As you may know, one of the most mysterious pieces of equipment in your Harley Davidson is the camshaft. There are what seem like a never ending selection of Harley Cams to choose from with everyone having their own favorites or opinions on which is best. This article will discuss one piece of the equation and how it will affect the characteristics of your camshaft.

Typically there are 3 numbers discussed by most when it comes to cams, duration, intake and exhaust lift. These give a general idea of what characteristics you might expect from that particular cam but there is much more to it than that. The cam timing plays a huge part in what the torque curve will look like and for most street driven Harley Davidsons, you really should build more for a broad and flat torque curve. This will deliver a Harley that is an absolute JOY to ride and that will leave most Harley's in your dust. So try not to get hung up on duration, as it can be misleading and should only be considered a general guideline as to the personality of that particular camshaft.

Cam Timing

The best indicator of what your torque curve will look like is the actual timing of the intake and exhaust valves. When do they open, how long are they open and when do they close in relation to each other will play a large factor in the "personality" of that camshaft. This sounds complicated, and it is, but if you follow these general guidelines you will have an idea of whether a particular cam might be a good choice for you.

Intake: 24˚ BTDC/48˚ ABDC
Exhaust: 59˚ BBDC/21˚ ATDC

Take a look at the timing specs above, these timing figures are referring to are degrees of rotation and these reference to the matching "stroke" of the crankshaft. For example, the intake timing degree number tells you the open and close points in relation to the intake stroke and the exhaust degree numbers do the same thing but in comparison to the exhaust stroke. In our example above we have a cam that an intake valve opening at 24˚ BTDC, which means that the intake valve starts to open 24˚ degrees before TDC of the intake stroke and the intake valve closes at 48˚ After Bottom Dead Center.

The intake closing point (relative to bottom center) listed in the table is the best indicator of the shape of the torque curve. If you have a cam that has earlier intake closing values then this will result in a higher torque value at lower engine speeds (along with lower horsepower at higher engine speeds). The later that intake values close then you will typically end up with higher horsepower at higher engine speeds (along with lower torque at low engine speeds). Basically the power band moves up or down the rpm range depending on your valve timing.

Cam Timing and Engine Compression

When building your engine, you want to make sure you match your cam and compression ratio very well as this will greatly impact the driveability of the motorcycle. You would typically choose your compression ratio first based on available fuel and power expectations and then match the camshaft to it. If you are an advanced user, then you can compensate for too much or too little compression with your camshaft but for most of us, determine your compression and then find the best cam to match from there.

This is important because the earlier you close that intake valve, then the sooner you start compressing the charge. Your static compression ratio needs to be right on if you are closing that valve early otherwise you may end up with too much compression and you will get spark knock. On the flip side, the later you close that intake valve then you risk not building enough compression and having a poorly burning cylinder. The later the valve closes, the more RPM you are going to need to fill the cylinder, which is why your riding style is so important to cam selection.

Whether you want valve timing that closes earlier or later etc depends on where you want the power peak of your engine to be. Higher revving engines will need to close the intake later, lower revving engines want to get that valve closed up early so you can build compression. Complicated? Yes but just understand that no particular way is right or wrong for all engines, you just need to make sure that you match your components well and all will be good.

Also bear in mind that intake closing values are only one indicator of the actual shape of the torque curve. The actual cam lift and duration along with compression and cylinder head flow will also affect it but if all else is equal, then you will get an idea of how each cam would react given the same engine, same heads etc.

Intake Valve Timing
Less than 30° Stock Street Type Cams - Strong Low End Torque.
30° – 50° Medium intake closing indicates a low to mid-range torque cam.
Greater than 50° Hot Street/Race type CAM.

If you are comparing camshafts and want a general guideline to go by, then you can count on the fact that typical hot street cams will fall into the mid 40's degree range for the intake closing point. If you creep up into the 50ish degree closing point then this will be considered a hotter street cam, even a mild race cam. Tread cautiously in this range as you will need to select your parts wisely. If you make it into the 60's then this is pure race cam territory and isn't a good selection for a street driven Harley Davidson.

Harley Cam Timing Summary

I know this can all be quite confusing, which is why you are smart to be doing research but simply understand that these general guidelines will get you in the ballpark and then you can fine tune your choice from there. Reality is that if you have 3 cams from 3 different manufacturers with similar numbers, then they are all going to be very close to each other in performance. It is quite difficult to tell the difference between two cams with very similar specs so don't get hung up on the numbers.

If you find a cam that is in the range you want and is made by a reputable cam company then you should be good to go. Double check with your builder before spending your hard earned money and then get to building.