2
are you thinking of ways to maximize the readings? You might want to log your
load and duty cycle to see if they are also limiting your boost (or capping off the maf readings) since the maf readings is a function of boost and rpm - and boost is controlled by load requirements specially on the dbw cars.
In other words, you might install a free flow system somewhere (intake path, exhaust path) thinking you'll get higher maf readings, only to find out that it might have gone lower. But overall, you might be making the same power but just more efficiently, so the software programming told the turbos to chill a bit.
In other words, you might install a free flow system somewhere (intake path, exhaust path) thinking you'll get higher maf readings, only to find out that it might have gone lower. But overall, you might be making the same power but just more efficiently, so the software programming told the turbos to chill a bit.
05-23-2005, 07:38 AM
#3
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Got it.
How would I tell if the load was limiting boost or capping MAF radings (or is this a question for the VAG forum?)? I took my readings while accelerating WOT up an uphill onramp onto a flat section of freeway, and see engine load numbers as high as 192. Not sure what this indicates - I'll try to do some searches.
I asked the original question because I know some other drivers who are running the same setup as I am, same software, same hardware (except they have milltek cats whereas I have magnaflow / carsound cats). They are making more power, though, and their g/s readings are closer to 300 whereas mine top out at about 270 - 280. When looking at my VAG logs, both suggested something might be wrong with my MAF, because they were getting higher readings (I swapped my MAF out with a new one (borrowed) and got the same readings).
Some others have suggested that the difference in power might be due to the cats being too restrictive. This got me wondering if there might be a connection, and hence the question about what determines air flow. Given your post, if the air flow is a function of boost and RPM, could it be possible that the ECU is detecting back-pressure from the cats, therefore scaling back boost a tad bit, resulting in lower air flow?
It's funny - here I am with the ultimate aim of figuring out why the car's not making as much power as it's sister cars, but I seem to be getting hung up on MAF readings. Maybe I'm just barking up a totally wrong tree...
Anyhow, thanks for your earlier answer,
C.
I asked the original question because I know some other drivers who are running the same setup as I am, same software, same hardware (except they have milltek cats whereas I have magnaflow / carsound cats). They are making more power, though, and their g/s readings are closer to 300 whereas mine top out at about 270 - 280. When looking at my VAG logs, both suggested something might be wrong with my MAF, because they were getting higher readings (I swapped my MAF out with a new one (borrowed) and got the same readings).
Some others have suggested that the difference in power might be due to the cats being too restrictive. This got me wondering if there might be a connection, and hence the question about what determines air flow. Given your post, if the air flow is a function of boost and RPM, could it be possible that the ECU is detecting back-pressure from the cats, therefore scaling back boost a tad bit, resulting in lower air flow?
It's funny - here I am with the ultimate aim of figuring out why the car's not making as much power as it's sister cars, but I seem to be getting hung up on MAF readings. Maybe I'm just barking up a totally wrong tree...
Anyhow, thanks for your earlier answer,
C.
05-23-2005, 04:53 PM
#4
what parameters have you logged? I can take a look and see if there's something there..
I don't think the ECU can detect back pressure per se, but any factors related to it will be measured and compensated for. For example, excessive back pressure can increase EGT. The 2.7T engine has an EGT sensor. It may be backing off boost and richening the mixture if high EGT's are obtained. Another thing to log I guess.
Another thing, excessive back pressure can cause higher or richer O2 readings since the present mixture is stuck a bit longer than something that will flow out more freely.
Excessive back pressure may cause higher engine temps. Anything more efficient means less heat since there is less work required to produce the same result. So, with higher coolant temps, ECU might scale back timing and boost.
This is pure speculation of course, but just pointing a few of possible many factors monitored by the ECU and adjusting performance based on that.
To tell what your load is and if it's capping off your N75 duty cycle (lower duty means lower boost) is to measure Spec load, Load Correction and Actual Load. Spec load IMO should be configured by the chip tuner. Load correction is the enemy - or the ECU sees something not right and then backs off the load from spec. Actual is the end result.
You should see the N75 corresponding well with Actual load. If your N75 duty cycle is lower than other folks, then you know that your ECU is cutting back somewhere.
