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How to select what size turbo...

Old 06-26-2006, 03:23 PM
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Default How to select what size turbo...

This was posted on a local site, and I find its a good informative piece so here ya go!

Compressor Selection

When using the formula's below, you will need to use compressor flow maps and work with the formulas until you size the compressor that will work for your application. Compressor flow maps are available from the manufacturer, or do a search on the web, you'll find that they are readily available. On the flow maps, the airflow requirements should fall somewhere between the surge line and the 60% efficiency line, the goal should be to get in the peak efficiency range at the point of your power peak. In this article I will walk through an example as I explain it, once you understand it, you can get the the formula's in the Sizing Formula's tech article for quicker reference.

Engine Airflow Requirements

In order to select a turbocharger, you must know how much air it must flow to reach your goal. You first need to figure the cubic feet per minute of air flowing through the engine at maximum rpm. The the formula to to this for a 4 stroke engine is:

(CID RPM) 3456 = CFM

For a 2 stroke you divide by 1728 rather than 3456. Lets assume that you are turbocharging a 350 cubic inch engine That will redline at 6000 rpm.

(350 6000) 3456 = 607.6 CFM

The engine will flow 607.6 CFM of air assuming a 100% volumetric efficiency. Most street engines will have an 80-90% VE, so the CFM will need to be adjusted. Lets assume our 350 has an 85% VE.

607.6 0.85 = 516.5 CFM

Our 350 will actually flow 516.5 CFM with an 85% VE.

Presure Ratio

The pressure ratio is simply the pressure in compared to the pressure out of the turbocharger. The pressure in is usually atmospheric pressure, but may be slightly lower if the intake system before the turbo is restrictive, the inlet pressure could be higher than atmospheric if there is more than 1 turbocharger in series. In that case the inlet let pressure will be the outlet pressure of the turbo before it. If we want 10 psi of boost with atmospheric pressure as the inlet pressure, the formula would look like this:

(10 + 14.7) 14.7 = 1.68:1 pressure ratio

Temperature Rise

A compressor will raise the temperature of air as it compresses it. As temperature increases, the volume of air also increases. There is an ideal temperature rise which is a temperature rise equivalent to the amount of work that it takes to compress the air. The formula to figure the ideal outlet temperature is:

T2 = T1 (P2 P1)0.283

Where:
T2 = Outlet Temperature R
T1 = Inlet Temperature R
R = F + 460
P1 = Inlet Pressure Absolute
P2 = Outlet Pressure Absolute

Lets assume that the inlet temperature is 75 F and we're going to want 10 psi of boost pressure. To figure T1 in R, you will do this:

T1 = 75 + 460 = 535R

The P1 inlet pressure will be atmospheric in our case and the P2 outlet pressure will be 10 psi above atmospheric. Atmospheric pressure is 14.7 psi, so the inlet pressure will be 14.7 psi, to figure the outlet pressure add the boost pressure to the inlet pressure.

P2 = 14.7 + 10 = 24.7 psi

For our example, we now have everything we need to figure out the ideal outlet temperature. We must plug this info into out formula to figure out T2:

T1 = 75
P1 = 14.7
P2 = 24.7

The formula will now look like this:

T2 = 535 (24.7 14.7)0.283 = 620 R

You then need to subtract 460 to get F, so simply do this:

620 - 460 = 160 F Ideal Outlet Temperature

This is a temperature rise of 85 F.

Adiabatic Efficiency

The above formula assumes a 100% adiabatic efficiency (AE), no loss or gain of heat. The actual temperature rise will certainly be higher than that. How much higher will depend on the adiabatic efficiency of the compressor, usually 60-75%. To figure the actual outlet temperature, you need this formula:

Ideal Outlet Temperature Rise AE = Actual Outlet Temperature Rise

Lets assume the compressor we are looking at has a 70% adiabatic efficiency at the pressure ratio and flow range we're dealing with. The outlet temperature will then be 30% higher than ideal. So at 70% it using our example, we'd need to do this:

85 0.7 = 121 F Actual Outlet Temperature Rise

Now we must add the temperature rise to the inlet temperature:

75 + 121 = 196 F Actual Outlet Temperature

Density Ratio

As air is heated it expands and becomes less dense. This makes an increase in volume and flow. To compare the inlet to outlet air flow, you must know the density ratio. To figure out this ratio, use this formula:

(Inlet R Outlet R) (Outlet Pressure Inlet Pressure) = Density Ratio

We have everything we need to figure this out. For our 350 example the formula will look like this:

(535 656) (24.7 14.7) = 1.37 Density Ratio

Compressor Inlet Airflow

Using all the above information, you can figure out what the actual inlet flow in in CFM. Do do this, use this formula:

Outlet CFM Density Ratio = Actual Inlet CFM

Using the same 350 in our examples, it would look like this:

516.5 CFM 1.37 = 707.6 CFM Inlet Air Flow

That is about a 37% increase in airflow and the potential for 37% more power. When comparing to a compressor flow map that is in Pounds per Minute (lbs/min), multiply CFM by 0.069 to convert CFM to lbs/min.

707.6 CFM 0.069 = 48.8 lbs/min

Now you can use these formula's along with flow maps to select a compressor to match your engine. You should play with a few adiabatic efficiency numbers and pressure ratios to get good results. For twin turbo's, remember that each turbo will only flow 1/2 the total airflow.

