Diesel Performance Theory

Beginner's Guide to Diesel Performance

Engineering a diesel engine for mass production is a balancing act of cost, performance, fuel economy, and emissions, amongst other considerations. As a result, your engine is not optimized for maximum horsepower and torque and modern diesel engines contain huge performance potentials that can be unlocked with aftermarket modifications. While tuning alone pays homage to this potential, it's important to understand the various principles surrounding diesel performance to better plan the depth and thoroughness of your upgrades.

Fuel and Airflow

The most obvious performance increase comes from increasing the amount of fuel and air in the cylinders during combustion. This may come in mechanical forms, such as high capacity injectors or increasing turbocharger boost, or electronic forms, such as tuning that modifies injector timing and pulse width. Additional power can also be found from boosting fuel pressure to promote complete atomization during an injection event.

An increase in only fuel or only airflow is counterintuitive - an increase in fuel requires a corresponding increase in airflow to maximize performance, and visa versa. Too much fuel and/or not enough airflow results in higher exhaust gas temperatures and copious amounts of black smoke, both signals of highly inefficient conditions.

Pumping Losses

Any internal combustion engine can be modeled as a simple pump; air is pumped into the engine, compressed, then it is pumped out. Since most diesels are turbocharged, pumping losses during the intake stroke are minimal. On the exhaust stroke, the factory exhaust and emissions system provide significant restriction as the engine pushes exhaust gases out. For this reason, there are many exhaust systems and emission delete kits that reduce this restriction and let the engine breathe easier. This typically broadens the torque curve and provides a noticeable improvement in overall torque. Reducing exhaust restriction also lowers exhaust gas temperatures and decreases turbocharger spool times.

On the air side, aftermarket intake kits, improved intake manifolds, and air horns can increase the efficiency of the turbocharger, decreasing lag times and reaching max boost sooner. These items also help improve airflow overall, which is beneficial in increasing horsepower and torque.

Volumetric Efficiency

In an internal combustion engine, volumetric efficiency is the ratio of the volume of air that is drawn or forced into a cylinder (measured at atmospheric pressure) versus the actual volume of the cylinder. It is used to measure the efficiency with which the engine can move/force air into the cylinders. For example, a single cylinder of a 5.9L Cummins has a maximum displacement of 59.8 cubic inches. If 100 cubic inches of air is forced into the cylinder during each intake stroke, the volumetric efficiency would be 100/59.8 = 1.7.

Forced induction engines will always have a volumetric efficiency greater than 1 while producing boost because the air charge is pressurized. Installing a more efficient turbocharger, intercooling, and reducing intake restrictions may all improve volumetric efficiency. Economical options are more limited for naturally aspirated engines, although aftermarket turbocharger conversion kits are readily available for many NA applications.

Naturally Aspirated Diesel Performance

Performance options for naturally aspirated diesel engines are limited unless they are converted to a turbodiesel. This is primarily due to the limited amount of air that can be drawn in. Volumetric efficiencies above 1 are achievable with naturally aspirated engines, but will never reach the VEs of forced induction engines. Adding fuel is relatively easy with mechanical injection pumps, but the maximum setting will be reached quickly as the rich mixture proves impractical for everyday operation.

The high compression ratios of a naturally aspirated diesel typically limit maximum safe boost levels to around 10-15 psi. In the end, a NA diesel simply won't compete with a modern turbodiesel, all things considered. But what a NA diesel lacks in performance, it more than makes up for in simplicity, and the lack of emissions components make them a very potent platform in terms of fuel economy.

Diesel Performance, Longevity, and Reliability

Increasing performance will reduce the longevity of your engine, but the effect may be negligible depending on your setup and how your truck is used. It's basic physics; a 300 hp engine will outlive a 500 hp engine subjected to the same conditions. Given the strength and ruggedness of most diesel engines, the difference is likely negligible in this example. If the same 300 hp engine is compared to a 1000 hp engine, the differences in life expectancy become more clear. On the other hand, some modifications, such as emission deletes and head studs, may increase the longevity and reliability of your engine. Additionally, how you use and how often you service your truck is of equal importance.