Comp Cams on Cam Lope

Competition Camps, manufacturers of cam shafts, posted an excellent before-and-after video of a cam swap, to give us an idea on how sound changes after installing cams. Give it a listen:

Comp Camps also has this to say about the “lope” of the sound when adding a cam:

Cam “lope” is caused by the overlap when the exhaust valve is opening and the intake valve is closing. The more of an overlap, the longer and lower the sound of the “lope” will be. This is usually a result of a longer exhaust duration.

To get more in-depth, valve overlap causes misfires at idle and that makes the lope or choppy idle. It causes a lot of intake reversion which causes the engine to basically choke on it’s own exhaust during idle. The more the overlap the more the lope will be.

If your internal combustion engine mechanics need a review, just click here. In HowStuffWorks’ depiction, you can see there is no overlap between the intake and exhaust valves’ openings.

If you know the specifications of the cam shaft, you can calculate the amount of overlap in degrees, with the following equation:

Intake duration + Exaust duration, divided by 4, – LSA, x 2.

A cam is typically specified like this: Di/De Li/Le LSA where D is Duration, L is Lift, and LSA is Lobe Separation Angle (and sub i is intake and sub e is exhaust.) Look at the bottom of this page for a visualization of this. So, for example, take the Comp Cams’ 280HR cam on a Chevy 4.3L V6. Clicking on the specs link above, we see that Di and De are both 280, and that LSA is 110°. From there, we can calculate that the overlap in degrees is ((280+280/4)-110)*2, or 60° of overlap. The result sounds like this.
Comp Cams 280

Cool, right?