Last updated 12/28/02
Turbo Type ----------- Approx flow @ pressure
Stock Turbo ---------- 360 CFM at 14.7 PSI
IHI VF 25 ------------- 370 CFM at 14.7 PSI
IHI VF 26 ------------- 390 CFM at 14.7 PSI
T3 60 trim ----------- 400 CFM at 14.7 PSI
IHI VF 27 ------------- 400 CFM at 14.7 PSI
IHI VF 24/28/29 ----- 410 CFM at 14.7 PSI

========= 422 CFM max flow for a 2 Liter at .85 VE pressure ratio 2.0 (14.7 PSI) 7000 RPM =======

IHI VF 23 ------------- 423 CFM at 14.7 PSI
FP STOCK HYBRID -- 430 CFM at 14.7 PSI
IHI VF-30 ------------- 435 CFM at 14.7 PSI
SR 30 ----------------- 435 CFM at 14.7 PSI
IHI VF-22 ------------ 440 CFM at 14.7 PSI
T04E 40 trim -------- 460 CFM at 14.7 PSI

========= 464 CFM max flow for a 2.2 Liter at .85 VE pressure ratio 2.0 (14.7 PSI) 7000 rpm =======

PE1818 -------------- 490 CFM at 14.7 PSI
Small 16G ------------ 505 CFM at 14.7 PSI
ION Spec (stg 0) --- 525 CFM at 14.7 PSI

========= 526 CFM max flow for a 2.5 Liter at .85 VE pressure ratio 2.0 (14.7 PSI) 7000 RPM =======

Large 16G ----------- 550 CFM at 14.7 PSI
SR 40 ----------------- 595 CFM at 14.7 PSI
18G ------------------- 600 CFM at 14.7 PSI
PE 1820 -------------- 630 CFM at 14.7 PSI
20G ------------------ 650 CFM at 14.7 PSI
SR 50 ---------------- 710 CFM at 14.7 PSI
GT-30 ---------------- 725 CFM at 14.7 PSI
60-1 ----------------- 725 CFM at 14.7 PSI
GT-35R -------------- 820 CFM at 14.7 PSI
T72 ------------------ 920 CFM at 14.7 PSI <--- Note you would have to spin a 2.0 L engine at about 14,000 rpm to flow this much air.

IHI VF 25 ----------- 395 CFM at 18 PSI
IHI VF 26 ----------- 400 CFM at 18 PSI
T3 60 trim ---------- 410 CFM at 20 PSI
IHI VF 27 ----------- 420 CFM at 18 PSI
IHI VF 24/28/29 -- 425 CFM at 18 PSI
IHI VF 23 ----------- 430 CFM at 18 PSI
IHI VF-30 ----------- 460 CFM at 18.0 PSI
AVO 320HP -------- 465 CFM at 17.5 PSI
T04E 40 trim ------ 465 CFM at 22 PSI
FP STOCK HYBRID- 490 CFM at 18.0 PSI
IHI VF-22 ---------- 490 CFM at 18.0 PSI
SR 30 --------------- 490 CFM at 22 PSI
Small 16G ---------- 490 CFM at 22 PSI
ION Spec (stg 0) - 500 CFM at 19 PSI
PE1818 ------------ 515 CFM at 22 PSI
Large 16G --------- 520 CFM at 22 PSI

========= 526 CFM max flow for a 2 Liter at .85 VE pressure ratio 2.5 (22 PSI) 7000 rpm =======

========= 578 CFM max flow for a 2.2 Liter at .85 VE pressure ratio 2.5 (22 PSI) 7000 rpm =======

HKS GT2835 ------- 580 CFM at 22 PSI 400 hp
MRT 400 ------------ 580 CFM at 16 PSI
AVO 400HP -------- 580 CFM at 17.5 PSI
MRT 450 ------------ 650 CFM at 19 PSI
AVO 450HP -------- 650 CFM at 20.0 PSI
SR 40 ---------------- 650 CFM at 22 PSI

========= 658 CFM max flow for a 2.5 Liter at .85 VE pressure ratio 2.5 (22 PSI) 7000 rpm =======

HKS GT3037 ------ 670 CFM at 22 PSI 460 hp
PE 1820 ----------- 680 CFM at 22 PSI
20G ---------------- 695 CFM at 20.0 PSI
HKS GT3040 ----- 710 CFM at 22 PSI 490 hp
AVO 500HP ------ 770 CFM at 22 PSI
SR 50 ------------- 770 CFM at 22 PSI
GT-30 ------------- 790 CFM at 22 PSI
60-1 --------------- 800 CFM at 22 PSI
HKS GT3240 ----- 830 CFM at 22 PSI 570 hp
GT-35R ----------- 880 CFM at 22 PSI
T72 --------------- 1000 CFM at 22 PSI <--- note you would have to run a 2.0 L engine at >40 PSI boost to flow this much air

Conversions used where there was control over conversion factors:
1 HP approx equals 1.45 CFM

1 CFM approx equals 0.0745 lb of air/min

0.108 Lb/min approx equals 1 hp

1 Meter cubed/sec = 35.314 CFS = 2118.867 CFM

1 KG/sec = 132 lbs/min approx equals 1771.812 CFM

power coversions:
1 PS = 0.9859 HP = 75 Kgf m/sec
1.3405 HP = 1 KW
1 HP = 746 watts
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