By Toda Namiki, Editor, Japanese Auto World
Choose a truck's engine by the work it does, not by the headline figure in a listing. Match the engine to the heaviest regular load, the steepest regular hill and the distance you drive each day. A larger engine gives more pulling power and generally uses more fuel. A smaller engine costs less to run, but it works harder when it is loaded.
Key facts
- Diesel holds more energy per gallon than petrol. The U.S. Energy Information Administration lists 137,380 to 138,490 Btu per gallon for diesel, depending on sulfur grade, and 124,340 Btu per gallon for motor gasoline.
- The Alternative Fuels Data Center of the U.S. Department of Energy sets the truck classes by gross vehicle weight rating (GVWR). Class 1 is under 6,000 lb, Class 2 runs from 6,001 to 10,000 lb, and Class 3 from 10,001 to 14,000 lb.
- Isuzu Australia rates the N Series NLR at 4,500 kg GVM, with 110 kW and 375 Nm. The NQR is rated at 8,700 kg GVM, with 140 kW and 513 Nm, according to its N Series page.
What does engine size tell you?
Engine size, or displacement, is the total volume the pistons sweep, usually given in litres or cubic centimetres. Torque is the turning force that gets a loaded truck moving from rest and keeps it climbing. Power is how fast the engine can do that work. For daily hauling, torque at low engine speed tells you more about how a truck will work than peak power does.
How does payload change the engine you need?
Start with the gross vehicle weight rating (GVWR) in the United States, or the gross vehicle mass (GVM) elsewhere. This is the maximum weight of the truck with its load, and it appears on the manufacturer's label. Payload is that rating minus the truck's own weight. Heavier ratings usually come with more torque. The NLR's 375 Nm serves a 4,500 kg truck, while the NQR's 513 Nm serves an 8,700 kg one.
Check the rating on the truck itself. The N Series page shows ranges for several models, so the exact figure depends on the variant. For the history of this range, read why the Isuzu Elf sells so well around the world.
Diesel or petrol: which engine suits your work?
Diesel carries about 10 percent more energy per gallon than petrol, and diesel engines are built to produce strong torque at lower speeds. That suits heavy loads, frequent hills and long daily distances, which is why diesel is the usual choice for medium and heavy trucks. Petrol suits lighter loads and shorter routes, where the engine spends less time under heavy strain.
Smaller trucks raise their own questions. The kei truck explainer covers the smallest class of trucks in Japan.
Does a bigger engine always mean a better truck?
No. An oversized engine on a short urban route spends most of its life at light load, and its extra capacity burns fuel without earning it. An undersized engine pulling a full load runs hard for long periods, and the strain shows in the gearbox, clutch and cooling system. Choose the engine that handles the regular work at a comfortable load, with room for the hard days.
What should you check on the engine of a used truck?
- Ask for the engine hours and the service records. For diesel trucks, hours show wear more reliably than the odometer alone.
- Start the engine cold and watch the exhaust. Blue smoke suggests oil is burning. Heavy black smoke under load can point to a fuel or air-supply problem.
- Look for oil leaks around the valve cover, the sump and the turbocharger, if one is fitted.
- Check the coolant level and the cooling system for leaks or a history of overheating.
- Compare the engine number on the paperwork with the engine in the truck.
For a full checklist, see the used truck maintenance guide. The used truck buying points guide covers the rest of the inspection.
Frequently asked questions
Is a bigger engine always better for a work truck?
No. A bigger engine helps when loads are heavy and routes are hilly or long. On light, short work, it adds running costs without adding useful capacity.
Do diesel engines use less fuel than petrol engines?
Often, under load, because diesel holds more energy per gallon and its engines are built for torque at low speed. Real figures depend on the engine, the load and the route, so compare fuel use on the same work.
Where do I find a truck's GVWR or GVM?
Look for the manufacturer's compliance label, usually on the driver's door frame. It lists the maximum weight of the truck with its load.
Does a turbocharger change the engine size?
No. A turbocharger forces more air into the engine, which can raise power and torque without changing displacement. Turbo and non-turbo versions of the same engine often carry different ratings, so check the figures for the version you are buying.
Is a small engine enough for a light delivery truck?
It can be, on short routes where loads stay within the rated payload. Check the GVWR and the torque figure before you decide, and test the truck loaded if you can.
Verdict
Size the engine for the hardest regular day of work, not the average one. Start with the load, the hills and the distance, then check the torque figure for that exact model against the fuel you can buy where you operate. New to buying trucks? The first-time truck buyer guide sets out the basics.
