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Old 06-15-2008, 02:07 PM
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Default How To Size A Turbo-Compressor

Credit:Cliff Wilson "Diesel Freak"
Here is a short lesson on how to size a turbo-compressor (I did not say turbo) for your truck and how to read a compressor map.

Click the image to open in full size.

Basically, the area left of the surge line is no mans land where crazy flow irregularities and the infamous turbo bark run rampant. The area to the right of the right most efficiency line is the choke line. If you are operating here the turbo is too small.

All the garbage in between is where you should operate a turbo.

The 'x' axis is air flow

The 'y' axis is pressure ratio in atmospheres. One atmosphere is 14.7 psi absolute. What you read on your boost gauge is in gauge pressure.

P absolute = P gauge + 14.7

The numbers in thousands is turbo RPM.

The percentages are the turbo compression efficiencies at those specified flow rates and pressures.

When computing temperatures and pressures, you MUST be in an absolute scale. I use the Rankine scale for temperatures (Kelvin can also be used) Using Fahrenheit or Celsius will give you incorrect results.

T Rankine = T Fahrenheit + 459

To get the nitty gritty of the compressor, you need a few things. First the correct part number and the Compressor map for that part (however you can fudge a bit here to get close if you know the basic compressor trim and flow rate. Also, there are limits to single stage compression efficiency.

Compressor trim is a ratio of area between the inducer and exducer. The inducer is what you see when you look at a turbo-compressor. It is the small diameter. The exducer is what you do not see....the part that is inside the housing. This is the large diameter. The turbine trim is the exact opposite.

Here are the basic equations to get you close to the actual post compression and post intercooled temperatures.

P1 = turbo inlet pressure
P2 = turbo outlet pressure
T1 = turbo inlet temperature
T2 = turbo outlet temp from basic adiabatic compression
T3 = turbo outlet temp from compressor inefficiency
CE = Compressor efficiency
IE = Intercooler efficiency
Tin = T3
Tout= aftercooled air temp
Tamb = T1 = the air that the turbo is sucking in

T2 = T1 x (P2/P1)^(0.286)

T3 = T1 + (T2-T1)/CE

Tout = Tin - IE x (Tin-Tamb)




To get density ratio (the condition of the air at the discharge of the turbo or after intercooling referenced to ambient) you need the following equation.

DR = T1/T3 x P2/P1

T3 can be substituted with Tout to find aftercooled density.

Manifold pressure is slightly less than P2, but usually no more than 4 to 6 psi on a 500 HP truck with the stock intercooler.

Also, I normally use an intercooler efficiency of 74%. On a dyno, CAC efficiency is crap, but on the highway it is quite good.

Now that you have density ratio, you can calculate the air demand of the engine.

(0.1038 x RPM x VE x DR) / 14.5 = Mass flow rate in LBM/Min for a 359 cubic inch displacement engine.

This enough info to size a turbo for a Cummins B series engine. VE is always less than 1.00 when used in the above equation. I use 0.92 for a stock ISB head and cam.

To add one more little tidbit. Density ratio is directly related to the ammount of fuel an engine can burn....hence the higher the density ratio the more fuel that can be burned.....and the end result is more power. If Air/fuel ratio is kept above 18:1 on a diesel you should see very little smoke. Most HP ratings of turbos that are used for gasoline engines are based on a A/F ratio between 12:1 and 14:1
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Old 06-16-2008, 09:36 AM
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