Who's right about flow rates?

[nb07bcar]

Ok I though I had the ANSWER, BUT NOW I DON'T KNOW. I was looking at turbo maps and trying to understand them. I read the section on them and tried to calculate the volume of air for the KA at 6000rpm. Now in the stick I found that it said the KA at 6000rpm is 26lb/min. Now when I convert lb/min=cfm*.07...... I get a number of 350 or so cfm. But then I go to the steal316 and use there formul of VAF = (L/28.317) x (RPM/2) x VE. I come up with 241.55cfm. Which number is right, cuz it makes a big difference when reading the compressor maps.

[PencilShavings]

I also got around 250cfm at 6000rpm

[nb07bcar]

So why does it say 26lb/min on the sticky. I don't think thats right. It might be if I'm using the wrong formula to convert lb/min to cfm.

[KATwo40]

The conversion between lb/min and CFM is relative to the atmospheric conditions in which the measurements are taken or calculated.

For example, say you have a figure of 250CFM and want to convert to lb/min at approx. 1,000ft above sea level. You'd multiply 250x.0997 to get the lbs/min.

However, if you were in Aspen, CO, which is about 8,000ft above sea level, you'd multiply 250x.0972 because now the air density has drastically changed.

Here's a for calculating CFM requirements of any engine at any given RPM. This is from Corky Bell's "Maximum Boost." While I don't quite agree with Corky's theories and suggestions 100%, I find this formula very useful, and he is a very knowledgeable person.

Airflow = Cubic Inch Displacement (CID)x RPM x 0.5 (two revolutions per cycle) x VE / 1728 (to convert to CFM)

So here we go:

Airflow = CID x RPM x 0.5 x VE / 1728 = CFM

= 146 x 6800 x 0.5 x .85 / 1728 = 244.18

Then you have to convert to lb/min via your relative air density.

In my case, I'm about 1100 ft. above sea level.

Airflow = CFM x Relative Air Density = lb/min

= 244.18 x 0.0997 = 24.34lb/min.

Now, if you incorporate boost, you multiply by the pressure ratio.

Say you have a pressure ratio of 1.72 (about 9psi).

lb/min x 1.72 = lb/min under boost.

24.34 x 1.72 = 41.86 lb/min @ 6800rpm

Hope this is useful to you.

[nb07bcar]

Thanks for the info, But then I have another question. if you have 41.86lb/min at 6800rpm. What is the measurment that you read on a turbo map. According to the sticky you would take the 24.34 measument? Is that right or do you have to multiply it by the amount of boost you plan on running and then look to see where you efficiency is?

[KATwo40]

24.34 is the N/A application. You'd multiply that number by the Pressure Ratio (boost) to find your required lb/min.

If you want 9psi (PR=1.72) you'd multiply 24.34x1.72 = 41.86 lb/min with 9psi @ 6800rpm.

[PencilShavings]

this is what I went by, and I put it into an excel spreadsheet

http://www.gnttype.org/techare....html

KATwo240, I found that at 6800rpm and 9psi I got around 34lbs/min (with 80 degree ambient temp, but also no elevation correction Im aware of) when I put the ambient temp to 0 degrees it brought up my lbs/min to around 40.

anyway, I guess I was just suprised the results were that different..

[KATwo40]

Interesting. What PR did you use for 9psi?

Thanks again for sharing and discussing.

[nb07bcar]

Ok So here is the turbo I'm running. its in cfm. I'm rumming 12 psi. so I take 24.34*1.85=45lb/min. now if I convert that to cfm its arond 451 cfm. Now at 6800rpm my motor is pumping 451 cfm. So do I use that for the end point on the turbo map? If that right i am way past my efficency because My turbo can only pump 428cfm at 15psi. Is this correct?

http://www.stealth316.com/images/td04-15g-cfm.gif

[Edub1]

This should help.

zerothread?id=152550

[PencilShavings]

Interesting. What PR did you use for 9psi?

Thanks again for sharing and discussing.

good call, i checked and my spreadsheet gives me a 1.61 pressure ratio for 9psi. I did (9+14.7)/14.7.

