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Old 01-10-2013, 08:37 PM   #1
downforce22's Avatar
Join Date: Aug 2009
Location: Colorado Springs, CO
Posts: 2,884
Volumetric Efficiency

I am trying to look at turbo compressor maps for turbo selection. The formula is straightforward for airflow of a piston engine.

CFM = L x RPM x VE x Pr
The problem I am having is trying to get realistic numbers from this formula for a boosted engine. Since forced induction can increase the volumetric efficiency of an engine the amount of air an engine can flow (CFM) should not only increase based on RPM, but also Volumetric Efficiency (VE). Displacement and Pressure Ratio stay the same. A soild value for 2 valves VE is 85%, and 90-95% for 4 valve heads.

What I am missing is where the engine begins to increase over 100% volumetric efficiency.

For Example an M20B25 at 10 PSI at Sea Level:
PR = 14.7 psi + 10 psi/ 14.7 psi = 1.68

(2.5 L x 2500 RPM x 85 VE x 1.68 )/5660 = 157.7 CFM

Now at the RPM we want to be spooled by, say 3500 RPM

(2.5 L x 3500 RPM x 85 VE x 1.68 )/5660 = 220.7 CFM

And Now the same engine at 6000 RPM

(2.5 L x 6000 RPM x 85 VE x 1.68 )/5660 = 378.5 CFM

But shouldn't the engine begin building boost, say at 3500 RPM, and somewhere cross 100% VE and begin nearing 110% VE or higher?

Let assume we get to 100% VE while building boost at 3500 RPM.

(2.5 L x 3500 RPM x 100 VE x 1.68 )/5660 = 259.7 CFM

And now the same engine at 6000 RPM assuming we are now at 110% VE.

(2.5 L x 6000 RPM x 110 VE x 1.68 )/5660 = 489.8 CFM

While spooling this is a 17.7% increase in flow and near redline this is 29.4% more flow. I belive this is an under approximation of the VE, but the point is the same, typical engine flow calculations should be higher due to the turbo's effect on the engines VE. Take a look at the map below at a 1.68 PR and the difference in flow on the compressor map. This turbo is either correct or may be too small for the m20, but with more accurate calculations shouldn't the effect of the turbo on the VE incorporated into the calculations make turbo selection more accurate? The PR is incorporated but it doesn't take into account how it affects the VE, just the AMOUNT of air in the engine, not the EFFICIENCY. I know im spewing, but what do you guys think?


Volumetric Flow rates for some BMW V12 Engines. Interesting read, considered looking at the M70 engine specifically as it is basically two M20B25s siamesed together. It has very good info on some turbo charged V12s.

Last edited by downforce22; 01-10-2013 at 10:00 PM.
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