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?

Question

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?

In: 87

Answer 1

Horsepower is a marketing number. The number that actually matters is torque. Horsepower is a measurement of torque over time and distance.

One thing a lot of people don’t realize is that you can have big fancy horsepower numbers and have them be almost entirely useless.

Your 2.0 and 3.0 examples may have the same PEAK horsepower, but where that peak is in the RPM range is going to be wildly different.

What you really want to look for is a flat “torque curve” that travels as far into the upper RPM range as possible. That is the engine that is going to give you the most all-around fun because it’ll have more available “work” at whatever RPM you’re at.

There’s a lot of other variables like vehicle weight, transmission options, etc etc etc that can make the driving experience wildly different. My 1976 280Z makes like 150 horse, but only weighs like 2600lbs after eliminating things, but the weight distribution is 50/50 and it’s agile as hell, so it’s a quick bit of fun despite not being super fast.

All things the same though, if you have a daily driver that is going to be used at all RPM ranges, and you specifically want the most available power whenever you press the go-go pedal, the larger displacement engine will give you that. It’ll cost you in terms of overall fuel efficiency, because larger displacement engines don’t tend to be pushed up the thermal efficiency ladder as much, but that’s a different consideration.

The smaller displacement engine will be able to make the same horsepower, but it’ll need higher RPMs to do it, and the RPM range where that power is is narrower.

For an extreme example, the BMW M12 for the F1 in the 80s was a 1.6 liter inline 4 cylinder that produced nearly 1400 horse power around 15000 RPMs. It made between 175 and 980 lb-ft of torque between 11k and 15k rpm. But under 11k RPM the car was virtually undrivable.

That said, you can have a little 1.8L 4 cylinder that is moderately drivable at lower RPMs but has a turbo on it so when you floor it, you’re suddenly making 300 horsepower. But the necessity of the turbo means your giggle zone is going to be narrower and higher RPM.