But direct comparisons of what? The problem with hp/l is that it's an arbitrary ratio with no real-world implications at all. All you have said is that it is an indicator of the technology in an engine... well I'm going to have to disagree. Technology can be manifested in different ways. All that matters in an engine are power, efficiency, size, weight and possibly cost (for a road car). The LS6 for example uses technological advances to reduce the weight and increase the power without affecting size and with only a slight effect on efficiency.
You answered your own question...it's comparing the power output standardized to a set displacement. It may not have real-world applications to us consumers, but in the design and engineering world, it has some meaning. If you read through science and medical journals, you'll see so many arbitrary ratios and statistical calculations that make absolutely no sense to most. But a lot of that does have real-world meaning in the long run. I'm no car engineer, but I am well-versed in medicine and scientific principles, and have worked in research labs.
In science and research, you have to standardize values so you can make direct comparisons. Just like in miles per gallon. If you have two cars that each go 200 miles to a tank, you can't say which one is more fuel efficient unless you standardize it to an arbitrary ratio. One could have a larger tank than another. I'm certain that engine designers cannot just go and make an engine any which way they want. They have to design an engine under given parameters, and one of them is displacement. So they will maximize power, economy, emissions, torque, etc. with what they are given. To do part of that, they will try to maximize the hp/liter ratio. This could be used as a benchmark or "goal" or a datapoint for engineers when they are designing an engine, and comparing the specific outputs of various head/cam profiles. How many times, when you are talking about engines, will you hear someone say "my engine puts out X horsepower"...one of the first things will be "how big is it?"...whether you like it or not, you just indirectly requested the hp/liter ratio.
If you think arbitrary values like that are useless, then talk to Ford, since in the brochure for my Lightning truck, they mention the hp/liter of it's engine (70 hp/liter). It must have some meaning to them. And talk to NASA, where they use a very similar arbitrary ratio called "horsepower per pound". If you are familiar with the STS technical files in the government section of your library, you can read up about the amazing technology they put into the Space Shuttle. One of which is the fuel pump (turbopump) used to feed the main engines. Each fuel pump puts out 76,000 horsepower continuously until main tank separation. And it only weighs 700 pounds. They actually mention that they strived to exceed 100 horsepower per pound of machinery. Does that arbitrary value have any meaning to you? Well it means something to the rocket scientists at NASA.
I said Hp/liter was AN indicator of technology, not the ONLY indicator of technology. I agree that technology can be defined in numerous ways, and that's beyond the scope of this thread.
No not really, which is why bikes aren't relevant here. Smaller displacement motors are extremely easy to pull huge hp/l numbers from because they can rotate much faster (due to smaller cylinders through which to propagate the fuel burn and less mass to spin among other reasons)... the same engine design applied to larger displacement engines doesn't produce anything close to the smaller displacement's numbers. You would never get a street engine with 8 liters of displacement to spin at 13000 rpm reliably. You might as well try to apply model airplane engine design to an 8L engine because after all, 500hp/l is pretty good!
You must not be very familiar with the different types of bike engines out there. Let's consider the V-twin sportbikes. A good one is the RC-51. 1100 cc's, 2 cylinder, DOHC, 135 crank hp. So that's 123 hp/liter. (The Busa, a DOHC inline-4 at 1300 cc's, is 135 hp/liter). Pretty close ratios, huh? And still above the Viper's ratio of 56 hp/liter. Well, the RC-51's cylinders are about twice as large as those in the Busa, and are putting out comparable numbers. And the RC-51 cylinders aren't that much smaller than what you can find in a car engine. Let's make a V8 from the RC-51 engine: 1.1 liters X 4 (to make a V8), comes out to 4.4 liters. So that theoretical 4.4 liter V8 could be producing around 540 hp. Are ~4 liter V8s produced? Maserati, Toyota, and Ford make them. And don't forget, the Viper engine, if you removed two cylinders to "make" it a V8, is only at 6.4 liters, and to me, that's not a very big difference anymore. So the difference in piston sizes between hi-perf V-twin DOHC bikes and car engines is not as big as you think. And hence, to me, bike engines are still relevant, and their head/cam technology can still be applied.
The Nissan Titan DOHC 5.6 liter V8 truck engine makes 68 lb-ft/liter of torque. More than the Viper's 56 lb-ft/liter. And it's just a truck engine, not a performance engine. So let's see, it makes more specific (not overall) torque than a Viper motor, but it's not that much smaller, and uses DOHC. So should they scrap the DOHC and go to pushrods?
The 1300 cc inline-4 Busa engine is essentially half of a CART racing V8 engine (2.65 liters by last year's or so rules). Of course, the stock Busa engine is much less powerful. So you mean to tell me that car makers, like Toyota and Honda, absolutely do not use the technology they get from their tiny "bike-sized" CART engines, and apply them to their larger street car engines? They may not use everything from their racing engines, but I bet you some of that DOHC technology "trickles down" to passenger and sport engines.
The model airplane engine analogy is silly, since the displacement difference between a bike and car engine is much much smaller than the difference between a car and 0.90 cubic inch model airplane engine (we're talking a factor of two vs a factor of 200, respectively). I do agree that (really) small engines can create big power for their size.
Why is it good to minimize displacement?
You'll have to ask the car makers that one. I like big motors. Let me restate my point you quoted. Let's say you had a contest, where you had an engine limit of let's say, one liter, and the winner was the one who made the most power, standardized by using same fuel, normally aspirated. You can use any piston/head technology out there. Other parameters, like fuel efficiency, emissions are out. It doesn't take a rocket scientist to see that for a given fixed displacement, the engine with more power is probably a little more advanced than the one making less power. And if you try to increase total power of an engine by increasing it's displacement alone, rather than keeping the same (smaller) displacement and then re-designing the heads/cams for more power, the latter, for me, requires more technology.
I'm tired now...
