Techy Goodness: A Deeper Look at the Lamborghini Aventador’s New V12 and ISR Gearbox
The new engine, not the same as the old engine: Lamborghini claims that the new Aventador LP700-4 that debuted at the 2011 Geneva auto show comes with an all-new 6.5-liter V-12 engine, codenamed L539, and that it shares no parts with any other engine in the entire VW Group. (Check out our previous breakdown of the new powertrain.) Research and development director Maurizio Reggiani says that the new powerplant doesn’t even share the bore center spacing of the outgoing V-12 or any other corporate engine. The 60-degree V angle, however, is unchanged from the Murciélago’s twelve-cylinder. “The perfect angle,” says Reggiani with a smile.
We won’t go over every single change to the new engine, but there are a few highlights. Peak power is 691 hp at 8250 rpm, peak torque is 509 at 5500 rpm, and maximum engine speed is 8500 rpm. To achieve such a high-revving output, the L539 uses an oversquare bore and stroke of 95 x 76.4 mm. The outgoing Murciélago LP670-4 SV was nearly square at 88 x 89 mm. The shorter stroke helps keep piston speed down, reducing inertia and heat from internal friction. The pistons glide up and down inside cast-iron liners; the V-10 in the Gallardo uses a co-casting technique to line the cylinder, but the sleeves in the V-12 allow for more lightweight construction. Compared to the Murci’s V-12, power is increased 8 percent and fuel consumption is reduced 20 percent thanks mostly to an impressive 11.8:1 compression ratio.
Even with such a high compression ratio, the L539 engine uses port fuel injection and not direct injection. Reggiani explains that if the company employed direct injection, the engine would have needed either a particulate trap or a secondary port fuel-injection system to meet upcoming Euro6 emissions standards. A particulate trap adds weight and increases cylinder back pressure, which makes the performance targets more difficult. A secondary fuel system would help emissions on cold start, but also add weight and complexity.
And Then There’s the Gearbox
The other jewel in the Aventador’s powertrain is the ISR automated manual transmission, short for Independent Shifting Rods. With a claimed shift time of 50 milliseconds (as measured by the amount of time the flow of torque is interrupted), the ISR is the fastest production transmission in the West—and the East, for that matter. For comparison, Reggiani says that a Formula 1 car shifts in 40 milliseconds, and the fastest the Gallardo can achieve a gear swap is 120 milliseconds.
This is done while still using only one clutch. Weight, again, was a huge factor and a dual-clutch was deemed too heavy. As it is, Lamborghini says the new transmission represents a 26-pound weight savings. So how does it change gears? Despite the single clutch, the idea is similar to a dual-clutch. In a conventional manual transmission, gears are stacked on a shaft in order. A four-speed, for instance, has first and second on one shaft and third and fourth on another shaft
This also is where the independent shifting rods come in; the ISR transmission has four shifting rods to carry out the work (the aforementioned old-fashioned four-speed gets by with two). Further helping the speed is the lack of any external hydraulic lines. By making the pressurized lines part of the housing casting, Lamborghini eliminated any spring effect that can happen with a metal hose. The Aventador’s transmission moves the odd- and even-numbered gears to different shafts (as in a dual-clutch automatic), so that one gear can be engaged while the other is being disengaged, ensuring the absolute minimum amount of time that a gear is not engaged. To facilitate smoother shifting and better wide-open throttle performance, the Aventador can reduce engine spark in the middle of the shift; this briefly reduces engine torque and allows the gear change to happen with a reduced load on the gears and synchros.
The big question with the transmission is whether or not it will cause the car to lurch during lower-speed urban driving. Single-clutch automated manuals are not known to be particularly smooth in such situations. We’ll find out in a few months if the new bull’s transmission has been tamed. Stay tuned.