can anyone tell me exactly what a portal is? thanks

There are two gears in a box at the end of the axle tubes. Raises the center line of the axle up for ground clearence. Search in the Mercedes section and you will find all the info you will ever need. Good Luck.

It is an axle where the hub centerline is below the axle shaft centerline.

Somewhat related question: Do portal axles have really big spring perches or special control arms to prevent spring wrap? I would think their urge to twist would be even greater than regular axles (longer moment arm?)...

Yes, the torsion effects of a portal axel are greater than a straight axel. However that increased force is not normally as much as one might think. Think of a straight axel with 6" lift blocks. You are creating the same forces and many times you don't see the addition of a track bar on a block lift.

But on that subject take a look at the pics. You can see it is a triangulated 4 link and the lower links are mounted at the Tube fork. Thus eliminating much of the negative effects.

http://www.pirate4x4.com/forum/attachment.php?attachmentid=65183&stc=1

http://www.pirate4x4.com/forum/attachment.php?attachmentid=65835&stc=1

http://www.stazworks.com/images/axle2.gif
http://www.stazworks.com/images/axle6.gif

http://www.stazworks.com/images/axle7.gif

http://www.pirate4x4.com/forum/showthread.php?t=142401

How much clearance is gained by your custom axles (Hummer portals?)
In other words, how "tall" are they with your custom mounts? The same as a H1?

Well first there not mine. Follow the link and you will see the original post the pics came from.

And to the question;
I think it’s something like 4 or 5". Now that doesn’t seam like much but it makes a big difference.
And an h1 really doesn’t gain much ground clearance from the portal hub. Since they use Independent Suspension the portal hub really just reduces the CV angles.