DOHC vs. Pushrod vs. Diesel: Which Engine Technology Has Improved the Most

In a world with multiple competing engine technologies there are likewise multiple different metrics to statistically judge which engine technology is the best. Horsepower per liter, torque per liter, horsepower per pound of engine weight, brake specific fuel consumption, and other metrics are often brought up in discussions about various engine technologies. For now we will disregard most of those metrics and instead look to see which engine technologies have improved the most over the past decade by comparing the hp/liter and torque/liter of three different generations of DOHC, pushrod and diesel engines. Looking over the data* it is easy to see that turbo diesels have made the most significant leap in horsepower and torque output since the 2000 model year. While pushrod and DOHC engines improved engine output by approximately 10% with every new generation, diesel engines on average improved their horsepower output by 16% with very new generation. The discrepancy in torque improvement turned out to be even more visible as the gasoline engine only improved their torque output by 4-5% while diesels averaged 17%. While the numbers certainly stand out, the reasoning behind them makes sense as non high RPM naturally aspirated gasoline engines are experiencing diminishing returns on improvement as it becomes harder and harder for manufacturers to extract additional horsepower from the same displacement. On the other hand it wasn't long ago that passenger diesels delivered about half the horsepower per liter compared to gasoline competition. With new diesel technology this is no longer the case with several diesel engines delivering the same horsepower per liter (and significantly more torque) as their gasoline competition. A good example is the Audi A6 that features a 310 horsepower gasoline supercharged 3 liter V6 and a 313 horsepower 3 liter twin-turbo diesel V6.

*DOHC category (Audi V8, BMW I6, Porsche H6), pushrod category (Chevy 5.3 smallblock, Corvette smallblock, Chrysler pushrod engines) Diesel category (I4, V6 and V8 TDI engines)