Eli5: 2 cars have the same Horsepower but different engine in terms of displacement (2.0L TDI vs 3.0L TDI) but what difference does it make and which one is better?
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Manufacturers are moving more to software changes between models instead of hardware.
When a company used to do 1.4, 1.6, 1.8, 2.0, 2.5, 3.0 etc, they’ll just more to a 2.0 and a 3.0 and do them in different levels of tuning. You don’t need a huge variety of different engine blocks and parts, they’ll often share similar transmissions, and you’d only have two main “projects” on the go, rather than 5-6.
There often will be different bolt on parts, maybe a slightly smaller turbo on the lowered powered versions, maybe bigger injectors on the higher powered version, but generally they are pretty close.
Often two side-by-side versions might share all the same parts, and it’s just tuning levels in the engine ECU.
Torque tends to be related to the engine size, HP is more linked to how hard they push it and rev it too. You can always increase those by pushing them harder, but in general you’ll tend to find that the bigger engine makes more torque at lower revs.
Current “averages” are about 190hp / 420Nm from a 2.0 diesel, and 270hp/500Nm from a 3.0d. Often anything making higher figures than that tend to be classed as the more sporty models, maybe twin turbos too.
You probably will see lower powered versions as well, often with a “fake” badge on the back, like 280D on a 3.0, where they are trying to suggest they are aiming for 2.8 litre performance.
The higher powered ones might be faked upwards, BMW has 640d which is a twin turbo 3.0 diesel, so they are hinting it makes more power than a 630d. They even went all out with a triple turbo 3.0 called the M50d, to suggest you can expect 5.0 litre diesel performance.
This means you can get into a situation where a smaller engine pushed more than normal has a similar power to a larger detuned version. The detuned version will still probably have a lot more torque low down in cruising driving RPMs, 1800-2000rpm, so it’ll still feel stronger during most of your normal driving.
It’s a more lazy, less stressed engine, the torque would be more artificially limited at higher engine speeds so the maximum power ends up about the same.
It would however end up with less MPG, it’s going to be a bigger engine to lug around, more moving parts, and likely to be less efficient, but it’ll probably last longer, and less chance of something breaking.
This assumes they are engineered to a similar standard – there’s nothing to say a manufacturer could make a bigger engine, mild outputs, but really screw up on some parts and it falls to bits pretty easy. A manufacturer would probably have ranges of strength and durability targets, eg, they’ll know the power, peak cylinder pressures etc, and calculate how strong the other bits like piston rods should be. You might find those parts might be shared on all power versions, the lower ones have stuff 4 times stronger than they need to be, and the top rated ones are on the target. If they have an unusually higher powered version, they might have some one-off stronger parts just in that one model.
As a whole though, the engineering team would probably be told to make everything capable of handling say, 50% more stress than calculated, and those targets would be shared over everything they do
Going back to the 2.0d vs 3.0d, 250hp from a 2.0d would be a push, that engine would be working quite hard, but a 3.0d would do that without breaking a sweat, and you’d consider it detuned compared to what you’d expect a modern 3.0 diesel to do.
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