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How to Select a Cam
Selecting the Appropriate Cam for Your Engine.
What is the desired rev range? The lower number of your range will be close to your peak torque engine speed and will define the roughly the duration that you will need.
Ball-park Intake Duration (in degrees) = (Peak Torque engine speed - 3151)/32.53
You can further refine this by looking at the top of your desired rev range? This number should be close to your peak HP engine speed. You can use the following formula to get a rough idea of how much overlap you should shoot for:
Ballpark Exhaust duration (in degrees) = (Peak HP + 9083 )/ 66.62
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? Here are some starting points
For engines that will be emissions tested, keep overlap less then 10 degrees. Also if you are using any sort of intake system that uses a common plenum such as CIS or EFI, you'll want to keep the overlap to less then 10 degrees since more then that will hurt both your part throttle drivability and potentially your peak HP.
For Autocross and DE use on an engine with carbs, MFI or individual throttle bodies, target an overlap of 60 degrees or less.
For Full Race use on an engine with carbs, MFI or individual throttle bodies, you'll want to target overlaps ranging from 40 degrees up to over 80 degrees.
What sort of fuel will be used - specifically what octane? The dicussion of fuel octane merits a whole different paper of its own. From a camshaft discussion there are a few things to consider.
Long duration camshafts can support higher compression ratios, and in many cases run better with a higher compression ratio. If you are using race fuel, you can go higher still.
On the other hand short duration camshafts with little or no overlap can create extreme static cylinder pressures in engines with high compression ratios. Steps should be taken to manage this by either adjusting the ignition timing, the cam timing, the fuel octane or some combination of these three.
If you have access to some key measurements, it's possible to take these thoughts one step further by calculating your engine's dynamic compression ratio. You can do this by calculating the swept volume from when the intake valve closes and adding any clearance volume in the combustion chamber, and divide this by the clearance volume. Typically the dynamic compression ratio for engines on pump fuel is 7.2:1 up to 7.5:1. For racing engines this dynamic compression ratio is generally more then 9:1.
While there are still volumes more that could be learned about the "black art" camshaft selection and design, hopefully you will find these rules helpful in peeling back the some of the mystery of selecting a camshaft.