When technical specifications of the new M5 leaked in mid-February, the curb weight left enthusiasts worried. BMW confirmed our worst fears when it unveiled the G90 earlier this week. Heavier than even the fully electric i5 M60, the seventh-gen sports sedan tips the scales at 5,390 pounds (2,445 kilograms) in the US and 5,368 lbs (2,435 kg) in Europe. Why is it so heavy? Although the car is larger than its predecessor, the weight penalty is primarily caused by the adoption of a plug-in hybrid setup.

In an interview with Top Gear magazine, the BMW M boss said the electric bits have added about 400 kilograms (882 pounds) of fat. Mind you, the new M5 has actually put on 474 kg (1,045 pounds) if we’re comparing the US-spec G90 to the F90 before it. Frank van Meel explains why they went with a plug-in hybrid setup instead of a regular hybrid with a smaller battery that would’ve been lighter.

“For us, it makes sense to ‘go the whole way’ and get the full advantage [of electrification] rather than accept setbacks. Yes, the [PHEV] system adds 400 kg of weight, but a normal hybrid system already adds 150-200 kg. We wrote down the consequences of the weight and worked out where we needed to be to have a different window of performance. In motor racing, ballast is added to the floor of the car [to even up] the balance of performance. That’s where we added our ‘ballast’ – the battery is in the floor with a lower center of gravity [than the old M5].”

Although the M5 has a smaller battery than the XM with which it shares the plug-in hybrid V8, the lithium-ion pack still has a useable energy content of 18.6 kWh. It takes 3 hours and 15 minutes to fully charge it at 7.4 kW. BMW considered making the car fully electric, which would’ve obviously had a bigger battery, resulting in an even heavier car.

Frank van Meel also touched on downsizing, saying it would be “crazy to have a four-cylinder” engine inside the M5. He also ruled out an inline-six as that would’ve meant a long hood. BMW also needed a sturdier automatic transmission that could handle 737 lb-ft (1,000 Nm) of torque. Cars like the M3 and M4 don’t go over 479 lb-ft (650 Nm), even the hotter CS variants.

In the new M5, the combustion engine alone produces 553 lb-ft (750 Nm), with another 206 lb-ft (280 Nm) generated by the electric motor. However, there’s a pre-gearing stage permitting 332 lb-ft (450 Nm) at the transmission input.

Expect the upcoming M5 Touring to have an identical powertrain, but possibly even more weight. Wagons tend to be slightly heavier than the sedans they’re based upon. For example, an M3 Competition xDrive Sedan weighs 1,780 kg (3,924 lbs) in Europe whereas the M3 Touring is 1,865 kg (4,111 lbs). That’s an extra 85 kg (187 lbs) the more practical body style carries around.

Source: Top Gear