One of the biggest misconceptions is that Pro Circuit reaches a "final" setting. Although the crew does wind up with a baseline setting, suspension is very personal and everyone requires different adjustments. Yes, there is a time when they stop going to the track to develop their settings, but after Bones gets his baseline settings, he then uses feedback from his customers to make updates.
Developing The MotorFor 2005, the Yamaha YZ125 has one of the strongest powerplants. The new motor is the only one in its class that is comparable to the almighty KTM 125 SX mill. The YZ has decent bottom-end that carries over to a strong midrange that pulls into a hard-hitting top-end with a nice dose of overrev. Over the years, Pro Circuit's Dave Chase has earned a great reputation as one of the premier engine builders. Chase took on the task of diving headfirst into the new YZ125 engine, and surprisingly, the process is similar to the suspension modifications-PC spends the first few days at the track getting as much feedback as possible regarding everything from overall performance to jetting specs. Afterward, the bike is put through several dyno runs with a variety of jetting to get a maximum horsepower number and to find out the motor's strengths and weaknesses. The dynamometer is a great tool for discovering where the motor needs help the most, but PC always backs up the numbers with track testing. Using the data from the dyno and the track testing, Chase gets a good feel for what the motor needs and where to begin making improvements.
The first step is to come up with a good exhaust system that will complement the stock motor. "We built three different pipes to start with, and from those three, we were able to come up with a baseline setting," Chase explains. "We need to have a pipe and silencer combo that will work well with the stock motor and with our motor modifications. We spend quite a bit of time testing at the track and on the dyno with the pipe combos. The minute we have a favorite, we begin degreeing out the motor, so we can see what the port timing is; from there we start making changes to the port timing, the compression ratio, the cylinder-head shape and the squish. Next, we focus on getting the motor to do what we want it to and go after the power gains we think it needs. The new YZ125 motor is really good, but it needs a bit of help in the midrange and a bit more power up top, so that's what we focused on."
During this process Chase will start off with small steps, say, grinding the cylinder and machining the head, and then move onto bigger strides as he goes, usually writing off a cylinder or two. Chase is able to push the limits with seemingly endless resources which leads to a great end result. While going after more power, Chase pays close attention to making the performance changes with longevity and reliability in mind. Even with big modifications to the motor and what most would consider a full-blown race engine, PC designs it to be able to withstand hours of abuse.