I've been looking around at the VE settings for the LT1.

In looking at the VE tables, I got the bright idea of trying to calculate the VE of my engine at a particular RPM by looking at the data from my Autotap logs. Using the data from several sources(good one here.....);

http://www.installuniversity.com/ins...n_9.012000.htm

I did calculate the VE of my engine(when it was stock) at 800 rpm.

Now, I went to the tables in the computer and got a shock. I expected the value of 800 rpm @ ~30 kpa to be close to the value I calculated(which was around 17%). The value in the table was ~65.

So....my question is; what am I missing? I was hoping to be able to roughly calculate out the VE of my engine with the CC503 by using the sensor data of the Autotap logs. However this has confused me. How is the VE calculated for the ECM VE table?

 

You calculated that the cylinders were only filling up 17% at 800rpm?

 

Using the webpage above and this data...calculate for VE.

RPM = 800
MAF = 0.998 lb/min
IAT Temp = 91 F

(459.67 +32)/(459.67+91) = d2/0.0808
491.67/550.67 = d2/0.0808
0.8929 = d2/0.0808
d2 = 0.0721

AVF = 0.998 / 0.0721
AVF = 13.8419

TAF = ((350)(800)(1))/((2)(1728))
TAF = 280000/3456
TAF = 81.0185

VE = (AVF/TAF)(100%)
VE = (13.8419/81.0185)(100%)
VE = 17.08%

So....the MAP at 0.998lb/min was ~31kpa. Looking at the VE table in the topic for 800rpm@30kpa I was expecting around 17 as the value....it was not. It was 65.

I really feel like I'm missing or misunderstanding something about the VE table.

 

Here's another site for calculating VE w/o the IAT factor ?? , I'm using tunercat MAS = at about 10 gms/sec at 800 rpm. = 600 gms/min = 1.3 lb/min = 16 cubic ft/ min. If I crank my numbers it comes out around 19.9%.. way off from the tables.. " http://www.ajdesigner.com/phpengine/...efficiency.php " There must something we are missing from the tables..

 

Your air flow looks right.... 0.998 #/Hr is 7.3 Gr/Sec. I have a spreadsheet I built for analyzing VE. When I put in the number you posted ( 0.998 #/Min, 9.0"Hg (31kPa), 91*F) I get a VE of 56.4%

Appears you corrected only for absolute temperature. You also need to correct for MAP, which would triple your calculated VE. If you corrected for MAP, you new air mass should be 0.02166 #/CF

 

Injuneer,

Is your Excel sheet based around this?

Volumetric Efficiency:

It was determined by John Lamb that the value is defined by the following relationship.




VE = ((MAF in g/s)/ (RPM/120 * Air Density * 5.733))


Air density can be determined using:

(MAP – Vapor pressure(H2O))
---------------------------
0.287 * IAT(in Kelvin)


I got the above from here....
http://www.akmcables.com/LT1Edit_Glossary.htm


I'm trying to find a way to calculate the new VE of my engine using sensor data.....I'm finding out that it is not cut and dry at all.

 

Calculate the density of the air using the following formula.

Density = kpa/(0.287*temp(kelvin) you will get kg/cubic meter. The pressure is the barometric pressure the car reports, usually around 100, 101.3 is sea level for this. Use your iat reading in C+273 to get it in kelvin.

Convert this to specific volume by dividing 1 by the density.

Take your maf readings, and divide by 1000 to make g/sec into kg/sec.

Multiply the specific volume (m^3/kg) times mass flow (kg/sec) and you get volumetric flow rate (m^3/sec)

Now forget all that for now.

Take your engine size in cubic inches as closely as you can convert it to cubic meters.

Now take the rpm and divide it 120 to get intake cycles per second. Multiply this value times your cubic meter number and you have now calculated the 100% volumetric flow value for your engine.

Now take the value I told you to forget that you calculated from the maf reading and divide it by the 100% value and you have now calculated ve for your engine. Multiply by 100 of course to take it from decimal to %.

Doesnt get much more cut and dried than that. The other way to do it is to find the mass flow rate the engine would be capable of at 100% and use the maf readings directly. Alot of engineers I work with prefer the ve mass method rather than the volume method, I showed you volume because its easier for most people to think in terms of volume. They like to measure fuel flow rate and air fuel ratio on the gas bench to calculate ve mass and thermal efficiency and a whole bunch of other things nobody cares about besides engineers.

Btw unless your long term fuel trims are 128 for both sides your maf is full of bull and you will have error in your calculation.

The ve tables are not relevant for your engine unless you have recalibrated them to match your modifications. If you want to make a new map of it just set it into speed density and use ve master. Its way way labor intensive to sort through a bunch of exported datamaster stuff to determine workable rough ve table based on mass airflow.

