Why 1 5/8"

I noticed a lot of the twin turbo headers are 1 5/8" primaries. If that is too small for a mildly modded lt1, why would it be effective on a turbo'd car?

1 5/8" primaries minimize the volume of the exhaust between the ports and the turbine housing, decreasing spool time and allowing for more stable boost since you're dealing with a smaller volume of gases. These are good on a mild setup. The heavily built cars intended for high-rpm, high-boost usage use larger primaries in order to move more exhaust gases. They suffer from more lag, but benefit in a stronger top-end charge. The primaries and turbine housing size usually need to be considered in conjunction to get the results you're looking for. A bad combo can give you issues of unstable boost and boost creep (boost not being constant).

Sure you could use larger primaries and a smaller turbine housing or vice-versa.... but trying to describe all of those considerations is a little tedious. Check into some books or lots of internet searching and you'll get the finer details.

P.S.- one good rule of thumb i've seen is that you want your primaries to be 100-110% the diameter of your exhaust valve.

1.6 x 1.10= 1.76 :)