FAMILY AFFAIR

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With today’s turbocharged engines, extracting more power to suit an enhanced model line-up can be as simple as turning up the boost and re-mapping the management to suit. You might assume that McLaren’s 675LT, unveiled at the Geneva Motor Show in March, could easily have had a similar upgrade, especially as its healthy 675hp (684ps) output from its 3,799cc base is only 25ps up on the already feisty 650S.

But such a straightforward engineering upgrade was not the case for the 675, says Mark Vinnels, McLaren’s executive director for program development, who’s also responsible for the new, long-tailed racer. “It’s not just a software tune: 50% of the parts are new, with new part numbers. Some of it was to reduce the weight of the reciprocating parts, such as connecting rods, which are 13% lighter although still made of steel, and there are new, lighter camshafts with a diamond-like carbon coating. It means the rate of change of engine speed can now be 30% quicker, up to 31,000rpm per second.”

The ‘new’ engine is different enough to merit a new designation, M838TL, with the ‘L’ referring to low weight and low inertia. In the 650S it’s dubbed M838T: McLaren, eight cylinders, 3.8-liters, turbocharged. One of the most obvious visual differences is in the exhaust system, now with fabricated manifolds instead of cast ones, sporting equal-length runners. The new design with its crossover pipes feeds into a new titanium silencer. “It’s a work of art,” Vinnels enthuses, “and it weighs just 3kg.”

Brilliant baritone Not everyone has been a fan of the McLaren engine’s sound print to date, but the 675LT’s exhaust is intended to change that. “We wanted to deliver a unique sound, so we’ve played with the orders in the harmonics,” adds Vinnels. “It’s purer and more tuneful but not too high-pitched – still a baritone.” The new camshafts make a small sound difference, too. Their profiles are unchanged but the valve timing is slightly retarded to give more of a rising torque curve. Torque peak has risen by 22Nm to 700Nm, but now plateaus between 5,500rpm and 6,500rpm instead of peaking at 6,000rpm.

Maximum power arrives at 7,100rpm instead of 7,500rpm. “Driver engagement is increased because the rate of acceleration feels as if it’s rising. If the torque curve is flat, as in the P1, the car still accelerates but it doesn’t feel as if the rate accelerates,” says Vinnels.

Another major new component is the turbocharger, which boasts a casing machinedfrom solid rather than cast, and a new titanium aluminite turbine. It spins at up to 190,000rpm, a gain of 10,000rpm all in all. Fuel injection pressure has risen to 7.5 bar to increase the flow rate; still low in absolute terms, and a reminder that this is still an indirect-injection unit. “We don’t want to go to direct injection,” states Vinnels, “because it’s possible that it might need a particulate filter in the future. Besides, we’re still at 275g/km CO2, so we escape the US gas-guzzler tax.”

There are software changes too, with a new base map calibrated in-house by McLaren instead of by Bosch and Ricardo. There’s a cylindercut during upshifts, replacing the 650S’s inertia-push system (which is retained in ‘track’ mode), but only the ignition is cut. Fueling continues, calibrated to be minimal enough not to burn the catalysts, “so it makes a nice pop, too”, says Vinnels.

The 675LT weighs 100kg less than the 650S, and the cooling system accounts for one of the bigger weight losses. Smaller radiators were made more efficient by rotating them outboard by 2.5°, a small change that has made a big difference in conjunction with the re-shaped rear bodywork. Revised aerodynamics increase downforce by 40%, yet the top speed is still 330km/h (205mph) and 0.1 seconds has been shaved off the 0-100km/h time, nudging itunder 3 seconds. Is there any potential left in this engine for road use? “No,” admits Vinnels candidly, “given what we had to do to maintain efficiency. If we made it bigger, there would be more friction. It’s the ideal compromise.”

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