Can someone please explain camshaft overlap? Specifically, how does it effect timing, idle mixture and, the rumbly sound it produces? I believe I have a good understanding and want to get it all.
Thanks.

Overlap helps an engine to produce power by helping to lower the pressure in a cylinder during the intake stroke, similar to the way a room with a fan in the window will get a better breeze going if you open the door. At higher RPM this overlap effect, generated partially by an earlier intake valve opening point and partially by a later exhaust valve opening point, will help to produce more power by allowing the engine to consume more air. The more overlap there is the higher the RPM that the engine will see peak volumetric efficiency/torque will be.

At idle, when the air is moving slower, this will cause issues with exhaust gasses getting sucked back into the engine at idle, and unburned air/fuel mix getting sent out of the exhaust. You end up with a stinkier idle, and need to richen the car up to compensate. The car will also like more timing, it needs more time to ignite and burn the crappy rich and exhaust filled mixture at idle.

In addition, most cams with a lot of overlap have a generally larger intake lobe duration-wise, and will typically have a later intake valve closing point as well which will cause a lower cylinder pressure at idle. (although this may not be the case if a cam has more overlap due to a tighter LSA but is going to use the same installed intake centerline angle) This is because at higher RPM the cylinder can continue to fill even as the piston travels upward on the compression stroke. The engine will also like more timing to compensate for the lower cylinder pressure at idle.

Visualize 2 triangles or pyramids viewed from the side. Left one intake, right one exhaust. The top of each triangle is max valve opening, the bottom left corner is where they begin to open, and the right bottom angle is where they're closed. As you move them together to where the bottom corners overlap each other, that's your Overlap. There's always some overlap. The distance between the top points is the lobe center measurement. In practical application, a race cam for a naturally-aspirated (no forced induction, like a blower) is usually around 106-108 degrees, hi-perf 108-112, milder cams 112-115. You can see that narrowing the lobe centers increases the overlap. Overlap will bleed off some compression, and also push some of the fuel through the exhaust, and will tend to move the HP & Torque peaks up the RPM scale. That's why cams with a lot of overlap can give pretty weak part-throttle/low (street) RPM response. This is a simplified explanation and there are ways to "trick" the motor into a wider/lower powerband, but will give you an idea of the basics.

Thanks for the great replies.
The cam in my 440 has a lobe seperation of 110*. I don't know what the overlap is. Will this cause the exhaust to smell bad while idling?

Degrees of overlap should be on the cam card, or you can get that info from the cam manufacturer. Exhaust shouldn't smell "bad" - which I take to mean rich - if the carb & timing are right.

My cam card doesn't have anything about overlap just lobe seperation. The "bad"smell is a rich smell. I guess it's back to the tuning fun.

You should be able to get any engine to idle decently clean, but an aggressively cammed engine will need more timing and idle RPM.

Do you have a Holley Double Pumper? These carbs are set up to have an especially rich idle circuit for cars with aggressive cams.

What duration is it/what are the other cam specs? Years ago a cam with a 110 LSA may have been slightly on the aggressive side but nowadays it seems that pretty street able cams are coming with LSAs that tight. You can't determine overlap with LSA alone.

I do have a holly double pumper. The cam's lift is .480 and duration is .280. It's a Compcams 280H-10.
I've been suspecting that my carb is the problem with my idle. I just wanted to understand overlap a little more and I now do.

That cam is fairly decent sized, with 60 degrees of overlap. 80 degrees is a pretty aggressive cam that would be typical of your basic 500 horsepower 440, and something around 35 degrees of overlap is something like a stock 350 hp 440 (not sure what the overlap specs on those are off the top of my head, but think Imperial smooth idle), just to put things in comparison. So you're right in the middle.

I'd probably run a double pumper on that if it were my car and it had headers and a decent intake and some gear, but expect a little bit of a stinkier idle, but I'm always more concerned about performance. DPs will always idle on the stinkier side of things though. MOST people would probably be happier with a vacuum secondary job on something like that though. More timing will help clean it up if that's what you're after. What are you currently running for initial timing and total mechanical timing (without vacuum advance)?

I'd take a guess that your setup would like 18-22 degrees of initial timing best.

The timing is at 15 degrees. The engine idles smoother and pulls more vacuum with the timing advanced more but then the engine fights the starter when starting.

Usually they'll take a little more before fighting with that much cam. Is your compression ratio pretty high?

In this case I would run manifold vacuum for your vacuum advance, assuming you're running vacuum advance. This will be the port on the front of the baseplate of the carb, rather than the one off of the passenger side of the front metering block.

I don't know what the compression ratio is.
The vacuum advance is disconnected for now. I've been reading about how it works and what it does. I'm going to be hooking it up soon to tinker with it. I will hook it up to manifold vacuum. How I understand it, vacuum advance should always be connected to manifold vacuum. I know many people disagree with this.
I'm thinking, hoping, that the engine will start easy with the timing at 15* the way it is now and then the vacuum will advance the timing after the engine starts causing the idle to get better.

I agree an always prefer manifold vacuum. That's what was used for years before ported vacuum was used for emissions related reasons. And yes it should do exactly that

On the vacuum source there are two approaches. Chevy set initial mechanical timing low and used manifold vacuum to advance timing at idle.

MOPAR/Ford used ported vacuum so idle timing was controlled only by mechanical settings. Vacuum advance only kicked in at light load cruise.

Worth a try to use manifold vacuum... that said manifold vacuum will dance around a bit and may make idle less stable. Vacuum advance is also hard to tune.