Popping the hood of a new Mercedes-Benz 300SL Gullwing must have been quite an experience for most enthusiasts in the Fifties. For starters, the 3.0-liter straight-six sat canted at a fifty degree angle. Above the lump, a huge cast-aluminum intake manifold without a carburetor in sight. The Gullwing’s long intake runners looked unlike anything else used at the time and served as a bellwether for high-performance internal-combustion engines.
In a car's intake manifold, air enters at atmospheric pressure, rushing toward the intake valves at tremendous speed. But when the intake valves close completely, that onrushing air reaches a dead end and effectively bounces back, creating a pressure wave. If you time the frequency of this wave and the timing of the valves just right, you can get this pressure wave to head towards the intake valve just as its opening. This allows air headed into the combustion chamber to pressurize beyond atmospheric pressure. That pressure isn’t a ton higher, but it's enough to create a "supercharging" effect, improving the head’s volumetric efficiency.
With a short intake runner, the pressure wave at the intake valve bounces back and forth at a high frequency, offering this supercharging effect when an engine’s rpm’s climb. With a longer runner, the pressure wave has to travel a further distance, creating a lower-frequency pulse of air.
"Basically, longer makes more low-end power, and shorter makes more high-end power," says longtime BMW and European car tuner and race engineer Steve Dinan. "The reason is at higher RPM, you have less time [for a pulse of pressurized air to arrive at the rapidly closing valve]. So you have to make the intake manifold shorter so the pulse can get there." Per Autozine, the runners in the 300SL were long to help with low-end power. (Remember that this was an adaptation of the 3.0-liter 'six from Mercedes' big sedan, not a high-revving race-bred screamer.)
If you want to take advantage of the supercharging effect across a wider range of engine speeds, you ideally need intake runners of different lengths. Mercedes received a patent in the Fifties for a variable-length intake manifold system, and in the May, 1966 issue of Road & Track, we detailed a number of experimental variable-length intake systems. It wasn't until the Eighties, however, that automakers began using variable-length intake manifolds.
A great early example is the Porsche 928 S4. Below 3500 rpm, the 5.0-liter V-8 was fed through a longer intake tract. Above 3500 rpm—and depending on throttle position—a vacuum-operated butterfly valve opened and the engine breathed through a shorter tract. This gave the 928 S4 at least 300 lb-ft of torque between 2700 and 4500 rpm. Porsche did something similar with the 964-gen 911, though instead of using runners of different lengths, it used pipes of different diameters connecting the plenums above either cylinder bank. These pipes produced different resonances that helped increase power at different points in the powerband.
For one of the last iterations of its air-cooled flat-six, the 1996 911 Carrera received a new intake system called Varioram, which combined Porsche’s resonance system with pipes of different lengths. That offered the engine three different intake modes. Below 5000 rpm, the engine breathes through longer pipes; Above 5000 rpm, the engine switches to shorter pipes and a valve opens to fill the resonance system; Above 5800 rpm, the resonance system opens to feed the cylinders even more air. As Paul Frere noted in his totemic Porsche 911 Story, Varioram provided for a significant torque boost in the mid-range, but the system was complex. Porsche abandoned Varioram along with its switch to water-cooled flat-sixes. The M96 and M97 flat-sixes which appeared in the 996- and 997-generation 911s and the 986- and 987-generation Boxster/Cayman used a resonance intake system with just two pipes of differing lengths, which connected the two plenums.
Ferrari utilized a similar solution with its Nineties engines. The 550 Maranello, for example—with its V-12 illustrated at the top of this piece—used a system with 12 butterfly valves that effectively lengthened the intake tract for each cylinder. To this day, Ferrari uses similar systems on its V-12s. A particular highlight, the continuously variable-length intake of LaFerrari, which uses a pinion gear system to give a range of intake-runner lengths. It's not just exotica, either. Ford's 2.5-liter Duratec V-6 had a system with sets of long and short pipes, while Honda used variable-length intakes for a number of years. BMW's N62 V-8, used in its larger cars in the early 2000s, had a continuously variable intake system too.
Perhaps the most spectacular system was deployed in racing, with Mazda's four-rotor R26. Rotary engines don't produce a lot of low-end torque, and produce most of their power at high revs. Mazda came up with a wild system that could change the length of its four intake trumpets, turning them into a sort of intake trombone. In the 787B, the engine made 690 hp at 9000 rpm and 448 lb-ft of torque at 6000 rpm, and its excellent fuel economy helped deliver Mazda its first and only Le Mans win, in 1991.
You can also get the exhaust paired to these resonant intake systems. When expelled air from the engine reaches the exhaust collector, it creates an area of low pressure. Dinan explains that this negative pressure rises as air travels through the exhaust primaries and back through the exhaust valve. If you have a bit of valve overlap—when the exhaust valve opens and the intake valve closes—you can use that exhaust pulse to help pull more air into the cylinder.
"That negative pressure wave goes up through the exhaust valve…up to the end of the intake runner, and creates an area of negative pressure that increases airflow to above 100-percent efficiency," Dinan says.
While the aforementioned article from our May, 1966, issue detailed concepts for a variable-length exhaust manifold, none were produced because, well, the exhaust manifold is hot and mechanical systems don’t thrive in that environment. With variable cam timing, however, you can time the valve overlap so that negative pressure wave arrives at the intake valve just when you need it.
Dinan notes that in naturally aspirated engines, this wave effect is useful, but in the age of turbochargers, when upping boost pressure produces huge gains in volumetric efficiency, it's mostly pointless. Still, there are a few examples of automakers tuning their intakes. The new Corvette Z06, for example, has a plenum for each cylinder bank connected via three pipes, each with its own butterfly valve. Those valves open and close in various combinations depending on engine speed, drive mode, and throttle position to give the LT6 strong low- and mid-range torque and a hard charge to its 8600-rpm redline.
Like the Mercedes Gullwing before it, the system is spectacular to look at, and incredible in action.Looking to purchase a car? Find your match on the MSN Autos Marketplace 2023-03-03T21:21:55Z dg43tfdfdgfd