Probably the most popular class of street bike that is purchased these days (and for many days past) is the 600 sport bike. After the initial purchase, and sometimes before it even leaves the shop, a slip-on muffler replaces the heavier, quieter, and in some cases, ground clearance limiting OE muffler. Generally, the modifier wants more / better sound coming from their bike, and power gains (if any) are a secondary consideration.
Often, fitting an aftermarket silencer causes the bike to run unacceptably, introducing surging, hesitation and abrupt throttle response. Additionally, the engine runs hotter as a result of the leaner mixture. The cure is to fit and dial in a jet kit if your bike still has those old-fashioned carburetors, or an aftermarket fuel injection module for the newer bikes to alter the fuel map contained in the bike’s black box. Doing this improves drivability over stock, improves torque, can improve gas mileage, and can even add a horse or two if the bike’s fueling was off far enough.
The senior editor had a Yoshimura Tri-Oval slip-on and EMS fitted sometime back to his personal 2002 Honda CBR600F4i, with an accessory hub that allowed selecting one of three fuel maps. I had ridden the modded 600 on several occasions and always thought it felt stronger than a stock 600 normally would, but didn’t have a way to verify my seat-of-the-pants evaluation, since I ride an open class bike.
Thankfully, a solution was at hand. My girlfriend happens to ride a 2001 F4i (49 state version) that also happens to have the stock exhaust and no aftermarket fuel injection module in place. With identical, stock gearing and similar mileage showing on the odometer, it would serve as the perfect test subject to highlight the real world effects of the modifications to the editor’s F4i.
Dirck and I rode out to an area we deemed suitable, and we began some roll-on tests (rider’s weights are within 15 pounds of each other, which was further narrowed by the seven pound weight loss Dirck’s bike enjoyed, thanks to Yoshimura’s lighter slip-on). A speed of 60 mph was set as the starting point for acceleration in third, fourth, fifth and sixth gears.
The first test (sixth gear) had 4900 rpm showing on the tachometer and when the throttles were whacked open, not much happened acceleration-wise, but we did gather momentum (these are 600s, after all). The Yoshimura modified bike did so in a comparatively more urgent manner, though. It pulled a bike length, and as the revs rose through 5K to 7K, it continued to gap the stock F4i to the tune of 3 bike lengths. After 7K, the acceleration of both bikes evened out.
Starting off in fifth gear, with 5400 rpm on the tach, pretty much the same thing happened, only with a little more urgency, but the Yosh F4i only managed 2.5 bike lengths before reaching 7K this time. Fourth gear was more of the same, starting off at 6000 rpm. This time the modded F4i could only manage a bike length. A pattern had emerged. Third gear acceleration was commenced with 7000 rpm showing and the bikes were in a dead heat as the revs climbed to 13K, when we closed the throttles.
With 7000 rpm or less showing on the tachometer, the Yoshimura modified F4i offers more power and acceleration for the rider. This is important while aboard a 600, as these supersport machines have peaky little engines shoehorned into their frames. When I’m in town, where 45 mph speed limits are the rule, or just commuting on the highway, any gains below 8000 rpm are most welcome, because I don’t ride one of these in it’s sweet spot 100% of the time and I’m betting that in this environment, most of you don’t, either.
What the acceleration tests, and any dyno for that matter, don’t show is the sharper throttle response from the modified F4i. They also don’t convey the extra willingness with which the modified 600 revs (but take a look at the original review of the Yoshimura products – note particularly the partial-throttle dyno chart). You could be fooled into thinking the crankshaft had been lightened. Tooling around town reveals a much more agreeable mount with which to commute aboard, and you don’t need to wring it’s neck (as much) or wait (as long) for the revs to climb before grabbing the next gear. It’s not an open class sort of acceleration, but it is a noticeable improvement over the stock bike.
The question is, then, are the modifications effective? Certainly. Are the modifications worth the money? You have to make the decision on that, but my answer is yes.
