rpm when towing

4x4ord wrote:


Here is some facts for you:

Gasoline has more energy per pound than does diesel.

Diesel engines need more air to burn their fuel than do gasoline engines. So when comparing a gasoline engine to a diesel of the same displacement the gasoline engine will make more torque and because it can be designed to rev much higher it will make much more Hp than the diesel.

The designer of the diesel engine (Rudolph Diesel) predicted that due to the slow burning of the fuel, the diesel engine would never be able to maintain operating speeds above 700 rpm. High pressure fuel injection has changed this.

Unlike gasoline engines, Diesel engines have been well suited for turbocharging for decades. The high torque of the diesel stems from turbo charging.

The reletvely recent development in direct injection gasoline engines is allowing gasoline engines to be effectively turbocharged. For instance the Honda 1.5 liter turbocharged gasoline engines make 108 lbft of torque per liter from 1700 rpm all the way to 5500 rpm. The HO Cummins makes 134 lbft of torque per liter at 1700 rpm but by 2800 rpm it is down to the same 108 lbft that the Honda makes. Peak HP of the Honda is 116 HP per liter. The HO Cummins is capable of only 54 HP per liter.

Although Diesel fuel has less energy than gasoline per pound it has more energy per gallon than does gasoline. This and the high compression ratio of the diesel engine contribute to its better fuel economy.

A high HP gasoline engine will beat a lower horsepower diesel up the hill everytime if the gasoline engine has the necessary gears to keep it in its peak hp rpm range.

Some of those aren't facts.

Diesel torque stems from its more explosive combustion which pushes the piston down with more force and the fact that diesels generally have longer strokes than gasoline engines which creates more low end torque, but also limits max engine speed. Turbos only make the output greater just like they do with gasoline engines.

You also can't use final displacement as a power comparison. You can have a long stroke 6.0L engine and it will have completely different characteristics than that of a short stroke 6.0L engine. Cams and what duty cycle the engine is tuned for will also make a big difference of how much output and engine has and when.

Comparing the the torque output of a Honda 1.5L to Cummins pickup engine is just plain silly. They are built and rated for two different things. The Cummins is de-rated significantly to keep exhaust and other temps in check for towing while the Honda is not. If you put that Honda engine in a tow vehicle application then I can guarantee that the torque rating will go down. The Cummins also has more restrictive emissions equipment limiting that power a long with turbos and cams made for a towing duty cycle.

If you want to make both engines that you used an example for a more even playing field, then you would use the QSB 6.7L which is not de-rated for towing and has about the same emissions equipment as the Honda. The QSB 6.7L has a max output of 542 hp @ 2,900-3,300 rpm and 1,250 lb-ft @ 2,000 rpm. It is down to 890 lb-ft by the time it hits max hp at 2,900 rpm. That would put it at 187 lb-ft per liter at peak torque and 132 lb-ft per liter when it hits max hp at 2,900 rpm.


Cummins QSB 6.7L Spec Sheet