If your N75 duty cycle looks the same or higher than comparable setups, yet you are getting lower MAF readings that might correspond to lower boost (also log the press @ intercooler or something) then it could be a few factors such as:
- your wastegates probably need fine tuning. I don't recommend you play with this stuff specially not knowing the target pressures vs wastegate opening the ECU software is programmed to. Typically it's 5-8 psi for fully open wastegates. This is why manual boost controllers make sense
- Something is causing your turbos to spin slower. Assuming your N75 is 100%, meaning wastegate alignment is not a factor - unless they are grossly off causing seepage. Then, something is hindering the spool. This could be backpressure, worn out turbos, heavy oil, not enough energy from the exhaust - many factors influence that (A/F, timing, etc)
- assuming your turbos are in full spool, and they are creating the proper pressure ratio (not just manifold pressure, but pressure ratio), then something else is causing the drop in manifold pressure. This could be leaks in the intake plumbing, a restrictive intake, excessive intake temps, etc.
Another thing, excessive back pressure can cause higher or richer O2 readings since the present mixture is stuck a bit longer than something that will flow out more freely.
Excessive back pressure may cause higher engine temps. Anything more efficient means less heat since there is less work required to produce the same result. So, with higher coolant temps, ECU might scale back timing and boost.
This is pure speculation of course, but just pointing a few of possible many factors monitored by the ECU and adjusting performance based on that.
To tell what your load is and if it's capping off your N75 duty cycle (lower duty means lower boost) is to measure Spec load, Load Correction and Actual Load. Spec load IMO should be configured by the chip tuner. Load correction is the enemy - or the ECU sees something not right and then backs off the load from spec. Actual is the end result.
You should see the N75 corresponding well with Actual load. If your N75 duty cycle is lower than other folks, then you know that your ECU is cutting back somewhere.
If your N75 duty cycle looks the same or higher than comparable setups, yet you are getting lower MAF readings that might correspond to lower boost (also log the press @ intercooler or something) then it could be a few factors such as:
- your wastegates probably need fine tuning. I don't recommend you play with this stuff specially not knowing the target pressures vs wastegate opening the ECU software is programmed to. Typically it's 5-8 psi for fully open wastegates. This is why manual boost controllers make sense
- Something is causing your turbos to spin slower. Assuming your N75 is 100%, meaning wastegate alignment is not a factor - unless they are grossly off causing seepage. Then, something is hindering the spool. This could be backpressure, worn out turbos, heavy oil, not enough energy from the exhaust - many factors influence that (A/F, timing, etc)
- assuming your turbos are in full spool, and they are creating the proper pressure ratio (not just manifold pressure, but pressure ratio), then something else is causing the drop in manifold pressure. This could be leaks in the intake plumbing, a restrictive intake, excessive intake temps, etc.
05-23-2005, 07:22 PM
#5
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I've only logged groups 002, 003, 020, 021, and 115
Kinda new to the VAG, and still learning what I can measure. If you want to look at them, here's the csv log (right click / save as) for <a href="http://audifahrer.com/aw/vag/002-003-115.csv">002/003/115</a>, and here' the log for <a href="http://audifahrer.com/aw/vag/020-021-115.csv">020/021/115</a>.
I'll do some searching to see how to log the other values you mention in your post - EGTs, O2 readings, N75 duty, spec, actual, and corrected load, and pressure at intercooler. Even if I don't get to the heart of this, at least I'll learn better logging skills.
Anyhow, I'll let you know what I come up with after I figure out how to log it, and thanks, again, for your input. It's much appreciated...
I'll do some searching to see how to log the other values you mention in your post - EGTs, O2 readings, N75 duty, spec, actual, and corrected load, and pressure at intercooler. Even if I don't get to the heart of this, at least I'll learn better logging skills.
Anyhow, I'll let you know what I come up with after I figure out how to log it, and thanks, again, for your input. It's much appreciated...
05-23-2005, 09:43 PM
#6
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Did some more logging...
To get the load readings alongside the N75 readings, I logged group <a href="http://audifahrer.com/aw/vag/114-118-119.csv">114/118/119</a>. Not terrific at reading these, but my guess would be that things look okay - corrected load it always close to spec'd, and the duty cycle seems to correspond with the actual load.
To measure intake temps, I logged <a href="http://audifahrer.com/aw/vag/002-011.csv">002/011</a>. To try to get some O2 readings, I logged <a href="http://audifahrer.com/aw/vag/031-032-033.csv">031/032/033</a>, <a href="http://audifahrer.com/aw/vag/034-035-036.csv">034/035/036</a>, and <a href="http://audifahrer.com/aw/vag/037-038-039.csv">037/038/039</a>. Finally, to try to get EGT readings, I logged <a href="http://audifahrer.com/aw/vag/043-044.csv">043/044</a> and <a href="http://audifahrer.com/aw/vag/111-112.csv">111/112</a>.
