In order to chose the right turbo for your car you have to be able to understand what kind of boost pressure you need to run in order to get the desire horsepower. This means you have to realize what kind of air flow properties you engine is currently operating at, as well as what it can handle.
The basic rule of thumb is for every 10hp made, it requires 1lb of air per min.
Example:
In order to change a 100 hp N/A, naturally aspirated, engine into a 200hp boosted engine you theoretically need a turbo that is capable of putting out about 20lb per minute.
This is not the same as psi, pounds per square inch. In order to figure out how many pounds per minute your putting out you first need to figure the CFM, cubic feet per min, by using the formula below:
CFM=(Cubic In. Displacement x RPM x . 5 x Volumetric Efficiency) / 1728
The . 5 is because a 4 stroke engine only takes a "Breathe" every other revolution of the crankshaft. (This variable could possibly change due to engine type Ex- Rotary Engines). While the 1728 converts cubic inches to cubic feet per minute. This being what you need to know for how much air is currently entering you engine before you conver it to pounds per minute later. Normally for most engines the Volumetric Efficiency, or VE, is around 80-87% depending on what work is done to the engine, Ex- porting of head, intake manifold, throttle body, or intake and exaust ports for Rotary engines.
Example of Formula:
If you were to insert an 85 percentage VE for a modern overheadcam motor of average output 2.7 Litre Toyota 4 cylinder Truck:
(165 CID x 6500RPM x . 5 x 85VE) / 1728 = 264 CFM
So at 80 degrees temp at sea level, 264 CFM converts to pounds per minute using this formula below:
lb/min=CFM x . 07
So for the Toyota engine 264 x . 07 = 18.48 lb/min. Using the basic rule of thumb this Toyota 2.7 Litre engine should produce approximately 185 N/A horsepower. Being: 18.48 lb/min x 10 hp = 184.8 rounded up to 185. Then from there you would find your designated horsepower and work the formula in order to figure out how much more pounds per min. you would need in order to acuire the hp. In order to find if the capability of each turbo you would need to look at the compressor map provided by the turbos production company or distributor.
The tricky part is figuring out how much of the boost is actually reaching the combustion chamber. You would figure if you have 10 psi coming from you intake turbine, you would have 10 psi reaching your combustion chamber. But unfortunatley it does not allways work like that. Typically less air than you might expect to (or even calculate) will make it through the engine. This is due to air density restrictions and reductions caused by altitude or theoretical heating of any compressors, plus the added heat contributed by the inefficiencies of a particular forced induction copressor, minus the heat removed by the intercooler. Also any piping that may be located near a heat source such as exhaust. All the added up heat reduces the density, which results in a reduction of pressure, resulting in less power than expected. This is why a lot of racers wrap their exhaust with a heat insulator to keep from unwanted heat reaching components such as air intake, and intercooler piping. Another big thing is finding an efficient intercooler. Intercoolers are essential parts of a forced induction system. The colder you can keep the air, the more dense it will be when entering the engine, resulting in more pressure and power. Some racers have found ways to cool the air so efficient that they have actually gained pressure by the time the air reaches the engine, because its so cold. Racers tend to use a larger intercooler, such as a 4core or bigger, and even spray nitrous on to intercoolers exterior casting to give an extra cold charge. The bottom line is if you cant get an efficient enough cooling agent, than you need to set up a larger boost setup coming from the turbo in order to compensate for the loss in pressure due to the various density restrictions. For the most part the thermal efficiency of a turbo will be in the 55-80 percentage area depending on where you are on a compressor map. But an Intercooler will definently help you regain some of the efficiency of a boost system.
Any thing you can do to limit air flow restrictions will help in gaining power. Porting a throttle body, intake manifold, and head, will all help aid in keeping the flow of air as smooth and dense as possible. The better the breathing efficiency of the engine the lower the boost required for a given airflow. Meaning more power with less boost.
In order to convert boost (or target boost) to a pressure ratio:
Pressure ratio = (PSI + 14.7 (atmospheric pressure)) / 14.7
Example-
( 10psi + 14.7) / 14.7 = 1.68 which converts to 68% higher pressure than atmospheric, or naturally aspiration.
