A four-valve head promotes combustion chamber swirl, and makes it easier to locate the sparkplug in the dead center of the combustion chamber.
Many years ago, I began jotting down factoids and short snippets of information I’d heard on the subject of motorcycle power, and thought were worth remembering. Some were snatches of conversation that I felt deserved further comment, some were just pithy remarks made by guys I respected. Since these tidbits make for interesting reading and will hopefully lead you to engage in further research, I thought I’d pass a few of them on. If nothing else, they’ll be great conversation starters at your next cocktail party.
Horsepower is made in the cylinder head and lost elsewhere.
An internal combustion engine is nothing more than an air pump. The more air you can flow, heat up, expand and evacuate in a given time at a given rpm, the more power you’ll make. Getting the air in and out of the engine is the trick, and that’s why tuners expend so much time and effort getting their engines to “breathe.”
It’s also why aftermarket induction and exhaust systems are so popular. Most stock air boxes and exhaust systems are somewhat restrictive, as they have to comply with federal noise and emission regulations, and that can hurt power.
The aftermarket is under no such constraint, at least not if their products are sold “for off-road use only”. The premise is that a less restrictive inlet and exhaust enhances airflow through the engine, which improves performance. In theory that premise is correct, but unfortunately, the aftermarket doesn’t always get it right. Airflow is a tricky thing, so some aftermarket pipes actually hurt performance or only improve it over a very narrow range. That being said, a well designed aftermarket exhaust, when used with the proper air intake and carburetor or EFI that’s been properly adjusted, can provide a lot of bang for the buck, and is one way to boost power quickly and cheaply.
So where is the horsepower lost? Anything in the engine that creates friction or drag robs horsepower, and that includes things we need to keep the engine running, like the water pump, oil pump and alternator. Even the oil in the sump can create a viscous drag on the crankshaft that steals power.
A four-valve head promotes...
A four-valve head promotes combustion chamber swirl, and makes it easier to locate the sparkplug in the dead center of the combustion chamber.
Good engine builders work hard to eliminate these parasitic losses. They polish and fit and shim to eliminate friction between rotating parts, at times going so far as to compromise engine life, as BMW did on their production racers back in the 70s, when they deleted the oil filter to gain 1/3 of a horsepower.
The rotating parts of the engine aren’t the only problem. External power losses occur through the driveline, from the clutch to the rear wheel, so paying careful attention to things like primary drive alignment and frictional losses in the transmission can set free a bunch of horsepower that might otherwise be lost. For example, a dirty, dry drive chain can require up to 20% more horsepower to turn the wheel than a clean, well-lubed one.
The bottom line here is that getting an engine to breathe and burn the charge more efficiently is the only way to create horsepower; reducing frictional losses throughout the engine and driveline doesn’t create horsepower, it just lets you use what you have to better advantage.
Horsepower = torque X rpm ÷ 5252
The above is an indisputable fact that describes the relationship between torque and horsepower, a subject we’ll be addressing in a few paragraphs.
To determine an engine’s horsepower you first need to know how much torque it produces. Once you know that, you can plug in the numbers using the equation: Horsepower = Torque x RPM/5252, and determine your engine’s peak horsepower. Why the sum is divided by 5252 isn’t important here (though the short answer is that it’s a mathematical constant). What is important is that as a description of the relationship between torque and horsepower, the formula has no exceptions, and that gives rise to several important points.
If you look at any dyno chart, you’ll see that torque and horsepower always cross at 5252 rpm. They’re equal at that point and, again, that’s a constant. Furthermore, below 5252 rpm, torque is always greater than horsepower, while above 5252 rpm, horsepower is always greater than torque. Lastly, you’ll notice that in most cases horsepower will climb slightly, even after the torque drops off; this is because horsepower is calculated on rpm, so even though the torque falls, the extra rpm allows horsepower to keep climbing.