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More Horsepower. Please.

There are various tried-and-true methods for making more horsepower, but they all carry a cost.

It’s a question that’s been around probably since the second engine was built: how can I get more horsepower than the other guys? I have been asked many times and the answer depends on who you are and what you ride. The usual answer heard at a performance shop is another question; “How much money you got?” 

While there is no set formula my guess is it would probably look something like this: 

More Power = % Gain X $ X $ X $

Of course the formula would not be linear—a little is usually inexpensive and only slightly illegal while a lot, well, the sky is the limit (maybe outer space even). 

It also depends very much on the bike. If you ride a heavyweight bagger whether you are a heavyweight yourself or not, you may need to settle for “relatively fast.” For some folks fast means quicker than the other bikes you normally ride with. Some bikes don’t need any help. 

A Kawasaki H2 with 200-plus horsepower, well, more is always possible but maybe you should think, ‘this is a street bike’ and ask yourself why? I know there are lots of 400-horsepower turbo Hayabusas on YouTube, their 200-kmh power wheelies on the freeway require power only the lunatic fringe can appreciate. And, of course, I guess there are the posers.

Weight has always been a performance factor; the old muscle cars had an option delete package. Take off the power everything (including the radio) and win! 

If you have a “computer bike” a chip, a Dyno Jet type tuner or computer re-flash will get you a noticeable improvement. On a carbureted vintage bike the same kinds of gains would involve re-jetting and manually adjusting the timing—more work and more trial and error. 

The modern sportbikes are hard to improve on: maybe a header, maybe a tuner although that might not improve some of the already crazy fast stockers. Maybe some carbon fibre. Drift guys and drag racers find a sprocket change makes for huge differences in acceleration. Let’s talk simpler bikes that would better respond to engine hot rodding: the cruisers, vintage, and café bikes.

The next steps involve getting yet more pressure in the cylinder, which means the piston gets pushed down with more force. That energy comes from good fuel combined with air in a perfect ratio with the correct compression ignited at the perfect time. (The more air you can get in the more fuel you can add.)

not necessarily the best way to use your extra horsepower

One option is an over-bore and new pistons. More bore means larger area to push down on: 

Force = pressure X area 

So we should get more torque, assuming good volumetric efficiency. Increasing the stroke by changing the crankshaft will also result in more displacement; both options mean there is more room for air and fuel in the cylinder so a larger amount of energy is released if everything works perfectly. There is real danger in stroking however; it means higher piston speed for a given rpm (the piston now has farther to travel) also more stress than the crankshaft might be able to handle.

While you are changing pistons increasing the compression ratio is an option. I have found Harleys like a bit more compression even if the displacement is left stock. Higher compression means more power but only if the fuel is good enough. Compression ratios are limited by how much octane is in the gas. Generally speaking, the higher the ratio the more power you can extract from the fuel until the fuel is not good enough and detonation (very destructive) sets in. There is as much power lurking in Regular gas but it’s more volatile than Premium, which gives a more controlled combustion, less chance of detonation, so more timing advance is possible. A stronger ignition system may at some point be required.

Another idea for more horsepower is make the engine breathe better, more airflow or CFM (cubic feet per minute). Head work, exotic valve train components, carb and exhaust help. These will likely benefit from a camshaft upgrade. 

The cam is the mechanical brain of the valve train. It determines when the valves open and close as well as how long they stay open and how far they open. This is known as duration and lift. A naturally aspirated engine depends on nothing more than atmospheric air pressure (14.7 psi at sea level, less at higher altitude) to push air and fuel into the cylinder when the piston strokes down on the intake stroke. That intake valve at thousands of rpm is open for only a tiny fraction of a second; it’s a challenge to get the cylinder full of air and fuel before it closes and compression starts to happen—100 per cent volumetric efficiency is what it’s called if you get in all the air and fuel the cylinder can hold. While some engines don’t quite make it happen, supercharged engines get more than 100 per cent. 

a little more horsepower can spin the tires

In any case the cam chosen is a big factor that also determines the power characteristics. The rpm of best volumetric efficiency is the rpm that you get the most torque. At best you will probably be using less than 50 per cent of the energy in the fuel, the rest is lost to heat and friction.

Then there are what some call power adders—nitrous oxide and the various types of supercharging systems. They work but do add complexity. So-called crate engines are also a popular option, some even have warrantees (future topics perhaps).

One thing to keep in mind is that none of these changes will add to reliability. Here’s another basic fact of engine building: the more power you make the more heat you generate. Upgraded cooling such as an oil-cooler may be a good idea, synthetic oil as well. Remember a hot running engine will detonate sooner than one running at its designed optimum temperature. Engine oil temperature is a good indicator:180 to 200F is considered normal. Polished engines do not shed heat well; the polishing process removes surface area so they run a little extra hot. 

For me, messing with engines is fun and satisfying (most of the time), but it isn’t cheap. Guess it’s celery sticks for lunch—saving money and weight. Aw, to heck with that! I’ll have a burger and add some more horsepower. 

by Rich Burgess Canadian Biker Issue #319


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