How to Choose a Camshaft

Choosing a camshaft is probably the most important part of designing an engine combination, yet it can be the most confusing and frustrating part of your engine building experience. One of the problems is the confusing and contradictory way that cams are advertised and the way the specifications are listed.

There are some basic principles that are universal, apply to all brands of camshafts and must be understood.

  1. Camshaft size: Bigger or smaller is based on duration, not lift, more specifically duration @ 0.050" tappet lift. Advertised duration figures are not reliable numbers when trying to compare one brand to another for these reasons: first, advertised duration numbers include clearance ramps, which have no positive effect on performance. Second, advertised duration figures are not always checked at the same point with every brand, so the same cam can have several advertised duration figures depending on who or how it is checked. Third, using the duration @ 0.050" is a reliable way to compare cam size from one brand to the next, a good apples to apples comparison.

    When selecting any camshaft for your engine/chassis combination, the size must be determined by the duration at .050" tappet (lifter) rise. Using a 1.5:1 rocker ratio this equals .075" valve lift. Any duration less than this has no positive effect on power.
  2. Understanding the duration at 0.050" may seem confusing but the important thing that must be remembered is: as the duration at 0.050" gets larger the camshaft gets larger and vise versa.
  3. The lower (or shorter) the duration at .050", the lower the RPM use, such as RVs, towing, stock engines, etc. As the duration at .050" increases, the power increases, however, the power band also moves up in the RPM range. This requires additional engine and chassis modifications to work best. One cam will not do everything well, you may have to make "tradeoffs". In other words, if you want to drag race with a cam that pulls at 6800 rpm, dont expect the engine to lug a trailer around at 2000 rpm or vice versa.
  4. Cams have power bands, "sweet spots" or RPM ranges that they work best in. This power band or "sweet spot" does not mean that the cam will not work above or below this range. If, for example, the sweet spot is rated at 2000-5700 RPM, the engine will still produce power above 5700 RPM (at least ours will), but above 5700 RPM, the next larger cam will produce more power. By the same token, cams will produce power below their sweet spot, but the smaller cam will have more power there .
  5. The cam size (duration at 0.050") determines where the "sweet spot" will be. All brands of cams of the same size will produce sweet spots in approximately the same rpm range.
  6. The "sweet spot" will be determined for the most part by the size (duration @ 0.050") of the intake lobe. Other factors such as the lobe separation angle, center line and exhaust lobes have some effect, but these are usually for custom cam installations.
  7. A change of approximately 5- 6 duration @ 0.050" is considered one (1) size or step.
  8. The power in the "sweet spot" is determined for the most part by the rate-of-lift or how quickly the valve is kicked open. The quicker the better (ours are the quickest).
  9. The best engine combination is when the sweet spots of all the components (camshaft, cylinder heads, intake manifold, carburetor size, compression ratio, headers and exhaust size) are in the same RPM range. For example, an RV torque cam with a large, single plane intake is a very poor combination.
  10. If you collect a very poor combination of "sweet spots" (also know as parts) and then try to pick a camshaft, you leave the cam supplier with an impossible job. We don't sell cams to people with really screwed up combinations unless they are willing to change some components.
  11. The first component you should pick when you build your engine is the camshaft. Add the other parts to compliment it, not fight it. Check our list of questions on choosing a camshaft before you call.
  12. Our charts lists general guide lines for engine/chassis combinations. There are exceptions, but "tradeoffs" are involved. The actual power ranges will vary somewhat with the engine size, compression ratio (cylinder pressure) and the intake/heads ability to flow air. If you need more for camshaft information, we will want to know these facts:
    1. Use of vehicle (What do you expect it to do)
    2. Altitude of track
    3. Weight of vehicle (with driver) if a driver is used
    4. Engine size (cubic inch displacement)
    5. Cranking cylinder pressure - checked with a compression gauge
    6. Static or Mechanical compression ratio
    7. Intake manifold
    8. Carburetor size
    9. Cylinder heads
    10. Cylinder head air flow numbers
    11. Exhaust type and size
    12. Stall speed of torque converter
    13. Final gear ratio
    14. Tire height and width
    15. Fuel type and octane
    16. Will nitrous oxide be used?
    17. Desired ET or MPH
    18. Present camshaft
    19. Rocker arm type and ratio

  13. Note: Please try to use numbers such as: 1600 RPM stall, not "stock stall" because Chrysler provided many varieties of stock parts.
  14. Remember, the more information you provide, the more accurate the recommendation. We can move the lobe separation angles and mix/match lobes for very special applications. If you follow these instructions, in most cases, you will choose the correct camshaft.