i being working on my 84 GSL-SE the past three days and one of the things i did was changing the cat to a presilencer. i had a turbo II main cat in my 84 -se and i put a dynomax muffler and before with the cat i was getting 5 psi and now with without the cat and a presilencer im get 10 psi. im impress because im a newbie on turbo and i have a ton of HP. i have a stock 84 GSLSE engine with a S4 turbo and im verry happy with the result

 

i did the same thing with replacing my 3" CAT with a racing beat presilencer and it went from 4.5# to 14#
i blew my engine after about 5000 miles..
with 3mm seals and a streeport.
the turbo is a hybrid stage 2
i tried to use a Manual Boost controller but it did not work..

Just remeber to port you wastegate.... on the S4 turbo
it is alot smaller than a S5

 

im getting a S5 turbo soon so im not worry about that