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 Post subject: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Wed Jun 09, 2010 3:51 am 
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HI,

I've seen 2 ways to compute for it. 1st is via Freon's formula:

VE = MAF(g/sec) / (2.457* A * RPM * 0.003871098 / (IAT + Temp))
A = (boost pressure + sea level pressure)/(14.7*760)
Temp = IAT in Celsius + 273.5
or in Fahrenheit it's (5/9)*(IAT-32)+273.15

Other is the one given to me

=Mass Air flow g/s/(0.9007*(Atmospheric pressure+mrp)*RPM/(Intake Air temp F+459.67))

The latter just computes lower a bit.

Question now is how do you know if your VE is already ok? Is going above 1 not good? If your boost is already maxed but VE is not, how do you increase VE? AVCS?

Attached is my log via OP2 Can with the 2 VE computations.


Here is the graph (using ecuedit) showing Freon's VE, MRP(corrected), RPM vs Time. I used time since OP2 can logs so fast that it causes rpm noise.

Image
Y1 is rpm(green), Y2 is mrp (blue), Y3 is red VE.

Will adding or reducing avcs help? Btw, this is not a full log til red line as I ran out of road.

VF52, avo v2 tmic, tgv delete, 8mm phenolic spacers,650cc injectors, FXT 09 edm 4eat.


Attachments:
3rd gear log1588 May 20 2010 Overboost.xls [161.5 KiB]
Downloaded 182 times
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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Wed Jun 09, 2010 4:53 am 
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I will log with AVCS right and left next time.

Here are more info based on the log posted. Y is RPM, X is Engine Load

MRP corrected
Image

VE
Image

Ignition Total Timing
Image


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Wed Jun 09, 2010 10:34 am 
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Location: Broomfield, CO
VE can be increased with AVCS tuning or things like head porting/cams.
A value over 1 is not bad per say, in fact higher values are always better. A value over one simply means one of two things: 1) Your MAF scaling is not 100% accurate, and is actually higher than it really should be because something like your injector scalar is off and you are compensating for it, or 2) your motor is actually sucking in more air than would be physically possible under steady state conditions, which can be achieved using careful runner length/size and cylinder timing.

I'm not sure why the two equations give different answers, mine is simply using the ideal gas law (it looks like Freon's is as well). For anybody who's interested, the derivation is below:

PV=mrT
P is absolute pressure in psi
V is the volumetric flow rate in L/sec
m is the mass flow rate in g/s (aka n/M, moles/s divided by the molecular weight of air)
T is the temperature in R
r is the gas constant, modified a bit to handle these unique units

Left side of the equation:
MAP (psia) * 2.5/2 (L/rev) * RPM (rev/min) * 1/60 (min/sec) = MAP * 2.5 * RPM / (2*60) (psia*L/sec)

Right side of the equation:
MAF (g/sec) * r (psia*L/(mol*R)) * 1/28.9645 (mol/g) * (T+459.67) (R) = MAF * r * (T+459.67) / 28.9645 (psia*L/sec)

r = 8.314472e-2 (L*bar/(mol*K)) * 1/1.8 (K/R) * 14.503 (psi/bar) = .669915 (psia*L/(mol*R)

Plugging it back in you get:

MAP * 2.5 * RPM / 120 = MAF * .669915 * (T+459.67) / 28.9645

Solving for MAF gives you

MAF = MAP * 2.5 * RPM * 28.9645 / (120 * .669915 * (T+459.67))

or

MAF = .9007 * MAP * RPM / (T+459.67)
Where MAP is the absolute pressure in psia and T is the IAT in F.


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Wed Jun 09, 2010 7:12 pm 
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From a post a made a while back:
Volumetric Efficiency = Actual Volumetric Flow / Theoretical Airflow x 100

Seems simple, but there are quite a few equations needed to calculate actual volumetric flow and theoretical airflow.

