Image/Mike Mavrigian

Have you ever wondered why one camshaft makes more power than the other?

Would you like to compare the behaviors of multiple cams without ever doing a cam swap?

Written descriptions of idle quality, power range, compression, gear, and converter requirements only go so far when the same cam fits in a 283 or 406 engine.

Fortunately, understanding camshaft specs and how they relate to specific valve events is a great way to benchmark an engine’s performance.

Note: This is part one of a two-part series on Comparing Camshafts. We take a much closer look at cam timing (including the four engine strokes and the effect of the pistons pumping air in and out of the intake and exhaust ports) in part two. You can read it here: Comparing Camshafts (Part 2): Consider Individual Timing Events When Choosing a Cam

But before we go further, let’s take a quick stroll down memory lane.

  • In the 1950s, cams were commonly called 1/2-race, 3/4-race, and full-race. The problem with that terminology is that it was often subjective—one person’s 1/2-race cam could be another’s 3/4-race, and so on.
  • During the 1970s, camshaft companies were using a spec called “advertised duration.” This was a step forward, but the companies still used different measurement standards, so one company’s “292” could be smaller than another company’s “288.”
  • By the 1980s and 90s, most companies were publishing “duration at .050 in. lift” numbers. Customers could use seat, .050 in. and .200 in. numbers to compare lobe intensity.

All these previous methods gave us these generally accepted truths:

  1. Wider lobe separation (less overlap) makes for a better idle than narrow lobe separation (more overlap).
  2. Advancing the cam makes more low end torque. Retarding results is more on the top end.
  3. Longer duration cams make more power up top, but suffer from a rough idle, and lack low-end.
  4. Nitrous and blower cams have wider lobe-separation numbers.

Unfortunately, this led to generalities like “Always install a cam 4 degrees advanced” and “Never run less than 110 degrees of lobe separation on the street.”

In reality, nothing could be further than the truth.

So, if you want something a little more dialed-in than a “3/4-race camshaft” for your engine—read on.

The engine cares only about individual valve timing events.

That’s because these four valve timing events create duration, lobe centerlines, lobe separation, cam advance, and overlap numbers….not the other way around!

What are these four important events?

  1. Exhaust valve opening
  2. Exhaust valve closing
  3. Intake valve opening
  4. Intake valve closing

By choosing these events independently, you’ll have more control of the engine’s behavior.

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Want to see how a camshaft is made? Check out this factory tour.

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Keywords
cam, camshaft, duration, valve timing
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Author: Brian Nutter

After a stint in the U.S. Air Force, Brian Nutter studied at the Houston, TX-based School of Automotive Machinists in 1997. The early part of his automotive career included working for engine builders Scott Shafiroff and C.J. Batten, followed by several years developing performance pistons at Wiseco Piston Co. Today, Brian develops performance parts for Summit Racing Equipment and is a regular OnAllCylinders contributor. For fun, he runs his 427-powered C5 Z06 in ECTA land-speed racing, at OPTIMA® street car events, and at a mix of autocross, drag racing, and track days.