Tips To Harley Camshaft Performance

Tips To Harley Camshaft PerformanceThe cam you are running will have a profound effect on the performance of your motor. This seems obvious, but the lack of understanding of how cams work, and what they do, can rob you of the full enjoyment of your ride.

A camshaft consists of hardened steel “lobes” that are arranged to raise and lower your pushrods, and open and close the motor’s valves. Depending on the dimensions of these lobes, the valves will be held open a lesser or greater distance, for a greater or lesser amount of time. The amount the cam’s lobes move the pushrods is called “lift”, and the length of time the valves are held open is called “duration.”

Stock HD cams are designed to provide a modest amount of lift for a factory-specified duration, and the stock design is conservative. It is meant to provide good performance without putting too much stress on the valve train. Without too much danger, however, you can improve the performance of your engine by installing a higher lift or longer duration cam. BE CAREFUL, however, not to install a new cam while your bike is under warranty. It may void the warranty!

There are as many cam designs as there are bikes, it seems. You can get great cams from Andrews, S&S, Crane, and a host of other manufacturers. There are so many options it is hard to make a choice. But you can get a rough idea of what you want if you know how cams work, and how they influence your motor’s characteristics.

If you want to ride around town on a Sunday or take leisurely trips in the countryside you may not even want to know what cam you are running. If the bike is stock you will be happy. However, if you want a little better performance you might want to get a little more aggressive. Using the Andrews cam numbers, you might want to run a J grind, an A grind, or maybe a B grind cam. The J grind will give you better power with a smooth idle, and is just a little better than the stock H grind. An A grind will raise your valves a little higher and keep them open a little longer. It can be bolted in without modifying your heads. An AB grind uses a longer duration exhaust to help your motor run a little cooler, and it gives a little more lift, which improves high RPM (high end) power somewhat.

If you have high-lift springs, and your valves have been properly clearanced, you can run a more aggressive cam like the Andrews C grind. This cam is about as tall as you can get, but you pay a price in more frequent repairs. Still, on the drag strip this cam will put out monster power where you want it – at around 5000 rpm on up.

In general, high lift long duration cams are for high power at high revs. The lower lift and shorter duration cams will skew the power curve down so you can get more low end torque at the expense of high-end power. With less aggressive cams you will also have less maintenance, because the springs and pushrods don’t do as much work. Less aggressive cams will also give you better gas mileage and a smoother idle. If you use a kicker you will find the bike easier to turn over as well, and because of the increased air velocity you will find the mixture ignites easier.

Don’t think that you need to use an aggressive cam if you are running a stroker motor. If you run a stock grind, or a less aggressive performance cam like the A grind, you will still get a power increase simply from the increased displacement. In addition, you will get improved idling because the higher volume of air flowing through a smaller valve opening will create a higher velocity in the carb at low revs. This improves the idle over what it would be if you were running a high lift cam. So don’t worry too much about matching the cam to the displacement or compression ratio when considering the use of a lower lift cam with high displacement engines.

IMPORTANT: The DURATION of a cam can be shorter or longer depending on your cam’s unique specifications. It is important to keep in mind that short duration cams can contribute to a problem known as DETONATION. Detonation occurs when your fuel mixture explodes rather than burns. The subject of detonation is so complex that a full page or web site would be required to explain it fully, so no attempt to do that will be made here. Suffice it to say that extreme detonation can destroy an engine in ten seconds!

A high combustion ratio (above 9:1) and/or a short duration (when the valves are kept open a shorter amount of time) can compress the mixture enough to make it explode rather than burn. Normal combustion is not an explosive process. Instead, a “flame front” is generated by the spark plug, and this flame front expands outward, burning fuel and providing energy as it does so. During detonation, however, the fuel explodes, often at the wrong time in the cycle. This puts enormous stresses on pistons and plugs, and can literally blow holes in pistons. Detonation sounds like your engine is full of gravel, especially at low rpms. Detonation can be cured by running the correct high octane fuel, reducing the compression ratio, adjusting for richer carburation, using cooler plugs, increasing the cam duration, adjusting timing, or a combination of these methods.

Using a high lift cam with a stock or lower displacement engine may be an exercise in futility. It is essential to follow the manufacturer’s instructions carefully, because cams often require head work in order to keep the valves from coming in contact with each other or with pistons. In addition, special springs, spacers and keepers may be necessary. Unless you are a drag racer, you should get all you want from your motor by running a stock or modest performance “bolt-in” cam.

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  1. Rob Fleming - August 30, 2011

    New blog post: Tips To Harley Camshaft Performance

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