Turbocharger Basics

Time To Choose
To make things easier, we've included a chart for street truck turbo selection, and an explanation of compressor maps (see pg. 46). Even with this information, you still have to be honest about how you're going to use your diesel. If you're looking for a bit more performance but still want to tow or drive your truck daily, we'd pick the smaller side of the turbo spectrum. If your engine has had head or cam work, and you plan on sled pulling or drag racing your vehicle, we'd go bigger. Remember, at the higher power levels (600 hp and up) plan on trying to turn some rpm to help those big chargers spool. Also, don't be afraid to call and talk to all of the different turbo manufacturers before you make a decision.

Compressor Maps
Probably the most important information you can acquire about a turbocharger comes on a little piece of paper. A compressor map is a chart that shows turbocharger efficiency ranges, surge lines, and maximum speed. It's also one of the best ways to show why you'll need an aftermarket turbocharger for your hopped-up diesel.

First comes first though, we'll teach you how to read a compressor map. On the left side of the chart extending upward is the turbocharger pressure ratio, also known as boost. A 2:1 pressure ratio means twice atmospheric pressure. Atmospheric pressure at sea level is roughly 14.7 psi, so a 2:1 pressure ratio would be: (atmospheric pressure x pressure ratio = turbocharger air pressure - atmospheric pressure = boost pressure) or (14.7 psi x 2 = 29.4 psi, 29.4 psi - 14.7 psi = 14.7 psi).

The second aspect of the chart is airflow, which involves engine size and engine speed. Normal engines are about 85 percent efficient, so theoretical engine airflow can be calculated as: (displacement x engine rpm / 3,456 x efficiency) or, in the case of a stock 5.9L (359ci) Cummins spinning 2,800 rpm, the engine would need a theoretical airflow of: (359 ci x 2,800 rpm / 3,456 x 0.85 = 247 cfm.)

Now comes the step that puts it all together. Since the maximum boost most 12-valve Cummins will make is about 20 psi (which is a 2.36:1 pressure ratio), we have to multiply 247 cfm by the 2.36:1 pressure ratio, which gives us 583 cfm. Most compressor maps come in a pounds/minute format, so we must multiply 583 cfm x 0.07 to get pounds/minute, which means at 2,800 rpm, at a 2.36:1 pressure ratio, the turbocharger will be flowing 40.8 pounds/minute. If we plot 2.36:1 pressure ratio and 40.8 pounds/minute on our chart, we can see it falls well within the efficiency range of a stock Cummins HX35 turbocharger.

When a diesel is under a high load (such as towing) the turbocharger will often spin faster and create more boost because the engine is creating more exhaust heat. Plot B is an example of an HX35 having to produce 29.4 psi (a 3:1 pressure ratio) at 1,800 rpm (which is close to the engine's torque peak). If we take this same setup and ask it to make 29.4 psi at 2,800 rpm at the dragstrip (plot C), you can now see that we've reached the limits of the stock turbocharger. Running an HX35 turbo at this level will be very inefficient (about 50 to 55 percent instead of 70 percent) and the turbo will be in danger of overspeeding. The lower efficiency of the turbocharger will also heat up the air more, resulting in a power loss compared to a properly sized turbocharger.

Speaking of properly sized turbos, plot C falls well within the efficiency range of the PowerMax Stage 3 Garrett turbo. In fact, the Garrett is safe all the way to 40 psi at 2,800 rpm (plot D). Since the turbocharger is still well within its efficiency range, the air will be denser and cooler, and will therefore make more power. Note that if the engine was any larger (say a 7.3L Power Stroke instead of a 5.9L Cummins) or spun any higher (Duramax) then a larger turbocharger would be needed to satisfy the added airflow requirement.