Tech / Tech Articles

Cam Glossary 2.0: The 5 Key Camshaft Specs You Need to Know

 

Summit Racing recently put together an A-to-Z glossary of camshaft and valvetrain terminology, which covers everything from advance to valve margin.

OK—so, it’s more like an A-to-V glossary, but you get the idea. It includes everything you need to translate the complex language of cam manufacturers into something you can understand. It’s a valuable tool for sure, but what if you were taking your Camshaft 101 final exam? Are some terms more important than others? Which terms should you know to make the correct cam choice?

According to the tech experts at Summit Racing, here are five key cam terms you need to be familiar with when shopping for a camshaft.

Valve Lift

  • What it is: Valve lift is the measurement of the distance a valve opens off its seat.
  • Why it matters: Because this spec tells us how much the valves are opening—the higher the lift, the more open the valve—it also gives us a clue as to how much air is flowing in and out of the cylinder. This has a direct effect on the amount of torque and power your engine will produce.
  • Rule of thumb: In general, higher lift cams will produce more mid- and high-rpm power. However, depending on the aggressiveness of the cam lobe profiles, high lift cams may also require stronger valve springs and lighter weight valvetrain components.

Duration @ .050”

  • What it is: Duration is the amount of time, measured in degrees of crankshaft rotation, in which a valve is off its seat (open). Duration @ .050” is the industry standard for determining duration and begins measuring the valve opening from the point of .050” lift.
  • Why it matters: Because duration is the amount of time an intake or exhaust valve is open, it is the determining factor for how much air/fuel fills the cylinders during each combustion cycle. This has a direct effect on combustion, fuel usage, and overall power output.
  • Rule of thumb: As engine rpm increases, the valves open and close more quickly, making it harder for the air/fuel charge to fill the cylinders. By increasing duration—keeping the valves open longer—you can increase the amount of air/fuel fed into the cylinders at high-rpm operation. Therefore, more duration is ideal for high-rpm power; less duration fosters more low-end torque.

Lobe Separation Angle (LSA)

  • What it is: Lobe separation angle is defined as the number of degrees separating the peak lift points of the intake and exhaust lobes.
  • Why it matters: This separation of the cam lobe peaks has an effect on idle quality, peak torque, engine vacuum, and cam rpm range—all the stuff you care about! Lobe separation also creates engine vacuum, which is important if you are using power brakes and an automatic transmission.
  • Rule of thumb: A cam with a wider lobe separation (112-116 degrees) allows torque to be spread over a larger portion of the engine’s rpm range and offers better power throughout the upper-rpm range. Narrower lobe separation (under 112 degrees) will foster good low-rpm torque and good acceleration but will generally keep peak torque concentrated in smaller area of the rpm range.

Overlap

  • What it is: Overlap is the point, measured in degrees of crankshaft rotation, when the intake and exhaust valves are open simultaneously. This occurs at the end of the exhaust stroke when the exhaust valve is closing and intake is opening.
  • Why it matters: By having the intake and exhaust valve open simultaneously, the open exhaust port can produce a scavenging effect that will help pull the air/fuel mixture into the combustion chamber. This can lead to increased combustion and horsepower.
  • Rule of thumb: Less overlap typically offers improved low-rpm response and increased fuel efficiency. More overlap provides an improved signal to the carburetor with high-rpm power potential.

 

This illustration gives you an idea of how lobe separation, overlap, and duration would look if plotted on a graph. Lobe separation is at the top and shows the difference in intake and exhaust centerlines as measured in crank rotation. Overlap is shown in the shaded area–the point where both the intake and exhaust lobes are open.

Intake Centerline

  • What it is: Intake centerline is defined as the point of peak lift on the intake lobe in relationship to top dead center (TDC). This can be adjusted when the camshaft is degreed with the timing set.
  • Why it matters: By advancing or retarding the intake centerline, you can alter your engine’s powerband.
  • Rule of thumb: By advancing intake centerline, you’ll shift the basic rpm range downward slightly from the original intended powerband. By retarding the centerline, you’ll move the rpm range upward.

Whether you opt for a hydraulic camshaft, mechanical cam, or roller camshaft, your cam manufacturer or sales rep can help you select the right specs for your engine’s powerband. Be ready to supply vital information, including engine size and compression, power adders, airflow, and other key factors in selecting a camshaft. Armed with that knowledge, you’re sure to get the ideal valve lift, duration, LSA, and overlap for your ride!

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