Honestly, at this point, I no longer know what I'm looking at or what I'm even looking for! Because I don't know the standard against which to measure these readings, I can't really make any sense of them. Anyhow, FWIW, not sure if any of this could be used to help me figure out what's behind the lower MAF readings, much less why I'm dyno'ing less power than other cars with the same setup.
Nevertheless, it is what it is. If you've got any inclination to serve as a translator for any of this, I'd appreciate it, but would of course understand if you've got better things to do. You've already helped me out a lot with your last post - thanks again for that.
Take it easy...
C.
To measure intake temps, I logged <a href="http://audifahrer.com/aw/vag/002-011.csv">002/011</a>. To try to get some O2 readings, I logged <a href="http://audifahrer.com/aw/vag/031-032-033.csv">031/032/033</a>, <a href="http://audifahrer.com/aw/vag/034-035-036.csv">034/035/036</a>, and <a href="http://audifahrer.com/aw/vag/037-038-039.csv">037/038/039</a>. Finally, to try to get EGT readings, I logged <a href="http://audifahrer.com/aw/vag/043-044.csv">043/044</a> and <a href="http://audifahrer.com/aw/vag/111-112.csv">111/112</a>.
Honestly, at this point, I no longer know what I'm looking at or what I'm even looking for! Because I don't know the standard against which to measure these readings, I can't really make any sense of them. Anyhow, FWIW, not sure if any of this could be used to help me figure out what's behind the lower MAF readings, much less why I'm dyno'ing less power than other cars with the same setup.
Nevertheless, it is what it is. If you've got any inclination to serve as a translator for any of this, I'd appreciate it, but would of course understand if you've got better things to do. You've already helped me out a lot with your last post - thanks again for that.
Take it easy...
C.
05-24-2005, 05:31 PM
#7
Pretty good results. How did you obtain these? Cruise around 3000 rpm and then punch it while
logging? I'll play with them tonight. At a glance, spec load matches actual load as you observed, requested boost matches actual boost - indicating good harmony between the mechanicals and software (wastegate settings, N75 operation, and ECU control).
If you think your actual boost is the same as your comparison, then the lower MAF readings could indicate a difference in pressure ratio.
Here's a simplified example.
Let's take a pressure ratio of 2:1
That means if pressure at the intake of the turbo is 14.5 (atmosphere), then the output is double that or 29psi. Manifold pressure is lower than this because of thermodynamics caused by anything in between the output and the manifold. Let's just say you measured 25 psi at the manifold, and that's what the ECU is targetting.
Now, Let's put restriction at the intake of the turbo. This drops effective atm pressure at the turbo inlet. Let's just say it drop by 1 psi. Now the outlet of the turbo is 27 psi. Let's say the manifold is measured at 23 psi. The ECU will then demand for more because it's trying to make 25 psi (spec load, spec psi). So, it will spin the turbos a bit faster. Now, the pressure ratio is not 2:1 because the turbo is spinning faster to achieve 29 psi. So 29/13.5 = 2.15:1
All other things being constant (ie timing, A/F), then the car is making the same power as the one with no pressure drop at the intake. What happens with the MAF? Look at turbo graphs, and it's obvious that you'll be sucking in more air at 2.15:1 vs 2:1. Again, making equal power.
It's not that simple since higher pressure ratios mean higher output temps, lower efficiency range, additional compensation to meet spec loads, etc. But hope you get the idea.
Now don't start stuffing velocity stacks in front of your turbo inlets. It's still better to have filtered air for longetivity
If you think your actual boost is the same as your comparison, then the lower MAF readings could indicate a difference in pressure ratio.
Here's a simplified example.
Let's take a pressure ratio of 2:1
That means if pressure at the intake of the turbo is 14.5 (atmosphere), then the output is double that or 29psi. Manifold pressure is lower than this because of thermodynamics caused by anything in between the output and the manifold. Let's just say you measured 25 psi at the manifold, and that's what the ECU is targetting.
