Q. No replacement for what?
Supposedly, there's no replacement for displacement, right? A larger engine will have a greater quantity of air and fuel to push the piston, so why is it that smaller engines tend to have greater efficiency?
For example, the Integra Type-R made roughly 109bhp per liter from the factory, though torque was only 71lb-ft per liter. The contender for American muscle I bring forward is the LS1 engine from the Corvette. Factory output: 350bhp and 360lb-ft of torque. That yields only 62 bhp per liter and 63lb-ft per liter from the 5.7-liter small block.
I understand an engine that revs high can make horsepower simply because of the definition of horsepower. But even based on torque, the relatively torque-less Integra is doing great for its displacement. What is it that decreases the efficiency of a big engine?
VTEC probably factors in because you get low- and high-rev cam profiles. I've heard rumors that the next-generation Corvette might have variable cam timing. Is there a particular reason pushrod engines can't utilize VTEC or something similar?
A. Efficiency can mean a lot of things: power per displacement, power per size or weight, power per unit of fuel burned... So don't fool yourself into thinking high specific output-power per displacement-necessarily means an engine is the most efficient in any other way.
The 'no replacement for displacement' nonsense is true only in terms of one kind of efficiency: power per engine. If you want an engine to make a lot of power above all else, there are few better ways than simply making it big.
Big has downsides, though. Big engines are often made of big parts-heavy parts that don't like to change direction 16,000 times per minute. As you pointed out, the definition of horsepower (torque x rpm/some number that depends on what units are used) means the most direct path to high specific power output is simply increasing the revs. But those big parts don't allow it.
There are more things distinguishing a B18C5 from an LS1 than displacement. The LS1's vintage two-valve pushrod arrangement limits breathing ability at high revs and makes precise valve control difficult at high speeds. The layout is well suited to mid-range output, so everything else in the engine is largely optimized for the middle of the tach.
For a big engine with the output character of a Type-R, build it like a Type-R engine-with small parts. Look at the 6.1-liter BMW V12 in a McLaren F1. Even without the benefit of variable valve lift, this big engine makes 103bhp per liter and an even more impressive 79lb-ft per liter.
Ignoring the engine speed at which it peaks, torque output is a good measure of how well an engine breathes. Torque is effectively a measure of output available from a single combustion event, while power is the sum of all the combustion events happening over a period of time. An engine with good volumetric efficiency (ability to fill its combustion chamber on the intake stroke and expel it on the exhaust stroke) will make a lot of torque.
The fact that the Type-R has better specific torque output may seem surprising, but remember, the engine is designed to breathe well at high revs, taking full advantage of the inertia of fast-moving intake flow and, of course, the kind of high-rev valve timing that wouldn't even run if the same cam profile had to be used at idle. This lets Honda eke out better than 100 percent volumetric efficiency at its 7500rpm torque peak, a feat the LS1 can't match with its fixed valve timing. Even taken to its extreme in the form of the 505bhp LS7, 67lb-ft per liter is all the ol' small block can do.
Variable valve timing is possible on underhead cam engines, but it's taken years for the mechanisms to be developed and for the benefits to outweigh the costs. The trick is that both intake and exhaust lobes must share one cam, but truly effective variable valve timing requires manipulating the two independently. The solution: concentric camshafts that can rotate a few degrees within each other. The cam lobes that follow the inner shaft then have to slide over the outer shaft with a bushing and link to the inner shaft with a pin.
This is exactly what the 2008 Dodge Viper does, though variable timing is limited to the exhaust lobes. This strategy helps this 600bhp behemoth rank as a low-emissions vehicle, but doesn't yield the earth-shattering increase in output one might hope for.
For that, the timing of the intake lobes needs to be varied as well, by adopting some kind of variable lift system. Fitting a VTEC-style variable lift to a pushrod engine hasn't been done, but it could work. Porsche's Variocam Plus, which mated two cam profiles to a single direct-acting valve bucket, could be mimicked relatively easily, with the pushrod's lifter controlling the same cam-switching action the direct-acting bucket did in the Porsche engine. When the pushrod crew will get around to doing this, however, is anyone's guess.