Rumors of the KTM RC 990 have been swirling for 9 months since the test mule was spotted in Europe. Punters and publications alike surmised that the RC 990 would be more powerful than the RC 8C, by way of the 990s extra cc's, but this ignores modern emissions standards. Now that the KTM Duke 990 has been released, we have a better idea about the upcoming RC 990's engine specifications. The Duke 990 uses a parallel twin engine with cylinder bore of 92.5mm and stroke of 70.4mm, displacing 947cc. The quoted brake horsepower is 123hp @ 9,500rpm which equates to dynamic compression of around 12.25 bar and mean piston speed of 22.3m/s. These figures are standard and unremarkable; however, the engine varies greatly from other middleweight bikes in a few key metrics.
The Euro 5 emissions standards and the upcoming Euro 6 standards are quite strict regarding unburned hydrocarbons so one of the design elements engineers are looking to minimize is the total piston circumference, where unburned hydrocarbons accumulate in the piston rings. The Japanese 600cc Supersport engines are all 67mm bore and have approximate total piston circumference of 84 linear centimeters. The upcoming Yamaha R9 will have 78mm bore and 73.5cm circumference. The KTM RC 990 will have just 58cm piston circumference. Additionally, the outgoing 600cc Supersport bikes have an operating rev range of about 13,000rpm-14,000rpm. Making peak power requires complex valve systems or valve overlap that increases unburned hydrocarbon emissions. The 600s also have superfluous piston surface area, meaning they can make more power than their spring-valves can actually allow. The KTM RC 990 will have an operating rev range of less than 10,000rpm, and while the KTM has less piston surface area than a 600cc supersport bike (135cm vs 141cm), the KTM can actually achieve max theoretical rpm and minimize the need for valve overlap.
Anyway, this is a layperson's understanding of how the sportbike industry is trending. Fewer pistons. More stroke and displacement. More midrange torque. Since motorcycle engines are quite oversquare to start, there is little worry about losing more heat through the cylinder walls as stroke increases. Since motorsport is looking to reduce cost at each class' performance threshold, these new kind of bikes are good for teams, as well. Time will tell whether street riders and motorsport fans accept the new direction the industry is taking.
