I've got the MRT turboback exhaust with the cat in the down pipe and twin tip muffler. It's about 2.5 years old. Today, I went in for Cobb Protune today on my 2002 WRX. HP was 20 less than the tuner usually sees for my kind of set up - VF34, MRT Turboback exhaust, Helix TMIC, gutted uppipe, STI injectors, walbro fuel pump, Vishnu pulley - and he thought the cat in the downpipe might be melted because the exhaust flow seemed restricted.

On the Dynojet and 91 octane, he got 240 whp, 230 torque, and at 95 octane he got it to 257 whp and 237 torque.

He suggested I take off the downpipe and inspect the cat.

What do you think of his theory on the exhaust?

 

if its the exhuast take it off and look at it