Why VBR in some cases, CBR in others?
 

For learning purposes, why am I using constant bitrate of 5000 for this video that I'm splitting up, and variable, 3200 avg, 4000 max, 1000 min for the 3 hr videos? What's the difference?

I notice for the 1 hr 40 min, CBR of 5000 its taking only 2 hrs to encode, where as the VBR for the 3 hr with the 3200 avg bitrate etc takes 10 hrs to encode...

At the moment (*), some of this concept is discussed here: http://www.digitalfaq.com/dvdguides/capture/intro.htm
Look for the MPEG bitrate information at the bottom of that page.

(*) I say "at the moment" because the entire digitalFAQ.com main site will be changing to an upgraded site any day now. Some of these multi-topic pages will be split up and expanded upon.

There are various factors that affect compression, and the returned image quality. This information is specific to MPEG, don't try to extrapolate this data onto other compression schemes.

• You have captured MPEG-2 at 15,000kbps (I-frame only preferred, when possible) because that is a ceiling of MP@ML MPEG-2, the profile used by standard DVD-Video MPEG. Yes, it is higher than DVD-Video specs, but the MP@ML is the important aspect here, as that's what your capture card actually adheres to. By capturing at this maximum bitrate, there is no chance at having compression artifacts visually creep into the image. If you use I-frame only, then it also eliminated temporal compression, thereby making the maximum bitrate all the more needed.
• When you compression to the 3500k-4000k range, you're starting to compress video. Rather than constant compression (CBR), we're trying to use more bits when needed, and less bites when not. This allows image quality to stay high, but by using a more restrictive bitrate. However, it takes longer to process, more than twice as long in some cases. But for the sake of quality, it is worth it. A CBR of 3500k-4000k would look fairly dismal compared to the 2-pass VBR.
• When you don't really need to compress highly, you can encode at the "superbit" ranges. For 352x480 live-action, superbit is about 5000kpbs. For some content, like cartoons, it's a bit lower, in the 4000-4500k range. Keeping that in mind, a 2-hour DVD is filled up with approximate 5000-5500kpbs range (about 5200k, give or take). Why not up it to the max, you ask? Well, there's no point. You're already on that plateau. And by "only" using 5000k here, you can still have room left for some nice menus. Never forget menus when calculating disc space!