Mild to wild: KTM introduce variable cam duration and lift for 1390 Super Duke R engine

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The latest KTM Super Duke R may have grabbed headlines for its increased engine capacity, but there’s some clever valve timing tech hidden inside the heads that’s worth talking about, too.

Variable valve timing is nothing new – Honda’s Hyper-VTEC design has been around for decades and more recently Suzuki used a clever mechanical set-up to skirt MotoGP rules that found its way into GSX-R1000 engines. Even 125-level machines in Yamaha’s range use the tech.

But it’s rarer to see bikes that can change the cam duration and lift, which is what KTM’s cam shifting system achieves – with the aim of achieving a broader spread of power and torque across the rev range. Not that the old 1290 range was ever lacking in that department…

KTM's new LC8 engine cutaway

‘Duration’ is how long the valve is held open for, and ‘lift’ is how far it gets opened. Lots of lift and duration lets you get more air and fuel into the cylinder, but smaller, briefer valve openings have a beneficial effect on low-rpm torque, emissions, and throttle response.

KTM’s design concept is similar to the BMW ShiftCam system, and like most motorcycle variable valve systems, it takes its lead from cars. This particular idea can be traced back to the Audi Valvelift System that debuted back in 2008.

It features an intake camshaft made up of a splined central shaft with the lobes mounted on a sliding sleeve that slots over it. There are two cam lobes for each valve – a ‘mild’ lobe with less lift and duration, and a ‘wild’ one with lots of lift and duration.

KTM's new 1390 LC8 engine

The trick is to switch between the two cam lobes incredibly quickly and at exactly the right moment to prevent fast moving engine components from (expensively) smashing into each other.

The engine has to make the switch while the intake valves are closed so there’s no pressure between the lobes and the finger-followers that sit between them and the valve stems. To reliably do that, KTM’s system – like Audi’s AVS and BMW’s ShiftCam – uses two solenoid actuators and a pair of spiral channels machined into the sleeve on the camshaft.

Activate one solenoid and it inserts a pin into the first spiral channel, pulling the sleeve carrying the cam lobes in one direction and engaging the mild cam lobe.

KTM variable timing exploded diagram

Put 12V to the second solenoid and it inserts a pin into the other spiral groove, moving the sleeve the other way to engage the racier cam profile. Because the spirals are cut into the same sleeve that carries the cam lobes, the movement is always timed to happen when the valves are closed, and the grooves are shaped to push the pins back out again when their work is done, within one rotation of the camshaft, so there’s no chance of both solenoids trying to engage simultaneously.

The solenoids themselves are fitted in a single unit called a ‘double pin actuator’, and KTM’s are actually from external supplier ETO Gruppe, with a switching time of less than 22 milliseconds. The whole lot is controlled by the engine ECU, using revs and throttle position to decide when to switch between the lobes.

The main difference between KTM’s system and the ShiftCam used on BMW’s boxer twins is the position of the actuator and spiral grooves.

KTM variable cam solenoid

BMW has an extension on the end of the camshaft for the mechanism, adding the cylinder head’s length but letting the actuator lie flat underneath the cam cover.

It would be exposed to potential damage and make the already wide engine even broader if it was on top of the cam cover. With a V-twin rather than a boxer, KTM’s priority is different, so the spiral channels are between the two sets of intake cam lobes and the actuators are mounted on top of the cam covers, keeping the rest of the head just as compact as those without a variable valve timing system.

KTM’s variable valve timing system in detail

  • Mild vs wild: There are two cam lobes for each intake valve – one mild, one wild. They’re all mounted on a sliding sleeve that slots onto a splined centre shaft attached to the camshaft sprocket.
  • Special spirals: Two opposing spiral slots machined into the middle of the camshaft sleeve are the key to the cam shifting system. A two-pin actuator is mounted in the cam cover above those slots.
  • Electricity to movement: The actuator contains two solenoids (switchable electromagnets) that can convert electricity into movement, in this case each solenoid has the ability to extend or retract one of the actuator’s two pins.
  • Switching sides: When one of the pins is extended, it slides into its corresponding slotted channel on the camshaft’s sleeve, and because the channel is spiral-shaped, the sleeve is moved from one side to the other, switching the cam lobe.
  • Move when closed: The spiral layout of those slots means the sleeve section will only move when valves are closed.