How to Pick a Correct Turbocharger Size

Assess your budget. Building a turbocharged engine isn't about just bolting a giant huffer to the exhaust manifolds and calling it a day. The turbo might only cost you $500, but a good install doesn't stop there. Turbochargers make power as a function of the engine's original horsepower and torque, so building an engine to make more power before bolting the turbo onto it will likely yield benefits that compensating with huge boost won't.

Determine the required airflow in cubic feet of air per minute. Boost doesn't make power, it just shoves more air through your engine. Because engines typically operate an air/fuel ratio of about 14 parts air to 1 part fuel, and because gasoline contains a certain amount of energy (about 114,000 British Thermal Units per gallon), you can make a direct correlation between airflow in cfm and horsepower. That ratio is about 150 cfm to 100 horsepower. As an example, let's put together a 900 horsepower Chevrolet 350: For this application, you'll need about 1,350 cfm of air.

Calculate your engine's non-turbo airflow in cfm. There are three ways to do this: You can either use an online cfm-to-horsepower calculator that takes engine displacement, efficiency and rpm into account, and you can extrapolate from the engine's stock horsepower; or you can take the engine to a dyno room and check it. For our example engine, we'll say that (in non-turbo form) it produces 300 horsepower at 5,500 rpm, at an 80 percent volumetric efficiency. The online calculator gives us 446 cfm airflow, and using the 150-cfm/100-horsepower ratio gives us 450 cfm.

Divide your required airflow by your engine's stock airflow to determine the required boost pressure ratio (the ratio of boost pressure to atmospheric pressure, which is about 14.7 psi). For the example engine, you arrive at a pressure ratio of exactly 3.00. Here's a bit of trickery, though: Dividing desired horsepower by non-turbo horsepower will give you the same pressure ratio figure as going through this long-form cfm-to-horsepower-to-pressure ratio calculation. You only went this far to understand the factors that you'll be dealing with in turbo selection from here on.

Look through a manufacturer's selection of "turbo maps." A turbo map is a graph that indexes airflow to pressure ratio, and gives a visual representation of turbo efficiency at a given pressure ratio and cfm. You'll see pressure ratio on the vertical axis and the airflow on the horizontal axis. A compressor map looks something like an elongated bulls-eye: the center of that bull's eye is the compressor's maximum efficiency range, which is where it makes boost without producing excess heat.

Compare your engine's required pressure ratio and airflow in cfm to various compressor maps and find one that puts your target airflow/pressure point in the center-to-upper-right-hand corner of the compressor's maximum efficiency range (the center of the bulls-eye). Many times you'll find airflow expressed in the metric "m3/s," or meters cubed per second. To convert cfm to m3/s, multiply cfm by 0.00047. For our example engine, we'll need to find a turbo that supplies full efficiency at a 3.00 pressure ratio at 0.6345 m3/s flow. Again, find a compressor where that point falls in the center-to-upper-right-hand corner of the turbo's maximum efficiency range.

Repeat Steps 2 through 7, using the engine's peak torque rpm. The Chevy 350 in our example makes its peak torque at 2,000 rpm, where (according to the stock dyno graph) it makes 140 horsepower. Apply the 150-cfm/100-horsepower rule and you'll find that this engine uses 210 cfm at that rpm. Multiply that airflow by the required pressure ratio (3.00) and you have your low-end boost response requirement. In addition to producing a 3.00 pressure ratio at 1,350 cfm (0.6345 m3/s), it should produce that same 3.00 PR at 630 cfm (0.2961).

Search and search some more until you find a turbo that's completely spooled up (producing a 3.00 PR, in this case) at your torque-peak airflow and maintains that PR through the engine's horsepower-peak airflow. You'll often find that, for larger engines like our 350, such turbos do not exist. No turbo out there will provide those PR and flow numbers over such a wide spectrum of airflow.

Re-calculate for a multiple-turbo setup. If you can't find a turbo to fit, divide your airflow figures by the number of turbos you want to use. Two turbos flow twice as much air as one, and smaller turbos have a wider efficiency range relative to absolute airflow than smaller ones. So, for our example 350, divide 1,350 cfm (0.6345 m3/s) and 630 cfm (0.2961) by two; now you need a pair of turbos that will provide a 3.00 PR at 675 cfm (0.3172 m3/s) to 315 cfm (0.1480 m3/s). That's a spread of only 360 cfm for the little twin-turbo setup, versus 720 cfm for the single, big turbo setup -- a much more achievable goal for any compressor.