You need this in order to interpret a compressor map that plots air flow at various pressure ratios and compressor speeds. In order to determine what compressor is right for you, showing at what speed with you achieve this desired ratio. Compressor maps typically show pressure ratios at outlet of turbine.
Example-
Lets say you were planning to turbocharge the 2.7 litre Toyota engine by 7.5 psi of boost with out an intercooler and wanted to compare some smaller compressors. The pressure ratio for 7.5 psi using the formula above would be approximately 1.5. ( (7.5 + 14.7) / 14.7, which comes to a rounded figure of 1.5) Then using the stock 2.7 airflow of 18.48 rounded to 18.5 pounds per minute, found at begining of article, at a pressure ratio of 1.5, you might expect the boosted airflow to increase to roughly 28 pounds per minute by multiplying 18.5 by 1.5. (note: most figures rounded). However if a turbocharger with no intercooler is just 70% efficient, the density ratio at sea level is really only about 1.3, which means airflow is no longer approxametly 28 lb/min but only 24 lb/min. So when looking at the compressor map youd plot the original pressure ratio of 1.5, but would use the derived density adjusted airflow of 24 lb/min instead of 28. On top of that if you were to intercool the compression, you would then find out how much density ratio you would regain in order to find out lb/min and actual pressure reaching the engine.
You need to calculate the above in order to find the correct boosted pressure your looking to run through your engine, if the compressor/ turbo is capable of putting it out, and how much pressure is really reaching your engine. After you figure that out its time to do some comparisson shopping, in order to find the turbo that fits you needs and driving style.
In order to get proper performance from a turbo one must look to what they plan to do and their driving needs. Not always should you jump to the biggest turbo with maximum power. One thing you must look at is lag time. This is the amount of time it takes for the turbo to spool up. Shorter lag time means faster turbo spool up, meaning faster responce and power at a lower rpm. Longer lag time means the turbo will take longer to spool up but will result with more power at higher rpms. Larger turbos put out more pressure resulting in more power, but it takes longer time for them to spool up taking away from acceleration and low end torque. Smaller turbos dont put out as much pressure and power as larger turbos, but spool up rather quickly and produce good acceleration and low end torque. But they also tend to fall off in power in later rpms.
So right now you must be thinking that bigger turbos are better because they dont loose power in higher rpms and result in more power than smaller turbos. This is true, if your on the track. The truth is if your planning on doing track racing the bigger turbo is most likely going to be your choice. But if your doing street racing, autocross, drifting, or just joy riding than you will hardly ever hit the power of the turbo. So in this case you would most likely want to go with a smaller turbo, as to where your almost allways into boost on the streets, and unless your on the high way you wont max out your tach.
Though there is a different route. Some people are leaning towards a sequential twin turbo set up in order to have a small quick spooling turbo in order to maintain good acceleration and low end torque, and a larger turbo that takes over in later rpm that provides high power. This provides a "Best of both worlds" so to speak. Also there are some hybrid turbos that use different size exhaust and intake turbines in order to allow different spool up speeds depending on how much of an output your engine can produce at low rpms.
Ignition timing and air fuel mixtures can have a big play on the efficiency of a turbo, since 80% of the exhaust turbines spool is from the exhaust temperature. The hotter the exhaust, the more the turbine will spool. Keeping in mind that the Intake turbine is right next door, and heating intake air will just reduce performance. But the right temperature combination will help maximize performance.
Things to Realize when choosing a Turbo:
-A smaller Turbine will restrict exhaust gases at higher rpms, creating maybe to much back pressure. A larger Turbine wont restrict as much and have less thermal loading.
-The power a Turbine will generate depends on 4 variables: Flow, Turbine Efficiency (including bearing losses), Turbine Inlet Temp, Turbine Expansion Ratio.
-A standard turbine is cast iron and reletively heavy, with high inertia. But Aftermarket turbines may include light wieght ceramic designs, and ball bearing shafts, which aid in the lag time by reducing the amount of force needed to spool the turbine.
-To perform on street premium gas with out detonation, look at 10psi with out an intercooler and 15psi with an intercooler.
-Be cautious with boost as to not surpass limit of fuel to where the engine starts detonating, which can cause premature engine damage.
-Larger turbos create less heat.
-A turbine cooling system must be sufficient enough to cool and lubricate the turbines and rods that can some times operate up to 300,000 rpm.