Shop Talk | Topics Of Discussion

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.

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.

I’d rather have torque than horsepower.

This is a popular topic whenever cruiser guys get together, especially when they’re discussing the merits of something like a large V-twin compared to a high-revving multi, but the fact of the matter is that by themselves, neither one does us much practical good. Like most things in life, there has to be a balance, so the first thing you have to wrap your head around is that when it comes to moving motorcycles down the road, torque without horsepower is as useless as teats on a fish.

Semantics are a large part of the problem. Formally, torque refers to a twisting or turning force and is measured in pounds-feet (lb-ft) or Newton meters (N·m). This is an important consideration for engineers to distinguish torque from work—work being defined as force measured over distance, which is measured in foot-pounds, (ft-lbs). What this means is that we can apply incredible amounts of torque to something—for instance a large, tight bolt—without moving it, so torque by itself is a useless commodity when we’re trying to move something like a motorcycle.

When motorcycle riders talk about torque, it’s in a different context. Typically, we’re using the word to describe how hard an engine pulls at a given point on the rpm scale, or in a larger sense, how easy the bike is to ride. In our parlance, bikes with lots of torque are easy to ride, while bikes without a lot of torque, even if they have high horsepower, are just the opposite.

Here’s the catch; horsepower is directly related to torque, and in a nutshell describes how much torque is developed over a given rpm. The relationship here can be confusing, but the easy way to think of horsepower as it relates to engine performance is as a measure of how long the engine continues to make torque, in terms of rpm. So all things being equal (between engines with similar torque curves), more horsepower is better, if only because it allows the engine to generate more torque for a longer period of time.

Let’s consider two hypothetical engines that have identical torque peaks of 50 foot-pounds. The first, an industrial engine, is capable of turning 2000 rpm. The second, a motorcycle engine, can rev to 8000 rpm. I’ll let you do the math, but the first engine has a horsepower rating of 19.04, while the second churns out a respectable 76.16. Intuitively, you can guess which bike would be more fun to ride.

Although the industrial engine would be fine turning a water pump or a generator, it just doesn’t make enough power over a wide-enough rpm band to be of much use in a motorcycle. Sure, it’d pull like a train off the bottom, but you’d need a 15-speed gearbox just to keep up with traffic.

The fact is that when we say we prefer torque to horsepower, what we really mean is that we prefer engines that make a lot of torque at the bottom of the power curve, and maintain a high level of torque right up to redline. Engines like those, which are typically found in cruisers, standards and touring bikes, are tractable, easy to ride, and relaxing, and that’s why we like them. Because the architecture used in such engines generally means they don’t rev very high, horsepower numbers are generally moderate, especially when compared to something like a 600cc, four-cylinder sport bike that can turn 14,000 rpm.

**Pushrod engines are done: These days you need overhead cams to generate serious horsepower. **

If you think pushrod engines can’t make power, I’d suggest you attend the next NASCAR, World of Outlaw Sprint Car or NHRA race that’s within reach. Pushrod motors can and do make big horsepower despite what their detractors say, and as far as motorcycles go, I’m hard-pressed to recall the last time an OHC-powered motorcycle won a Grand National dirt track championship (I’d guess 1987). Since then, it’s been a nonstop parade of pushrod engine-powered H-D XR750s. I’m not saying the pushrod engine is the best thing since sliced bread; it has issues just as overhead cam engines have theirs, but overall the design is still as viable as it ever was and should continue to be for the foreseeable future.

A four-valve engine isn’t necessarily superior to a two-valve engine.

I don’t recall when I wrote this one down but I suspect it was back in the day when four-valve engines were all the rage, and I was something of a Luddite regarding their advantages. These days I’ve come to believe that while a two-valve head isn’t necessarily a bad thing, especially when the engine is a relatively slow-turning one, in nearly every instance, because they promote better atomization of the fuel/air mix, the four-valve head works better. It produces more power, reduces emissions and improves economy, which I think we’d all agree are good things. That being said, I own two bikes that have two-valve heads on them, and both run really well (the newest one was built in 1970 though).

Small, unpolished ports often flow better than big shiny ones, especially for street use.

When I was a kid, it was all about porting and polishing. Everyone made the ports as big and shiny as they could and then wondered why their bikes weren’t as fast as the ones from California with tiny, rough ports. Here’s the skinny: While it’s true that big ports can flow more air, it’s only of benefit if the engine can turn enough rpm and create enough velocity through the ports to make use of them. At lower engine speeds, an overly large port can actually hurt performance because it allows the incoming air to stall, which hampers cylinder filling and kills low and midrange torque.

Conversely, a small port increases gas velocity in much the same way that restricting the size of a water hose causes the water to jet out with greater force. This helps fill the cylinder, especially at lower rpm, and increases torque. As to surfaces, a rough finish—particularly in the intake port—promotes better fuel atomization, and though it may seem counterintuitive, reduces drag through the port and increases flow, much the way the dimples of a golf ball do. As far as the exhaust port goes, a slight polish to prevent buildup in the port works fine, but there’s no need to buff the thing to a mirror finish.

So there you have it; the next time you find conversation lagging, you can trot out one of these moto-snippets and become the life of the party. Hell, it’s always worked for me, and in case you’re wondering, yes I’m still collecting them.