Large cube small cube engine fuel efficiency

[oleman]

Which engine would be more fuel efficient to produce 100 continuous horse power.
1. A Chevy small block 5.7 liter V8
2. A Nissan KA24E 2.4 liter 4 banger
It's my contention than a smaller engine running harder is more fuel efficient than a larger engine loafing along but I have no hard evidence to back me up.
Let's assume that both engines are modern fuel injected normally aspirated engines that are basically in the stock configuration.

[kouki munster]

This reminds me of the top gear episode where the fuel mileage of a prius and a m3 was tested around the test track. The prius was driven like it was stolen and the m3 just had to keep up with the prius, the prius did ~17mpg while the m3 did~19mpg.

[Dattebayo]

Why the hell would you compare those two engines?

[MinisterofDOOM]

There's a lot of "theoretical" involved in this. There have also been numerous 5.7 liter SBC variants over the last 54 years, so we'd need to pick one. Practically, if you were looking to make 100hp with each of those, you'd tune for it, so we'd be running at a designed constant RPM with the intention of being efficient. So the next question is: are we looking for an ideal tuned setup for that 100hp, or are we looking at out-of-the-box performance of factory street-car engines?

Horsepower = torque x RPM / 5252. So, in order to make power, if you lack torque you must rev higher. Or alternatively, if you're not particularly suited to high-revs, you must make more torque.
But 100hp isn't much, so it won't stress either capability too far. The real problem with the engines you've chosen to compare is that they both share the same strength: low-end torque. It's not like an F20C vs a Chrysler LA V10. We're not going to be comparing crazy revs from 4 cylinders vs ultra-low revs from 8 cylinders.

If you look at tractors, bulldozers and other modern work equipment, they usually use VERY high-torque diesel engines that run at VERY low rpm (well below 1000...in the realm of 150 to 300rpm) to drive hydraulic pumps which then operate the vehicle (including drivetrain). It doesn't take many revs to make 100hp if you have much torque.

An LS1, which is a modern, fuel-injected small block displacing 5.7 liters, makes 100hp at about 1700rpm. The KA, alternatively, doesn't make 100hp until about 3200rpm. But the KA doesn't use exactly the same amount of fuel per cylinder per ignition as the LS1, so that doesn't tell us everything we need to know.

Bore and stroke for the LS1 are 99mm and 92mm.
For the KA24DE they are 89 and 96 mm respectively.
So there's not really a whole lot of difference in displacement on a per-cylinder basis, so we can expect the LS1 to drink about twice as much per rev--probably a little more, realistically.

In the end, to answer the question, we'd need to know how much fuel is used to turn the engine one time.

These two particular engines happen to share the same strength: low-end torque. And 100hp is hardly a difficult mark to achieve. So it's not a very disparate fight. I'd put my money ont the KA.
It'd be more interesting to compare, for instance, an LS3 (6.2 liters) and an F20C (2 liters) with vastly different peak torque RPM. And shoot for a loftier goal: 200hp.
The LS3 makes 200hp at about 3200rpm. While the F20C makes 200hp at about 7500rpm. So more than twice the revs but a bit less than 1/3 the displacement.
Or the LA V10 I mentioned above, which weighs in at 8.4 liters in its biggest form, and makes 200hp at about 2250rpm and 100hp at about 1600.