Could be a couple things, potential boost leak somewhere... pull the cold air off and get a silicone boost leak tester and put air through the pipes and see if air leaks out anywhere. and i found this out the hard way, but there's a little hole on the bottom that suppose to be there. another common thing is the waste gate is internal on the 19t i believe. it could be out of place or loose and it may not be completely covering the hole and not building until high RPM and making little boost.
So, i have a 98 S70 T5 manual. its lowered on ipds and bilstein struts. cold air with down pipe and exhaust... my next plan is to do an ECU and an inter cooler and Reverse inter cooler piping kit. im planning on doing it in steps.. which should i do first or be more effective? i know the ECU would help big time but im gonna do that after i do the inter cooler or reverse inter cooler piping kit. so which one is more effective, just an aftermarket inter cooler or a reverse inter cooler piping kit?