What logged data is required:
Engine Speed (rpm)
Mass Airflow (g/s)
Intake Temp (F)

Abbreviations:
RPM = Engine Speed (rpm)
MAFa = Mass Airflow (g/s)
IAT = Intake Temp (F)
MAFb = Mass Airflow (lbs/min)
IAD = Intake Air Density (lbs/ft3)
AVF = Actual Volumetric Flow (ft3/min)
TAF = Theoretical Airflow (ft3)
VE = Volumetric Efficiency (%)

Calulated values and excel equations:
MAFb =MAFa*0.13224
IAD =(491.67/(459.67+IAT))*0.0808
AVF =MAFb/IAD
TAF =(346*(RPM))/3456
VE =(AVF/TAF)*100


Example - using the equations above:
RPM = 5446
MAFa = 285.27
IAT = 72

MAFb = MAFa*0.13224
MAFb = 285.27*0.13224
MAFb = 37.72

IAD = (491.67/(459.67+IAT))*0.0808
IAD = (491.67/(459.67+72))*0.0808
IAD = 0.0747

AVF = MAFb/IAD
AVF = 37.72/0.0747
AVF = 504.86

TAF = (346*(RPM))/3456
TAF = (346*(5446))/3456
TAF = 545.23

VE = (AVF/TAF)*100
VE = (504.86/545.23)*100
VE = 92.60%


Leslie

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Current Car: 2002 ADM WRX STi
Current Engine: EJ207
Current Mods: X-Force 3" TBE Exhaust, GCG "bolt-on" GT3076R, APS 3" Hard Turbo Inlet, Short Ram Pod, RomRaider/ECUFlash Tune
Current Power: 248kw@wheels (332whp)


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 1:54 am 
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Leslie,

Im surprised that boost is not part of the equation (like freon and suicidal eggroll). Also, can you post the link where this was earlier discussed?

Thanks guys for all the inputs. Hope to see MickeyD and some other guys share their thoughts on this especially how to tune avcs to make higher VE.

Regards


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 3:26 am 
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Consider the true form of VE, being the difference between actual volumetric flow rate and theoretical volumetric flow rate. Now consider boost, MRP and/or absolute pressure are not units used for measuring volume or flow - they measure pressure.

I suspect their calculations use absolute pressure to help calculate air density, but I am not exactly sure - They'll need to comment on that.

Leslie

_________________

Current Car: 2002 ADM WRX STi
Current Engine: EJ207
Current Mods: X-Force 3" TBE Exhaust, GCG "bolt-on" GT3076R, APS 3" Hard Turbo Inlet, Short Ram Pod, RomRaider/ECUFlash Tune
Current Power: 248kw@wheels (332whp)


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 3:36 am 
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wrxsti-l wrote:
Consider the true form of VE, being the difference between actual volumetric flow rate and theoretical volumetric flow rate. Now consider boost, MRP and/or absolute pressure are not units used for measuring volume or flow - they measure pressure.

I suspect their calculations use absolute pressure to help calculate air density, but I am not exactly sure - They'll need to comment on that.

Leslie


I'll use your formula and see what I get. Thanks!


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 10:53 am 
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Joined: Mon Oct 06, 2008 8:05 pm
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Location: Broomfield, CO
wrxsti-l wrote:
Consider the true form of VE, being the difference between actual volumetric flow rate and theoretical volumetric flow rate. Now consider boost, MRP and/or absolute pressure are not units used for measuring volume or flow - they measure pressure.

I suspect their calculations use absolute pressure to help calculate air density, but I am not exactly sure - They'll need to comment on that.

Leslie


Yes, the MAP is used to calculate air density, how can you calculate air density without it? I assume you're just using atmospheric pressure?

The question is where you draw your box, are you calculating the VE of the engine itself, or the VE of the entire system? Some people just calculate the VE of the entire system, but this doesn't tell you anything useful in a FI motor because your VE really just becomes a function of boost, not engine VE, and any fluctuations in MRP will affect your calculated "VE". Essentially, you get a number, but you can't use this number for anything useful.

If you instead calculate the VE of the motor itself, pulling the turbo out of the box, you must use MAP in your calculation to get the correct air density.

Take your set of equations:
RPM = 5446
MAFa = 285.27
IAT = 72
gives you a VE of 92.6%

Now say you raise the boost, and at the same RPM and IAT your MAFa rises to 350 g/s. Your equation shows the VE going to 113.6%. Your engine's VE didn't go up at all though, the VE of the entire system did, but only because you raised the MAP.

