You see a little bit better braking on 4WD because your two axles are coupled together. The front wheels (which receive the majority of your braking force) slow the front axle, which in turn will back up the driveline and slow the entire driveline, this includes the rear wheels too. Should you drive like a douche because of this? NO. It provides a SMALL amount of help, nothing very substantial. It still all comes down to available traction for the tires.
Most (as in 95% of trucks/SUVs except for a few newer ones) t cases do not have differential. They are all fully locked when you put it in 4hi or 4low. If you want to think of it like a spool, fine, its like a spool. When your “part time” light comes on it means you should only use it part time becuase your t case is locked splitting torque 50/50. When you shift the level, push button, whatever. You are actually engaging a slip shaft in the t case and have a mechanial link between front and rear driveshafts for a locked 50/50 split.
Even though this is a perfect bias, it never really is. You always have different amounts of torque actually making it to the wheels (for various reasons that are not important in this discussion). That is why you get “crow hopping” around turns (front wheels skipping) and other feels that don’t quite feel normal. This is the front axle not spinning at exactally the same speed as the rear axle thus your wheels are littereally fighting each other. This is why you should not use 4WD on dry or even rainy pavement. Your driveline does not like this!
Those 4 auto modes starting to pop up on newer vehicles do not lock the t case. They typically have an electronically activated clutch pack which can bias torque to whichever axle needs it. These can be driven on dry pavement because they are actually running in 2WD mode most of the time (or something similar, like a 90/10 split or something). The big advantage to these type is the computer can send power to whatever axle it needs to in milliseconds. The 2006 Explorer and soon to come out 2007 Sport Trac with these can send between 0-100% of the engines torque to either axle in (I forget how many milliseconds). They begin to bias torque when the computer sees just 1/4 turn of rear wheel slip. I’m not sure on others, but I’m sure they’re about the same.
Ranger’s still have an old style t case. Where 4hi and 4lo physically locks the gears together for a 50/50 bias. Same with the F-150. Expeditions use a combination of this and the electronic clutch for the 4 auto mode.
Alot of these systems are starting to use the clutch pack for 4hi mode too. When you put it in 4hi they just send juice to the clutch pack to grab all the time. This gives you close to your 50/50 split (depending how strong your clutch pack is and how the system is calibrated).
Some of the 4 auto modes, or AWD systems use a viscous coupling differential to bias power. This is a fluid filled mechanical device that operates on the principles of fluid mechanics. They have no electronic intervention. When they see a speed differential the blades in it begin to shear the fluid which expands to put pressure on the plates sending power to both driveshafts.
The final type of center differential is an open center diff, just like your diffs in your axles. These are set to a pre determined ratio (based on location/offset of the planetary gears in them, ie: its complicated).
I don’t know a ton about GM and Chrysler t cases which are made by New Venture Gear in Syracuse. I tore apart a few of them to benchmark against ours. Mainly I know Borg Warners t cases which Ford uses. They actually have the best electronic clutched automatic 4WD system out of the big 3 for trucks/SUVs right now. I’m not just saying that because I worked on them either.
