3 ways I squeezed more power from my race bike

In the endless pursuit of more speed aboard my 1986 Honda XR250R motard, it has now come down to searching for incremental gains in power. The aerodynamics are terrible, the suspension is pretty good, and the brakes are great. So that leaves horsepower as the low-hanging fruit. But to be clear, there is no truly low-hanging fruit here.

Regular readers will recall I converted this bike from an air-cooled thumper designed for trail riding and toodling around the farm into a dedicated road-racing machine. Sure, it’s an odd choice, but with lots of ground clearance and suspension travel, the XR250R is a forgiving and fun bike to ride. It’s just important to remember such use is far outside the engineering parameters of its early 1980s origins. The lack of horsepower means corner speed is the only way to decent lap times, and that requires real understanding of proper riding technique, which scales up to larger bikes that might one day be in my future.

After running three weekends with this bike last year, I realized that while chasing additional power would likely be a fool’s errand, there were some easy ways to eke out a little extra. Since bolt-on solutions do not exist for my “problems,” and I had to engineer my own performance package, I looked no further than the basics.

More air (and exhaust)

There’s an old saying that engines are nothing more than air pumps. More air in and more air out makes for higher power potential.

To get more air out, my first step was to grind down the welds inside the factory exhaust. This opened up nearly a quarter-inch of additional diameter, which is quite significant when talking about airflow. With a couple more paychecks saved up, I made the jump up to an even larger stainless-steel header pipe. Plenty of air leaving.

Sadly, there still wasn’t much going in. Despite my DIY porting of the cylinder head and adding a larger, 34-mm Mikuni carburetor, the whole operation was drawing through the drinking straw that is the factory air box. This meant all air had to travel through a rubber tube that changes shape and volume as it passes around the rear shock.

After finding a mangled airbox from a bike I parted out a while back, I had the idea to swap to a free-flowing pod filter. The guts of the old airbox would act perfectly as a deflector to protect the shock from debris flying off the rear tire, and I could keep open the left side, which has the number-plate mounting points. Because I put the pod filter right on the carb, the bike still has all the proper mounting points for the things it needs. Crucially, it also has a lot more airflow.

More fuel

Most motorcycles like this XR have simple fuel systems. Fuel tank, petcock shutoff valve, hose, carburetor. Having already changed the carb to a larger model, the only thing left here was the fuel hose, which in this application is actually something of a fuel pump. The stock XR250R is made for grunting around at low rpm and has a fairly large fuel bowl, meaning there is little risk of pulling fuel up through the main jet faster than it can refill from the tank.

In prepping the bike for racing, however, it only took a couple full-throttle pulls down the street to see I had fuel delivery problems. The amount of air being drawn into the engine corresponds to the amount of fuel being consumed, since we tune the engine to run on a particular fuel-air ratio.

With a significantly larger fuel hose, I essentially created a larger fuel bowl. The inlet into the carb is still a slight restriction but not nearly as tight as the petcock. No more fuel issues.

More compression

More air and more fuel are the basics of making more power from an engine. Increasing compression is a little more involved. This is the ratio between the volume of the cylinder with the piston at bottom dead center and the volume when the piston is at top dead center. The tighter you can squeeze the air and fuel mixture before lighting the spark plug and allowing the burning mixture to expand, the more energy can be converted from the potential in the fuel to the kinetic energy of piston movement.

To solve this, I changed the piston for one with a slight dome and also had the cylinder head decked to remove a couple thousandths of an inch. Think of this as lowering the ceiling in a room. The volume of the space changes, especially when the piston is at the top of its stroke. More squeeze, more power.

The change demands that I be careful about what fuel to run, as lower-octane fuel is more susceptible to igniting by itself when compressed. Factor in the added work of ensuring that valve-to-piston clearance is acceptable, and this step borders on “not easy.” The effort is worth the reward, though.

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The key to getting more power from your engine is to understand how it makes power—and what adjusting the factors of that equation can do. Sure, for most engines you can copy someone else’s parts list and assemble a similar mill that you know will work. But where’s the fun in simply being a credit-card mechanic?

Individuality requires knowledge. Everything you learn on one project can be used on future projects, and before you know it, you’ve spiraled up to building really interesting stuff. Start with the basics, and you never know where you’ll end up. That sounds a lot more fun than just ordering parts, right?

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