That equation would work fine for an N/A car, but throw FI into the mix and it becomes pretty useless. Make a pull, calculate VE, make another pull, recalculate VE, and you will end up with two completely different values depending on how early the motor spooled up and if you overboosted or if the boost oscillated at all.


I'd like to know where you got your equations for IAD and TAF, and what assumptions are being made in those numbers. Pressure has to come in at some point, even if it's just atmospheric pressure, but again it appears an assumption about your current atmospheric pressure is also being made. I also see no mention of engine displacement, are you assuming a 2.5L engine? Based on your results, it looks like you're assuming something much larger, because a 2.5L motor will not be drawing in 285 g/s at atmospheric pressure at 5400 RPM with a 92.6% VE, it would be more like 120 g/s. Based on my calculations, those equations are for a naturally aspirated 5.7L v8 at sea level.


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 9:23 pm 
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We've done some changes to avcs and boost already before reaching to where it is now. Have I reached the full potential or I can still make it go 1 all the way? Getting a VE closer to 1 or 1 plus translates to whp and torque or its nothing more than just VE? So whats the point of getting higher VE then?

AVCS tuning info is really scarce, hope to get more inputs about it.

Suicidal E, thanks for the info and time in explaining. Appreciate it :-)


Last edited by MFB on Fri Jun 11, 2010 1:19 am, edited 1 time in total.

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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Thu Jun 10, 2010 10:55 pm 
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I can't quite remember exactly where I got all the formulas from man. Some where from text books and others were from web articles etc.

After reading your post, I agree that the way I presented the VE calculation I use is not incorporating MAP properly and is only suitable for measuring VE in NA applications. I'm pretty sure I can slot MAP into it in place of one of the calculations to suit measuring VE in FI applications.

I'll have to revise the calculations used and will post again later.

_________________

Current Car: 2002 ADM WRX STi
Current Engine: EJ207
Current Mods: X-Force 3" TBE Exhaust, GCG "bolt-on" GT3076R, APS 3" Hard Turbo Inlet, Short Ram Pod, RomRaider/ECUFlash Tune
Current Power: 248kw@wheels (332whp)


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Fri Jun 11, 2010 10:39 am 
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Location: Broomfield, CO
MFB wrote:
We've done some changes to avcs and boost already before reaching to where it is now. Have I reached the full potential or I can still make it go 1 all the way? Getting a VE closer to 1 or 1 plus translates to whp and torque or its nothing more than just VE? So whats the point of getting higher VE then?


VE is a measure of how well your engine is breathing.
A 2.0L motor has .5L of displacement per piston. Under steady state conditions, each cylinder will be filled up with exactly .5L of air at whatever pressure and temperature is in the manifold. Knowing the volume, pressure, and temperature, you can calculate the mass of the air. Let's say for a quick example that this value is 5 grams.

Now our engines do not work under steady-state conditions, obviously, because if they were steady-state, it means they wouldn't be running. In the real world, the air does not have an infinite amount of time to blow into the cylinders. It has a very limited amount of time, and it depends on RPM. Let's say that at 5000 RPM the air only has 10 milliseconds to enter the cylinder, now if it had an infinite amount of time, all 5 grams of air would be able to enter the cylinder, but how much of that 5 grams is able to get into the cylinder in only 10 milliseconds before the intake valves close and the air supply is cut off? That's what the VE tells you, a 90% VE means you're only getting 4.5 of the available 5 grams into the cylinder. A 99% VE means you're getting 4.95 grams, etc. Increasing the size of the intake runners, or the intake valves, increasing the lift on the intake valves, or increasing the length of time that the intake valves are open, are all things that can increase your VE.

It is possible to get over 100% VE under certain conditions, the car can do this with a very well designed intake manifold and cylinder pulse timing...essentially the air in the manifold builds up momentum and it blows into the cylinder so fast that the air in the cylinder is actually at a slightly higher pressure than the air in the manifold. Obviously this couldn't last, and given some more time some of that air would then blow back into the manifold, but the system is timed such that the intake valves close before this happens, so you actually get more air in the cylinders than would be possible under steady state conditions.