All you have to do to solve this or other thermodynamic type problems is to determine what your next move by keeping track of the units you have currently and where you are trying to go with them. Just keep the definitions in mind and what information you have and you will complete it every time quickly and correctly.

Hope I didnt bore you too much .

 

LOL!

One of my shortcomings is thermodynamics! DOH! I had a couple of classes that were geared towards this stuff, but electricity is my bread and butter. When it comes to the programming, of this ECM, I've now got to learn some of this stuff.

I don't have a datamaster, but how accurate is it when it comes to making a new VE table? Will a datamaster work on ODBII?

I have found so many formulas floating around on various boards for VE.....none of them are working like I expected. I do wonder about my data logs too(Autotap, GM enhanced parameters). At this point, when taking sensor data value, I think I need to get my data from a steady state value for a few seconds(ie holding at 2500 rpms for 10 seconds). When I go modulating the throttle, I'm noticing all these different transient states. I think that jacks around with the flow rates and can lead to screwy readings when calculating VE from the sensors.

 

The VE's for the tables were probably arrived at by the factory by running the engine on a dyno, and making the flow measurements as outlined in WS6's writeup. What you are doing now is trying to "back into" the numbers using much cruder measurements than the factory used. For example, how would your analysis account for any incoming air that blows through to the exhaust during overlap? How would it account for any exhaust gasses remaining in the cylinder, as a function of the back-pressure on the exhaust? How would it account for the continued pressure loss and temperature increase the air experiences on its journey from the intake manifold (where you are taking the measurements) to the cylinder bore? How do you account for the calibration of the MAF sensor, and the changes in calibration that may have resulted from modifications to the air intake ducting in front of the MAF?

Something like VEMaster helps you arrive at meaingful numbers, without getting too bogged down in the science. The caution about 128 BLM's is relavent, as long as the errors being corrected are the result of the MAF measurement. You have to insure you don't have other problems that are contributing to BLM disconnects.

I don't know what my spreadsheet is based on. I just sat in front of the PC and started punching in the formulas that I know and have used regularly. It calculates what I need to know and I've checked the results agains real-world data to make sure there are no errors. But is doesn't account for any of the things that I mentioned in the first paragraph. It is a simple model based on the swept cylinder volume (only reflecting nominal bore and stroke) and the perfect gas law. But its got all the units converions built in, to save going through the metric/English thing. A couple bonuses included like gas velocity through the MAF (variable diameter and free area ratio) and throttle bores (variable diameter). All results are expressed in both volume and mass flow rates, with a variety of units, both metric and English.

TTS DataMaster is only for OBD-I.

 

We have to calculate ve using the outside air conditions totally ignoring transient conditions that occur on the way to the cylinder. The assumption is that the temperature of the air on its way to the cylinder while reducing density would also equally alter the possible total trapped mass in the cylinder. I wanted to account for the same thing but one of the senior engineers in the lab instructed me otherwise and showed me a gigantic derivation that basicly said it didnt matter. Also if you think about it what you get from doing this is it allows us to have a standard and it helps give us a basis to correct our dyno numbers to sea level.

As for error of risidual exhuast or whatever, exhaust doesnt burn and if you think about it it will affect both numbers in the same way if its in there it takes up some volume in the theoretical value and it takes up the same in the actual trapped volume, being on top and bottom it cancels out.

I agree with short circuiting though, that can cause up to a 5% error depending on specifics. The stock camshaft and intake calibration and timing was built around minimizing unburned air/fuel in the exhaust system. Hence when people but big cams in the blms can get jacked around pretty good.

The trouble with doing this type of calculation is you need to design an algorithm that can sort the data about three times. You need to set it up to store data in a series of arrays the arrays will match the ve table. You would then sort based on map and rpm, then you would correct each value before putting it in the array for whatever cell. Then you can average the values. Basicly you need to reinvent ve master but collect maf data and then calculate ve.

The trouble is when you do this you are going to have to deal with non lab grade sensors. Deceleration will always be off in terms of blm and will throw off your average values decel values are unimportant and unwanted as a general rule. You're also going to have to deal with the switching the stock pcm does. I assume that the developement pcm that was used for these cars used some sort of ram that could be altered during operation in real time and there would have been no switching with the short terms throwing things off. The short terms cause the fueling to oscillate across about 1pt of a/f ratio.

I think you're going through a whole lot of brain damage in order to arrive at a table you will never use unless you have a maf sensor failure. The only numbers you should really care about are the ve numbers at wot anyways, that will tell you how good you're doing with your engine setup.

 

Yeah, I'm trying to back into it in some fashion.

This is just like with the efficiency of an electric motor.

eff = power out/power in

Same is true for an engine....VE = air out/air in; Wonder if I can throw a MAF sensor on the exhaust?

I guess more than anything I'm just trying to learn.

Have a look at this.....what do you guys think? Read his paper....

http://redhardsupra.blogspot.com/200...ity-works.html