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.

[Mrturner1]

Good info thanks Dave!

I have one wrench to toss in there

Would advancing or retarding your cam timing change the same parameters as having one cam with a 108 lsa and one with a 110 lsa?

[davepl]

Originally Posted by Mrturner1

Would advancing or retarding your cam timing change the same parameters as having one cam with a 108 lsa and one with a 110 lsa?

No, BUT. To keep this simple I'll use the hands on a clock. Well, simpler if not simple.

Think of the two lobes like the hands on a clock. One's at 10 o'clock and ones at 2. They are centered around 12.

If you tighten up the LSA, they're now at 11 and 1. But they're still centered around 12, you've just tightened up the LSA. That helps high RPM stuff.

What you could also change is the CENTERLINE. That's the point they're averaged around, or centered around.

So, instead of moving the hands (lobes) closer together, we could advance them one hour. So we go from 10 and 2 to 11 and 3, now centered around 1 o'clock.

I'm sure a picture would be better, but I can't draw.

So, lobe separation angle: how widely spaced the lobes are.
Centerline: center spot between two lobes

[Alex V.]

Interesting. How do you check cam timing during reassembly?

[davepl]

Originally Posted by Alex V.

Interesting. How do you check cam timing during reassembly?

You degree the cam with a degree wheel and dial indicator; you find where peak lift occurs and it should correspond to what you see on the cam card.

LSA is built into the cam and cannot be changed; intake centerline can be advanced or retarded (the whole cam, of course).

I've never checked LSA when degreeing a cam, but I suppose you could! My understanding of how cams are ground leads me to believe it's unlikely to be off unless they actually ground the wrong profile.

[Alex V.]

Simple enough. Good to know!

[Mrturner1]

Well that makes it a lot easier to understand. Just to clarify, the LSA applies to the 2 lobes on each individual cylinder then, affecting every combustion event? So tightening the LSA would give you more overlap between the intake lobe and exhaust lobe, causing more cylinder pressure and a real snotty idle? And widening the LSA would tend to bleed off some cylinder pressure (once you get to 110-112ish LSA) and be much more tune able, with a longer and more gradual power range.

The clock reference is perfect, really good way to simplify that

[davepl]

I was going to go with bunny ears but clock hands worked better :-)

Your description of LSA and idle is pretty close. I think tightening up the LSA increased overlap as you say, but that LOWERS cylinder pressure because it's not as efficient. It'll make more power up top but have a worse quality idle.

Loosening it reduces overlap which keeps more in the cylinder for higher pressure and a better quality idle, but loses a bit on the top end.

Otherwise (and unless I'm wrong) you've got it!

Remember that "rumpy" idles are because the motor's having trouble running in a stable fashion at idle. It's usually because, in the case of a tight LSA, exhaust gasses are getting "burped" back into the intake port, which is called reversion. So no two cylinders are quite alike and it idles bad (or awesome, from your perspective I think!).

Contrast that with 12-cylinder Rolls, which probably has no overlap, short timing events, and each cylinder at idle is a crisp one-of-twelve around the circle for a purring idle. That's what I like, but that cam timing is all done by about 3000rpm likely!