Chain Drive Cams S&S chaindrive cams are another way to save on a cam installation, since
After intake and exhaust, camshafts are the next most popular performance upgrade for Harley-Davidson motorcycles. They’re relatively inexpensive and comparatively easy to install. They’re also one of the most misunderstood items in the giant air pump we call an engine.
The most common misunderstanding we encounter is the notion that the biggest cam you can stuff in your engine is the best. Also commonly misunderstood is the difference between torque and horsepower. These are critical factors in cam selection because most cams are better for one than the other.
Torque is a twisting force that makes something turn, such as your rear wheel. How hard your engine can twist on your rear wheel determines how quickly your bike can accelerate. Horsepower is defined as the ability to do a certain amount of work in a given time. Like how long it will take to move 800 pounds of bagger from wherever you happen to be to somewhere a mile from there. The more horsepower you have, the faster that can happen. So torque equates to acceleration, and horsepower determines your top speed.
Depending on the type of bike you have and how you ride it, you need to decide which is more important to you. For 99 percent of the population, and particularly bagger riders, torque and acceleration are what you want, whether you realize it or not. How often do you ride at more than 100 mph compared to how often you need to get that hog up to freeway speed on an on-ramp? If you’re a drag racer, you may top 100 every weekend, so maximum peak horsepower will top your wish list. On the street, not so much.
Discussions of camshaft theory can get pretty long-winded … blah-blah-blah, and we don’t want you nodding off and drooling on your shirt, so for this article we’ll concentrate on the basics and extol the virtues of bolt-in cams for late-model big twins. If we win the arm-wrestling contest with the editor, we can go deeper into the theory next month. Let Baggers know if you want more!
Typically a bolt-in cam is defined as a cam that can be installed in an engine with stock heads and valve springs. The cam’s lift does not exceed the maximum lift capacity of the stock valve springs. That’s correct, but a true bolt-in cam is also designed to have the stock base circle dimension so you can use your stock pushrods.
Gear Drive Cams S&S Cycle’s patented gear drive cams are another option when installing pe
That part about the base circle may require a bit of demystifying. A cam lobe is shaped sort of like an egg when viewed from the side, but the fat end of a cam lobe is perfectly circular and is called the base circle. When the cam is turning and the lifter is rolling along base circle, the valve is closed. When the bump on the lobe (hence the origin of he vernacular camshaft term “bump-stick”) comes around, the lifter rides up it, pushes the pushrod up, which rocks the rocker arm, which opens the valve, and aspiration is achieved.
“So what?” you may ask. Well, it all comes down to cost. The price of a bolt-in cam and a high lift cam should be about the same, since they cost about the same to manufacture. However, true bolt-in cams are a lot cheaper to install since you can reuse all your other stock valvetrain components. Valve springs, pushrods, and rocker arms are expensive, but when you add the labor to install them, the bill mounts up. Some mechanics will save their customers a little time/cash when removing stock pushrods by using a specialty tool called a bolt cutter, but there is no way of installing a set of high-lift valve springs without removing the heads from the engine. Cha-ching!
Another consideration about bolt-in cams is that most of them are actually designed for an otherwise stock engine. The timing specifications aren’t so wild that a stocker will run like a pooch at low rpm. There are a variety of bolt-in cams available from S&S and other manufacturers generally advertised as either torque cams or horsepower cams. As mentioned earlier, most riders will find the torque cams more useful on the street and more fun to ride.
The majority of high-lift cams are really designed for modified engines with more compression, more displacement, and possibly ported heads. If your engine is still stock, you probably shouldn’t go there.
To sum up: Don’t just look at cam specs and choose the cam with the biggest numbers unless you have the engine to match it. If your engine is stock, a bolt-in cam is probably your best bet. If you are in doubt, you should ask someone who has the knowledge and experience to give you a straight answer. The boys down at the local watering hole are always full of advice, but we suggest that you ask a professional engine builder or give the S&S tech line a call for a recommendation for your application!