Is the KTM 390 Duke's engine the 200 Duke's cousin?
When you look at the engine of the KTM 390 Duke you could easily be misled into thinking that it is a simple bore and stroke job on the KTM 200 Duke. Indeed, that is actually how the platform may have started. But when you are out to extract big performance, the changes required can be drastic. How drastic? KTM R&D Manager (Street), Sebastian Sekira says only the finger followers are common to the Duke 200 and the new KTM Duke's 375cc engine.
So what is inside? The 200's 72mm piston has grown to a massive 89mm in the KTM 390 Duke while the stroke goes up from 49mm to 60mm. These are not small changes. KTM says this extremely oversquare combination has many advantages. The large bore allows the head to be spacious which means large valves can be put in to enhance breathing. This can be critical in high rev performance. What also helps is the other advantage of a large piston. A large bore allows a shorter stroke for the same displacement which means lower peak pistons speeds - as in the piston accelerates from zero to a lower speed before slowing again to zero as it travels between TDC and BDC.
But then in the hunt for performance, the compression ratio was raised from a high 11.3:1 for the Duke 200 to an astonishing 12.9:1 for the new engine. Let us understand what that means. In a piston engine (like the Duke's engine), compression is expressed as the ratio between the volume inside the cylinder when the piston is at the bottom of its cycle (BDC) and at the top of the cycle (TDC).
The higher the ratio, the better the engine is able to turn heat into mechanical work. Or in more complex words, the same combustion temperature can be reached with a smaller amount of air-fuel mixture. As in it allows the engine to more efficiently overall - make more power, run a bit cooler and so forth. The downside of a high compression are also more sensitive to lower octane and impure fuels, descending quickly into detonation or pinking.
In the KTM 390 Duke's case, the higher compression is one of the reasons why the engine produces such great power and torque and at the same time economy is not significantly different from the 200. In fact, in one of the test economy cycles, the Duke 200 returns 31.25 kmpl. The 390 surprises by managing, on the same cycle, 29.4kmpl!
Which is incredible when you factor in the 44PS the engine makes at 9500rpm - peak power arrives 500rpm earlier than on the 200 Duke. And also the 35Nm of peak torque which arrives at 7250rpm. Which is why the piston also changes in construction from a cast unit on the 200 Duke to a higher performance forged unit on the 390 Duke. The forged piston is a significant change because it can handle higher temperatures as well as have greater thermal stability. This is proper high performance equipment.
Just like the bigger everything, the Bosch fuel injection also has been upgraded. The throttle bodies grow from 38mm to 46mm - powerful engines are well-fed engines.
Then comes the gearbox. KTM has in the 390 Duke added a completely new gearbox with new ratios for all gears as well as a significantly taller sixth that makes it a brilliant highway cruise tool. 120kmph in sixth is just 6500rpm, and 100kmph is an all-day 5000rpm. We understand Bajaj and KTM may consider similarly altering the sixth gear on the 200 Duke as well, extending its highway abilities.
The external cases of the KTM 390 Duke may be the same or at least visually the same. But even the metal Y-shaped mounts that are so clearly visible on the 390 Duke are completely new. Finally, KTM have also given the 390 a significantly larger radiator to keep up with the new engine.
What that means, simply, is that it may look the same externally, but the 390 Duke engine is an all-new beast with almost nothing carried over from the Duke 200.
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