Inside the new 1250 engine
The new 1250 has a bigger bore (up to 102.5mm from 101mm) and a longer stroke (76mm from 73mm) than the previous 1200. This increases the engine’s capacity from 1170cc to 1254cc.
Increasing the stroke more than the bore makes the motor less oversquare – usually, something associated with a gruntier, less-revvy power delivery. Sure enough, the new engine’s peak torque arrives 250rpm lower, at 6250rpm
Maximum power jumps from 123bhp to 134bhp, while torque is up from 92lb·ft to 106lb·ft. So that’s 9% more power and 14% more torque from just 7% more capacity — which means the new engine isn’t just bigger but more efficient too.
On the face of it, plenty is unchanged. Valves remain the same diameter (40mm intake, 34mm exhaust) and sit at the same angle (8° intake, 10° exhaust) in the head. Throttle diameter is still 52mm. Compression ratio is an unchanged 12.5:1. Gearing is identical, from primary drive to the six-speed box to the shaft final-drive ratio.
On top of all this, there’s also a new variable valve timing system called Shiftcam. The intake camshaft now has two pairs of cam lobes: a smaller set designed for lazier running; and a pair of lumpier lobes for when you’re nailing it.
Unusually, the partial-load cams (which engage when you’re on a gentle throttle below 5000rpm), have independent timing and lift for the two intake valves in each head — one moves 2mm, the other 4mm. BMW says this swirls the air/fuel charge, filling the cylinder better and improving efficiency. The intention is not additional performance but better emissions, smoothness and economy.
When you open the throttle wide, or the revs rise above 5000rpm, Shiftcam switches to the full-load lobes. These lift the intake valves 11mm and hold them open longer, releasing the engine’s full performance. To switch lobes, the ECU sends a message to an electronic actuator, which fires a pin into a curved channel at the end of the cam. As the camshaft rotates, the pin forces the whole shaft to slide along. To switch back, a second pin is triggered, returning the cam to its original position. Shiftcam is electronic and requires no additional maintenance, according to BMW.