Cam timing vs ignition timing, and degreeing a cam

[davepl]

I got a PM asking about the differences in timing, why they're not related, and whether degreeing a cam is worth it. I said I'd post it in the open forum so people could correct me where I wander astray!

So, some basics:

Ignition timing: controls when the spark happens relative to TDC
Cam timing: controls when the valve opens relative to TDC

Now you can change both, and they're keyed off of top dead center, but otherwise they're unrelated - ignition timing is about when the spark happens, cam timing is about when the valve opens.

When you degree a cam, you are making sure that the stack of tolerances in your particular motor, combined with the errors in machining of that cam blank don't add up to shift the valve timing too far in one direction. It's a good idea and I always do it when changing a cam. Heck I do it twice, once on #1 and once on #6 to make sure I get the same numbers.

Is it worth it? Only once did I find a cam that I confirmed was out, and it was only out by 2 degrees, but I was able to compensate for that with an offset keyway.

What about advancing or retarding the cam?

Advancing the cam causes the valves to open a few degrees sooner, and close a few degrees sooner. You can't change duration or lift by changing the cam timing - just tweak where the valve events happen relative to the piston.

Conventional wisdom holds moving the cam one direction helps high RPM performance and the other way can help low-end torque. But you always lose on the one end what you gained on the other, I think.

You can buy cams that are already advanced/retarded, too. They're ground 2 or 4 or some similar number of degrees advanced, usually.

I've personally never felt that I knew enough about a cam to start changing the valve timing events. My guess is that if you need to tweak cam timing, you probably should have picked a different cam. So in general:

- Should I degree my cam?
- Yes, it's a good idea

- Should I advance/retard my cam?
- No, not unless you really know what and why

If there was a free lunch to be had merely by tweaking cam timing, they'd already build it in (which they sometimes do, as noted) and call it the Mega 3D Turbo Blaster 5000 Cam Series. But there's no free lunch.