Originally Posted By: HTSS_TR
I expected the auto industry should by now produce high efficient engines that can generate 140-150 HP per liter or more without resorting to turbo charged.
That's not a realistic expectation. To make power without forced induction (assuming deep breathing capabilities) you need one of two things:
1. RPM
2. Displacement
The BMW example I cited earlier, making over 100HP/L, did so out of a 5L engine. But it also is, like the S2000, relatively high strung with a redline north of 8,500RPM. While that may sound awesome, it made the car feel slower than the car it replaced, which was the E39 M5. It was of course actually faster, but perception is a big thing. And like the guys ragging on the S2000 for having to be wrung out to actually go anywhere, similar comparisons were drawn with the E60 vs the E39.
Remember, HP (work), in an automobile engine, is a product of torque and RPM. It is a measure of work being performed. The higher the rate, the smaller the units need to be to do the same amount of work. So you can provide a lot of force at a slow rate (low RPM, high torque) or a small amount of force at a high rate (high RPM, low torque) and end up with the same amount of work being performed. The S2000 doesn't make a lot of torque. Its power-band is very high RPM biased, which is how it is able to make the power it does. This means that low-speed performance suffers. As was pointed out, that kind of setup doesn't make sense for a regular production car.
Say you have a performance target of 300HP. Now, you can meet that performance target a variety of ways:
1. You can use boatload of displacement and produce a relatively RPM limited engine that has a strong low-speed power curve. Fuel economy may suffer a bit though. In this instance it could be like using a 5.4L V8.
2. You can use a compromise of displacement and RPM. You end up with a somewhat softer bottom-end but a healthy mid-range and higher RPM area. This would be good on fuel. Think 3.6L V6.
3. You can use a small displacement engine and rev it to the moon. You end up with an extremely weak bottom-end (makes no sense in a sedan for example unless you like driving a car that feels like it can't get out of its own way most of the time). Think 2.6L I4 or I6/V6. Driven in a manner as to not feel slow, fuel economy would suffer significantly.
4. You can use a small displacement engine with a turbo on it. You get a strong bottom-end, strong mid-range and a decent RPM ceiling. You don't need to rev it to 8 or 9K to extract maximum power and the power curve is very flat. Think 2.5L I4 or a small V6/I6. This engine would be excellent on fuel when not on boost.
Now of course variable cam timing has the ability to move the power curve around a bit to bump up the bottom-end and increase the top-end. I know, I have two cars with it
But ultimately the one engine still makes 380lb-ft of torque because it is a 5L V8. And it does so because that same cam profile, retarded, makes 400HP at 7,000RPM. So while you can move the power curve around a bit to make the torque curve flatter, ultimately total output is a compromise based on designed RPM range and displacement. The 5L V10 was designed to make 100HP more, but to do so, it had to rev almost 2,000RPM higher, which means that the torque curve couldn't come on as low or be as flat. The cam profile didn't allow for that.
So, if you wanted to fatten up the bottom-end, you'd have to throw displacement (or boost) at it. And there's still the compromise. Do we do high HP and a doggy bottom-end or do we want reasonable HP and enough torque to make grandpa feel like his neck is going to snap over the back of the seat? It takes aggressive camshaft profiles, deep breathing induction....etc to make that kind of power. Your average Joe does not want to have to rev his Camry to 10,000RPM for it to feel fast (which would be the case if you wanted 140HP/L) and it be a raging turd the rest of the time. And if we throw a pile of displacement at it, we are just moving the problem around. A 140HP/L 6.0L V8 would make 840HP at some insanely high RPM, still be a dog down low and be an absolute pig on fuel. It isn't practical.
THIS is why boosted engines are in vogue. Good fuel economy, good power output, great torque with a FLAT curve and a sane RPM range.
