I've decided on .125" thickness, but what's the best grade of aluminum to use? 6061-t6?
Any suggestions on a good mock up material for modeling the plenum? I've been using cardboard, but I'd prefer something a little more stiff. It's hard to get accurate with flimsy card board.

 

1/8th min, if you're going to run big boost go up to 3/16.

6061 is the common material, the temper doesn't much matter because it will be T0 once you're done welding anyway.

Cut some boxes apart and use that cardboard, don't use the thick paper stuff. You can also do it using a tape measure and a little thinking.

 

Use 1/8" beryllium.

 

You would be better to use a 5000 series aluminum if and pieces are going to be bent or formed!!

 

+1 on this one 6061 is a pain to bend i use 5000 series 1/8

 

also, you can go to an art store and grab some fiber board to use as mock up pieces....can't really bend those though.

 

maybe could help a bit but im building an intake manifold for a VW engine, using 5754 aluminium 4mm thickness..and any problems with bends..