Okay, I need a little help with this one. I have been reading Corky Bell's book and have been playing with the formulas he provides for reading compressor maps. From the beginning...

I'm starting with a pressure ratio of 2.0 (around 15 PSI of boost). Just to keep it simple. No problems here.

Next I'm trying to determine the airflow rate for the L61 normally aspirated. The book gives the following formula:

airflow rate= (CID x RPM x .5 x Ev)/1728

Here is my first question. Ev, Volumetric Effeciency, does it vary with RPM? Logically I would think yes. If so, where would I find an Ev vs. RPM chart for the L61 engine? If I can figure this out, most of the rest of my questions will also be answered. But I moved on...

I assumed max RPM of 6500 and took Ev's of 80% and 90%, and got AR's of between 200 to 230 CFM. Does that sound about right so far?

I have negligable density loss being close to sea level. Then, at the 2.0 pressure ratio, my required CFM, now called lbs/min, is between 400-460. Still okay?

Okay then, here is why I think I've got something wrong. I look at a Garrett Compressor map for the GT2871R, and 460 lbs/min is off the scale to the right.

I know that I come back into the map with a reduction of either RPM or Ev. I know that the engine accelerates to max RPM of 6500, so shifting through the gears at WOT, I'll be between 4500-6500, and at the lower end of that (4500 RPM), I'll be right in the middle of that map. It's the high end that concerns me.

 

4 valve per cylinder modern engines assume a VE of 85%

You are missing the conversion from CFM to Lbs/Min. You need to multiply your CFM by .0756 if your going to be looking at Garret compresor maps. Mitsubishi turbos use CFM for there compressor maps.

If your calculating 460CFM at 2.0 Pressure ratio (I'm not going to check your math) then you are looking at roughly 35 Lb/min airflow.

Remember to calculate where you expect to see boost begin (not your shift point), because you'll need to make sure you are staying far enough away from the surge line. I usually use 3500RPM as my low figure calculation. So calculate airflow at 3500 with a 2.0 pressure ratio, calculate airflow at 6500RPM with a 2.0 pressure ratio, plot those two points on the compressor map and connect the dots. Then from the 3500RPM dot, draw a line down to the origin of the map.

There you have it.

 

Question there. You are converting CFM to lbs/min using a constant I don't recognize (.0756). The Corky Bell book makes the adjustment based on deviation from the standard atmosphere, or more easily put, height above sea level. I'm pretty much at sea level, so there is no correction. I did take that in to account actually.

 

Read young padawan.

http://www.redz31.com/z31p/writeups/turboselection.html

 

To go from ft^3/Mins you need to take the cube root or x^1/3 power some where inthere I dont know if on the sea level deviation chart if thats already taken into account.

 

Cubic feet per minute is a measure of volume. Pounds per minute is a measurement of mass. The conversion factor varies a little bit but with everything i have ever done with turbos, the accepted converstion for air at STP (standard temp/pressure) is .0756. This takes CFM and makes it Lbs/Min.

 

I've always found max airflow for a given engine at redline, then started dividing it from there. Example: say 40lb@200kpa at redline, then 20lb@100kpa and so on. I'm not 100% if this is correct, but it seems to work for me.