Increasing your VE is pretty much guaranteed HP, close to 1:1. Increase your VE by 10% and you'll increase your whp by close to 10%. It's similar to raising your boost...all raising the boost does is help to blow more air into the cylinders, same as increasing VE. In fact you can increase your VE and turn down your boost and end up with a tune that makes the same amount of power, but with less stress on the turbo. I recently built my motor, and as part of that I installed cams, valves, valve springs, etc. Looking at the before/after logs, you can see that my engine is now flowing the same amount of air at 15 psi, as I was before the motor build at 19 psi, pretty much all of that can be attributed to the cams.

AVCS is a way to adjust your VE at low RPM. It can help with around-town drive ability and spool up time, but it won't help you with WOT power, because people have shown time and time again that you make the most power with AVCS set to 0 after you pass peak torque. The easiest way I've seen to adjust AVCS is pretty straightforward. You set your entire AVCS table to 0, go make 3 pulls in some gear. Set your entire AVCS table to 5, go make 3 pulls, then 10 degrees, then 15, 20, 25, etc. Eventually you'll have 3 WOT logs for each AVCS advance, then you compute the average power (or VE, but see below for some caveats with doing that) for the 3 runs, and overplot them on each other. What you'll have at the end is a HP vs RPM graph with 6-7 curves on it, showing the HP for each AVCS advance value. Then you simply pick and choose. At 1000 RPM, which AVCS advance value made the most power, and use that one. At 2000 RPM, which AVCS advance value made the most power, etc. You'll probably see that at low RPM it's the high AVCS advance that are making the most power, but as you approach 5000 RPM it will trail off and the lower advance values will make more power, until above 5k RPM it's the 0 advance table that's making the most power.

Now as for watching VE when tuning AVCS, ideally you could do this, but there is a problem. With AVCS you're changing how soon the intake valves open/close. A 30 deg advance means the intake valves are opening 30 degrees before normal, and closing 30 degrees before normal. At very large advances, you can actually have times when both the intake and exhaust valves are open, at the same time. When you're in vacuum at low RPM, this can help things, unburned air in the exhaust can recirculate back into the intake manifold and then back into the cylinder, which helps you get more air in there than you would normally. When you're in boost at high RPM, you can actually have the opposite situation, where air from the manifold literally blows through the intake valves, through the exhaust valves, and straight into the exhaust. The air is basically being wasted, it's not being burned, it's just being blown into the exhaust. According to your VE, things are good, you're flowing a ton of air into the cylinders, but the VE doesn't take into account the possibility for that air to actually just blow straight through into the exhaust. This is why you want to look at power, because VE might not be telling you the whole story.


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Sat Jun 12, 2010 9:20 pm 
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That's quite a handful but info very much appreciated. So far, most detailed info yet on RR about AVCS tuning!

Whats the formula to compute for power based on logs?

So AVCS values on higher load also work the same way with 0 being value for rpm past 5000? I've seen aggressive tables with 7 5 and 2 in past 5000 rpm. My current avcs table is like that as well but will try zeroing it out as per your suggestion.


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Sun Jun 13, 2010 11:06 am 
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MFB wrote:
That's quite a handful but info very much appreciated. So far, most detailed info yet on RR about AVCS tuning!

Whats the formula to compute for power based on logs?

So AVCS values on higher load also work the same way with 0 being value for rpm past 5000? I've seen aggressive tables with 7 5 and 2 in past 5000 rpm. My current avcs table is like that as well but will try zeroing it out as per your suggestion.


Power is quite a bit more complicated since you have to take the car's drag coefficient into account. I would just use Airboy's spreadsheet, it seems to work well. Although if you're just comparing the back to back power, all on the same car, all in the same gear, you could get a nice corollary using the rough acceleration rate (dRPM/dT), though you might want to smooth it a bit.

From what I've seen, most people get the best result with 0 advance at high load, high RPM. Your car could be different though, you might in fact pick up a couple of HP with a 2-3 deg advance up there. I was just explaining why you wouldn't want to use 30 deg advance at high load and high RPM, for example.


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 Post subject: Re: How to compute for VE? How to improve? Over 1 is bad?
PostPosted: Mon Jun 28, 2010 10:53 pm 
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How is having a better flowing exhaust related to avcs advance? Less back pressure means...?
Other than the dyno, can ve log and improvement of acceleration be a good measure for avcs tuning?


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