These days, I am thinking does aftermarket cat-back might damage stock engine?

I had HKS cat-back, 76mm, the sounds was really respectful but the hp feel decrease a lot. and many people suggest me to put HKS up pipe and Helix down pipe. I know these aftermarker exhaust system are good for big turbo suby, but how about stock turbo? Do you think if I only change HKS up-pipe with OEM cat-back and down-pipe, it will works better?

 

I'm surprised that the addition of a catback makes the car feel slower. If anything, it should feel pretty much the same or slightly stronger. Anything you do to reduce backpressure will help even the stock turbo. Several WRX owners have bolted on catless uppipes (on pre-'06 model WRXs) and complete turbo back exhausts for good gains. Personally when swapping downpipes I like to recommend getting engine management like the Cobb AccessPORT or an EcuTek reflash to maximize power and enable safer engine operation through precise fueling and ignition advance with the increased breathing.

Yes, a catless uppipe and your HKS catback should work well together, even if it won't show as big of a gain as swapping out the downpipe at the same time.

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i felt the same way but i actually attributed it to the fact that adding the aftermarket exhaust took away from the overall smoothness of acceleration and made the car feel like it was surging....