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18 Jul 2011 | Text & Photos by Benjamin G. Kline | Category: Car Technical Advice
Turbocharged cars have always been heralded for the explosive power they muster. But the lag experienced in a single turbo system continues to make it less attractive than a Naturally Aspirated car. We find out if twin turbos are the way to go...
While the single turbo gives incredible power, it however carries a penalty. Lag. Lag is the time it takes for the command from your right foot to garner a reaction from the engine. This delay is caused by the amount of time the turbine takes to spin up to generate enough boost for the explosive combustion.
In a highly modified car like the Evo 9 GSR I once had the chance of sampling, anticipating the power after a monumental lag can be both thrilling and a rather humbling experience. The mad Evo was sporting a highly tuned 2.0-litre under the hood that was hooked up to a gigantic Garrett snail. Apologies for not having any exact technical specs about the turbo besides measurements from my butt-o-metre.
Once the turbo kicked in (and I really do mean a violent kick in the nuts), the car accelerated relentlessly from 90kmh to 150kmh in inconceivable seconds. The only reason the car stopped accelerating was due to a lack of testicular fortitude (and losing my license) on my part than a lack of boost or power.
While thrilling in a straight line, large single turbos make it that much harder to drive when attacking the bends. More often than not, you might find yourself in a lower speed and too low down the rev range to have any effective boost.
For example, despite my love for the WRX, the lack of a twin turbo setup means you're caught out of boost at the most inopportune moments in a mid-corner attack where low-end torque is most useful in pulling out of a corner. This is a problem that while complicated to solve, is possible with an experienced tuner and fabricator that's able to work around the space constraints of a WRX's engine bay to fit a twin charger system.
Different twin turbo configurations - Sequential turbo system
When it comes to twin turbocharging, automobiles typically have two setups for such systems, sequential or parallel.
In the sequential method, the system uses a smaller turbo at lower engine speeds and a larger turbo at higher RPM. Larger turbos are less efficient at low RPM due to the greater inertia in their larger turbines which results in a lower intake manifold pressure. Whereas a smaller turbo spins freer with little inertia at lower RPM but are unable to provide enough air at higher RPM.
In the low to mid RPM, the available spent exhaust energy spins up the smaller turbo (primary) only. During this initial stage, all of the engine's spent exhaust energy is directed to the primary turbo only. As the RPM climbs, the larger turbo (secondary) is partially activated in order to spin up the turbine for impending operation. When the preset engine RPM and boost pressure is reached, valves controlling the compressor and turbine flow to the secondary engine will be fully open. Thus allowing the secondary turbo to be fully activated.
Some notable performance machines that utilise this method of turbocharging include the legendary Porsche 959, the iconic Toyota Supra as well as the FD3S Mazda RX-7.
Where older performance machines used to favour the sequential setup, the latest fire snoring steeds coming out of manufacturers are leaning more towards this parallel system. Unlike the sequential system, the twin turbos in the parallel system function simultaneously splitting the turbocharging duties equally.
In a parallel setup, both turbos are fed equally by the engine's exhaust. On an inline powerplant, both turbos will send their charged air to a common intake manifold which is then sent to individual cylinders. Turbochargers can be mounted to its own exhaust manifold but on inline engines, both turbos can be mounted on the same manifold.
On V-type engines, the twin turbos are mounted each to its own cylinder bank. This provides symmetry and simplifies the engine plumbing significantly.
Theoretically twin turbo systems should possess less turbo lag than a single turbo setup. But they would typically still have some turbo lag. The amount of lag is dependent on the size of the turbos used and the pressure setting. Using smaller turbos with a light pressure would garner better and more immediate response although this would sacrifice power at the top of the rev range. But it would still possess less lag than with a similar engine on a single turbo setup.
Some notable cars that utilise this system are the Nissan GTR, the R34 Skyline GTR as well as the BMW 1 -series M Coupe.
Parallel twin turbo systems can be applied to more than two turbochargers. The Bugatti Veyron utilises four turbochargers in its W16 powerplant and produces 1200bhp.
A variation of the system would be the variable geometry turbochargers. This system changes the angle of the guide vanes within the turbine according to exhaust pressure. This gives the system excellent power and response throughout the rev range. The current 997 generation Porsche 911 Turbo is one of the most famous examples of such a system.
Fitting a twin turbo system
While the benefits of a twin turbo system over a single turbo setup is obvious, the challenge of actually installing it into a stock car's engine bay poses enough of a challenge to turn most drivers away. Not to mention the cost of engineering such a huge undertaking.
Out in North Carolina in the US of A though, a tuning outfit called Underground Racing has developed a kit that takes supercars to hypercar status through the use of twin turbo systems. They've effectively engineered a twin turbo system that bolt on directly to a Lamborghini Gallardo's V10 powerplant that takes the 513bhp Italian supercar to over 1500bhp. Enough power to make even the great Valentino Balboni wet his undies.
The impressive thing about the system isn't just its functionality but its form as well. The R&D team has created a turbo system that not only fits nicely in the Gallardo's engine bay but has created a work of art in the process as well. The combination of symmetrical chrome pipes is as pleasing to the eyes as it is to the ears and is a testament to the tuner's craftsmanship.
Underground Racing is so confident about their twin charger kits for the Gallardo that they offer a 2 year / 40,000km warranty on it. Just to underline how insane the 'charged' bull is, the 390km/h Gallardo comes with a parachute attached at the rear.
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