Discussing What Makes A Cam Chop — Lobe Separation Vs. Overlap

Recently, we came across this video from Powell Machine, in which Daniel took umbrage with a TikTok video that calls out lobe separation angle as the source of the quintessential chop in a V8. While not technically incorrect, as Powellwill points out, the lobe separation angle is but one of the factors that creates a loping idle that many equate to big performance under the hood.

We have an article on lobe separation angle, which you can read here, but we’re focusing specifically on the subject of idle quality and sound right now. As we said earlier, lobe separation is a contributing factor, but only because it alters the actual source of the familiar chop chop: that’s valve overlap.

What is overlap? To put it succinctly, it’s when both the intake and exhaust valves are open at the same time. Why you might or might not want valve overlap is a whole discussion in and of itself, but right now, let’s just focus on the overlap itself. Expressed on a 720-degree graph often used to graph a camshaft’s valve events, overlap is represented by the small triangular area between the lobe traces when they, well, overlap.

As Powell explains in his video, if you only consider lobe separation angle and nothing else, as the TikTok he’s referencing implies, you can have a wider lobe separation angle camshaft that has a way choppier idle than one that is much tighter. Check out the graphs below that we’ve created in DynoSim5’s Cam Manager to better explain Powell’s point.

These are the 720-degree graphs from the two cams that Powell mentions in his video. As he explains, you can see that in this case, a 110-degree LSA cam has significantly less overlap than a 116-degree LSA cam, showcasing that there is more to the story than just lobe separation angle.

Now, to be fair to the TikTok being referenced, if all other variables within the cam lobe are equal, what they say is correct. A tighter/narrower lobe separation angle will result in a greater overlap area, and presumably a more rumbly idle. As we show in the graphs below, as the centerline of the two traces get closer together, the overlap area grows.

With all other variables equalized, yes, a tighter lobe separation angle will increase overlap area, as shown here, with a 116-degree LSA on the left and a 108-degree LSA on the right.

However, there are multiple factors that can affect the overlap area. Besides cam duration, you can also enlarge the overlap area with increased lift. While two cams with the same duration and lobe separation angle will have the same overlap measurement in degrees, as you can see in the graph below, the total overlap area is greater between the .400 lift trace and the .700 trace, even though the overlap number doesn’t change between the two graphs (also, because these are duration numbers at .050 inch, not seat-to-seat numbers).

Here, you can see that lift can play a part in the overlap area as well, albeit not as significantly as duration and lobe separation angle. Notice that the overlap measurement in degrees doesn't change, but there is clearly more area "under the curve" with the higher lift.

Ultimately, the point of this discussion isn’t to point out what’s wrong but rather to get everyone to understand that there is a lot more that goes into cam specs than just a couple of numbers. It’s an extremely complicated arena that people can spend their whole careers working in. That’s not said to scare you off, but rather to encourage you to open your mind to understanding what is an incredibly complex subject.

About the author

Greg Acosta

Greg has spent nineteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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