1.71 pres ratio is 10.5psi

[nb07bcar]

Thank that helped alot. I should be running a bigger turbo. I'm running in the 67-65% at 6000rpm. thanks

[KATwo40]

good call, i checked and my spreadsheet gives me a 1.61 pressure ratio for 9psi. I did (9+14.7)/14.7.

1.71 pres ratio is 10.5psi

You have to take into account the pressure differential between atmosphere and the compressor inlet. Since the inlet will have both an air filter AND a MAF, it's safe to say there's probably a pressure differential of at least 1psia.

So, your formula really should look more like this:

(9+14.7)/(14.7-1) = 1.73

Make sense? I'm willing to bet that the pressure differential actually caused by an air filter and the stock MAF is really more like 1.5-1.8psia, but I was being liberal with the numbers.

[fiznat]

Great discussion guys. Edited the title to better reflect the topic.

[C-Kwik]

Thank that helped alot. I should be running a bigger turbo. I'm running in the 67-65% at 6000rpm. thanks

While efficiency is important, you should consider not just where you are in the compressor map at 1 RPM point but rather over a broad range of them. A KA's transmission will probably require you to consider efficiencies you will see between 4000RPM and redline. I wouldn't jump on the 'I need a bigger turbo' bandwagon until you look at the overall efficiency you will see.

[nb07bcar]

I understand, but if you look at the turbo I'm running you will see that I could be running a much more efficient setup.

[savatoge]

Here is a graph I constructed for the KA against the TD04H-15GBlue line is 31 PSIOrange line is 23.2 PSIPink line is 15.5 PSIRed line is 7.7 PSI

Calculation points are at 1000, 2300, 2850, 3400, 4575, 5750, 6375, and 7000 RPM respectively. My calculations include 1.5 PSI pressure loss in the system and I do try to reasonably simulate a real life operational boost curve, ie 0-4 PSI by 2500 RPM, with max boost occuring later in the RPM range depending on the target max boost. I admit this can be somewhat subjective, but for the most part, the plot is pretty close to what you would see in the real world.

Bottom line is this compressor is too small for the KA
Modified by savatoge at 8:56 PM 3/24/2006

[PencilShavings]

You have to take into account the pressure differential between atmosphere and the compressor inlet. Since the inlet will have both an air filter AND a MAF, it's safe to say there's probably a pressure differential of at least 1psia.

So, your formula really should look more like this:

(9+14.7)/(14.7-1) = 1.73

Make sense? I'm willing to bet that the pressure differential actually caused by an air filter and the stock MAF is really more like 1.5-1.8psia, but I was being liberal with the numbers.

yes, makes sense. I may have to make some changes to my spreadsheet

[nb07bcar]

When I graphed it out it didn't look that bad. But I had to convert the graph from cfm to lb/min, so my conversion might be off. When I was calculating my flow for the ka at 5500 rpm i got 26.1lb/min at 1.85 bar. I see that your calculation even at 15.5 psi at 5700rpm you got less than I did like 22. Am I wrong?

Thanks for everyones help.

[savatoge]

When I graphed it out it didn't look that bad.

Sorry...my bad. I had forgot to reset my spreadsheet from my previous calculations, where I had my base variables setup for maps targeting 440 crank HP. So, I guess my previous graph demonstrates that if you're looking for 375 HP to the ground the TD04-15G isn't for you.

nb07bcar,Do you know what kind of crank (or wheel) HP you're looking for with the TD04-15G? Let me know and I'll be glad to produce another graph...it'll take me a couple minutes to change the crank HP variable and uplink the graph. Just to let you know though, I just reset my spreadsheet to 300 crank HP and the upper RPM ranges are still off the compressor map and extended well into the choke area. If I also drop the displacement down to 2.0 L, then things would be ok, up to about 15 PSI, but even then the upper RPM range is way over in the lower efficiency islands, almost verging on choke.

[nb07bcar]

I'm not looking at a specific hp. I'm just running what I have. I'm running that turbo with 42lb ingectors and 10-12 psi. I just wanted to figure out where my efficency was and if it was real bad I want to try to find a th04-19T turbo off another volvo. Its a bigger compressor and is rated for 325 hp on the 2.5L volvo engine. Really I just wanted to learn how to read a map and set one up. I think I have it down now, but when you put that map up there I was affraid I made a mistake. I estmate I'm making around 250hp. Thanks for the help.