DIESEL MATH
for 6:1. This can be very helpful in sizing turbos for your desired power levels.
Turbos and Airflow
We get the question all the time: How do you size turbochargers? In reality, the answer is simple. Turbos should be sized as small as possible for your desired horsepower. Luckily, airflow is one of those rare areas where the stars align, as roughly one pound per minute (lb/min) of air is equal to about 8 rearwheel horsepower on a good-running engine. That means a turbo that can flow 50 lb/min (we’re talking stock-ish here) would be good to about 400 rwhp. For those looking to be right at 1,000 rwhp an S480 is a popular choice, which at 120 lb/min (120 x 8 = 960 rwhp) we can see why. Note that it’s also popular to make more power than the number the formula yields, but it’s usually at the risk of overspeeding the turbo.
Fueling
Engine airflow math is fairly simple (with some exceptions, like intercooling), but fueling is a tough one to crack. Lots of things come into play, including injection pressure, timing, duration, nozzle size and so on. One thing we can calculate pretty easily, though, is the needs of a lift pump. Starting with a factory example, let’s take a truck that has a turbo that flows 50 lb/min of air and calculate the fuel it needs to support that power level. Most factory engines run at around a 20:1 air/fuel ratio (or even higher) to keep smoke to a minimum, so we can see that at that airflow level we’d need (50 / 20) 2.5 lb/ min or 0.36 gal/min (diesel is around 6.93 pounds in weight), or (0.36 x 60) 21.6 gph. Not very much. However, just enriching the air/fuel ratio to 14:1 increases that need to 30.9 gph. On a big-horsepower mechanical truck (let’s say 120 lb/min turbo, and 12:1 air/fuel ratio) we can see the need goes all the way up to 86 gallons per hour.
Now, you may be wondering why there are 100 and 150 gph pumps out there if there’s only a need for 86 gph, even on a hot engine. The answer has to do with pressure. As pressure goes up, flow decreases, as the pump has a harder and harder time pushing fuel to the engine. A pump that free-flows 150 gph might only flow 120 at 20 psi, 100 at 40 psi and 80 at 60 psi. Some pumps aren’t even made to run at those types of pressures, and might not get there at all. Also, the instant need for fuel once the pedal is mashed means that most people go with overkill so there’s no pressure drop when the engine suddenly demands fuel.
Nitrous Oxide
Nitrous oxide can be great fun on a diesel, or it can lead to a lot of