– Motorcycle News, Editorials, Product Reviews and Bike Reviews

Motorcycle News, Editorials, Product Reviews and Bike Reviews

Tuning for Mid-Range Power

In my recent article speculating on the future of sportbikes and their displacement, I mentioned that any manufacturer that accepted the peak horsepower levels currently produced by their sportbikes as more than adequate and instead directed the minds of their R&D engineers toward devoloping more mid-range (while maintaining constant peak power levels) could certainly produce a powerplant that had significantly more usable street power than current sportbikes. I felt that the topic was worthy of further discussion, and decided to delve deeper into how it can be done.

Many recent ‘standards’ (or ‘nakeds’) have used big-displacement sportbike motors that are allegedly ‘retuned’ for torque – the problem is, most of these naked bikes end up with nothing more than a choked-up powerplant that trades marginal increases in torque and mid-range horsepower for massive losses in top-end power. Is this the best the manufacturers can do? The answer is no. Most of the ‘retuning’ done to these bikes consists of fitting less-aggressive camshafts (which achieve their peak efficiency at a lower RPM – see my article on camshafts from last year), smaller-diameter throttle bodies, and other ‘band-aid’ tweaks. Truly re-tuning a motor to produce significantly more mid-range power requires a more in-depth effort, which the manufacturers apparently don’t want to pay for – not surprising, considering the budget price (when compared to the sportbikes they borrow parts from) of most big-bore nakeds. If their marketing departments were convinced that a more torque-laden powerplant would create sales success for their range-topping sportbike models, things might be different.

Yamaha actually provided a ‘proof of concept’ with their new 2007 R1. They started with a motor that was praised for its top-end rush, but criticised in magazine shootouts for its lack of motivation at lower, more streetable RPM levels. By employing technology that is well developed in the automotive world, they were able to achieve significant increases in midrange power and torque, while at the same time actually making a slight (claimed 5hp) improvement in peak power. How did they do it? By utilizing a variable-length intake tract. The length of the intake tract (along with the diameter) controls the RPM at which it achieves peak efficiency, and because the length of the intake port and the space required to accomodate the throttle-body mechanism is typically fixed by design and component specifications, changes to intake tract length are typically accomplished by differing length throttle body velocity stacks (also known as ‘intake trumpets’).

Off the top of my head, the first application of variable-length intake trumpets that I can recall was on a Can-Am racing car back in the 1960s or 1970s, so the idea isn’t exactly ground-breaking. For Yamaha, however, it was a new way forward – since sportbikes have typically featured intake tract lengths leaning towards the high end of the powerband (to help achieve the high peak numbers that are important to marketing types), the ability to build an engine that can switch between two completely different intake tract lengths allowed Yamaha to have a setup that produces significantly more efficiency in the midrange, and then switch to an uncompromised design focused purely on top-end power. The best of both worlds, and all for the price of a few servo motors and some more time spent on the engine dyno optimizing the motor for both trumpet lengths.

Now, imagine if this technology was applied to a motor that wasn’t starting off with a low-end powerband considered weak for its class – basically, any of the other Japanese literbikes, but particularly the torque-laden GSX-R1000. If we had the ability to call up the R&D department and yell “hey, 160 wheel horsepower is PLENTY, concentrate on the rest of the powerband!”, they could easily adopt technologies like these and make REAL improvements in low-end power.

Another easily available technology that could help in the quest for low-end and mid-range power gains is variable valve timing (for an in-depth exploration of this technology, see my article from last year). Dual-stage systems like Honda’s VTEC (the kind on their cars, not the kind on the Interceptor) allows engineers to design two cam profiles, each of which can be optimized to achieve its maximum efficiency at a specific RPM. Switching between these two cam profiles can allow the motor to retain the same high-end power (when using the more aggressive of the two available profiles) while making significant gains in low-end and midrange power and torque (when using the smaller, less aggressive of the two profiles, which helps maintain high air velocities in the engine at lower RPM).

The newer variable valve timing systems (like Honda’s I-VTEC) incorporate another system that allows the ECU to advance and retard the intake cam through a range as large as 50 degrees overall (based on crankshaft timing) to further optimize power production and engine efficiency based on RPM, throttle opening percentage, and speed. This further fine-tuning can increase fuel efficiency under cruising conditions and reduce emissions, as well as offer greater power production at almost any RPM.

The basic layout of an engine, meaning its bore and stroke dimensions, practically dictates the RPM at which it will achieve peak power. As long as the cylinder head and cam design can provide efficient flow at that RPM, the motor will make the power. The R&D teams at the big Japanese manufacturers have this down to a science, although they’re still constantly searching for the smallest of incremental gains. Maintaining the same basic engine design, but implementing some of the technologies described above would allow a design team to maintain or even slightly improve peak power output, while at the same time significantly increasing the breadth and depth of the powerband. Why aren’t the manufacturers pointing their R&D dollars in this direction? It has to do with cost and the choices made by you, the consumer.

It also has to do with us . . . motorcycle journalists. As we place greater emphasis on “real world power” usable on the street, and less emphasis on the lap times of professional riders on a race track, we will impact consumer preferences and, eventually, manufacturer priorities.

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