Now, Let's put restriction at the intake of the turbo. This drops effective atm pressure at the turbo inlet. Let's just say it drop by 1 psi. Now the outlet of the turbo is 27 psi. Let's say the manifold is measured at 23 psi. The ECU will then demand for more because it's trying to make 25 psi (spec load, spec psi). So, it will spin the turbos a bit faster. Now, the pressure ratio is not 2:1 because the turbo is spinning faster to achieve 29 psi. So 29/13.5 = 2.15:1
All other things being constant (ie timing, A/F), then the car is making the same power as the one with no pressure drop at the intake. What happens with the MAF? Look at turbo graphs, and it's obvious that you'll be sucking in more air at 2.15:1 vs 2:1. Again, making equal power.
It's not that simple since higher pressure ratios mean higher output temps, lower efficiency range, additional compensation to meet spec loads, etc. But hope you get the idea.
Now don't start stuffing velocity stacks in front of your turbo inlets. It's still better to have filtered air for longetivity
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05-24-2005, 07:35 PM
#8
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Actually, started logging when very slow in second, and punch it to redline in third...
I have an uphill onramp to a flat section of freeway (no merge - independent lane). At about 10MPH at the base of the onramp in second, I just started logging and floored it through third. Back and forth, back and forth.
I "think" I get what you're saying.
If I understand you correctly, increased air flow could in principle actually be the result of corrections implimented to counteract the effect of restriction. If this is true, then lower air flow measurements could potentially indicate greater system efficiency, assuming that power output was the same. Therefore, it can be problematic to assume that lower air flow readings indicate poorer performance, since, ultimately, airflow is a function of the system working to maintain targeted boost / pressure. Is this right?
If so, then in a way I'm back to the drawing board to figure out why I'm lacking 35HP in our dyno comparison. I never knew getting a car to run right could be such a bug hunt!
Anyow, thanks again for the illumination. If nothing else, it's really educational for me, and I apprecaite it!
I "think" I get what you're saying.
If I understand you correctly, increased air flow could in principle actually be the result of corrections implimented to counteract the effect of restriction. If this is true, then lower air flow measurements could potentially indicate greater system efficiency, assuming that power output was the same. Therefore, it can be problematic to assume that lower air flow readings indicate poorer performance, since, ultimately, airflow is a function of the system working to maintain targeted boost / pressure. Is this right?
If so, then in a way I'm back to the drawing board to figure out why I'm lacking 35HP in our dyno comparison. I never knew getting a car to run right could be such a bug hunt!
Anyow, thanks again for the illumination. If nothing else, it's really educational for me, and I apprecaite it!
05-26-2005, 04:08 PM
#9
35 hp is huge. Although I don't know if that was a back to back comparison
with other cars on similar equipment - including wheel and tire weight (wheel inertia) and pressures. However, if the other cars do make 30 g/s more than yours at the same RPM's (I think yours is slightly under 6000 rpm), then that's a sign that they must have a higher boost request to fulfill. It's hard to imagine since I think your duty cycle is already quite high, and your injectors are fully open.
I crashed my home system yesterday and I already started playing with your logs. I can't do much at work right now. I'll be sure to get my system up and play with numbers on the weekend. Have a great memorial day. To me, it's a trip to Fry's Electronics >(
If you are curious to see if you can get your duty cycle number higher, you can try logging your right foot vs throttle angle, and see if you get the spec load and corrected load numbers higher with backing off the accelerator a bit
I crashed my home system yesterday and I already started playing with your logs. I can't do much at work right now. I'll be sure to get my system up and play with numbers on the weekend. Have a great memorial day. To me, it's a trip to Fry's Electronics >(
If you are curious to see if you can get your duty cycle number higher, you can try logging your right foot vs throttle angle, and see if you get the spec load and corrected load numbers higher with backing off the accelerator a bit
05-26-2005, 06:40 PM
#10
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Dude, you're too good to me.
Thanks for your willingness to check out my numbers. Mui cool of you.
Good luck at Frys (I feel your pain) and a great memorial day to you as well. I might do what you say and check the numbers with less throttle - it IS weird - sometimes I feel like I get more power when I don't put the pedal all the way down.
Anyhow, have a good weekend - catch you on the rebound....
Good luck at Frys (I feel your pain) and a great memorial day to you as well. I might do what you say and check the numbers with less throttle - it IS weird - sometimes I feel like I get more power when I don't put the pedal all the way down.
Anyhow, have a good weekend - catch you on the rebound....