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How to Select a Cam

Step 1 - Define the Performance Parameters

Before you buy anything it is important to define a set of goals for your engine.
  • What is the desired rev range? For the sake of discussion it would be best to pick a 2000 RPM range where you want the engine to be strongest. The desired rev range is really a function of the intended gear box and the gap between gears. If you are running a close ratio gearbox you'll be able to optimize the engine's rev range for a smaller drop in rev's after every shift. If you are running the relatively wide gearbox from a 4 speed 911T on the other hand, you might actually consider targeting a wider rev range.
  • Where are you going to use it? On the street? Track? Race? Are emissions important?
  • What sort of induction system are you planning on using? Carbs? MFI? CIS? EFI or Individual Throttle Bodies?
  • What sort of fuel will be used - specifically what octane?
    All of the factors of a cam's design do only one thing when it comes to engine performance. That is to define the shape of the torque curve. The shape and rev range of the torque curve will by definition define the engine's horsepower since the two are related by the following equation:
    Horsepower = (RPM * torque)/5252

    Generally, an engine's peak horsepower engine speed will be about 1500 to 2000 RPM above the engine's peak torque engine speed. So the first parameter that we need to identify is where in the engine's rev band you want the peak torque to occur. I've included a copy of the 2.2 liter 911S's torque and horsepower curves below as an example.
  • On it you can see that see that the engine develops its peak torque of 20.3 mkp at 5200 RPM (the graph isn’t perfectly accurate). This is the “sweetspot” for the engine where the engine is pulling the strongest as a result all of the pieces being in tune and the cylinders getting the maximum charge. As the revs increase from that point the cylinder will no longer have enough time to fill completely and as a result the torque will begin to decrease. As long as the revs are increasing at a faster rate then the torque is dropping, the horsepower will increase. At 6500 RPM in the example, the torque begins to decay at a rate greater then 1 mkp (note the scale on the right) per 1000 RPM. At that point the torque is dropping off faster then the revs are increasing and so the HP begins to drop.

    Step 2 - Cam Parameters

    Selecting the Appropriate Cam for Your Engine.

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