people are telling me 3" exhaust is not recommended because of over boost, but i've been reading a lot, and most people has a 3" setup. downpipe, presilencer/straightpipe, then exhaust. So what is better 3 or 2.5?

 

RB 2.5" DP to an RB 2.5" MP to a 3" Corksport catback. Works for me

 

if you still have the stock intake then a 3inch should give you about 11psi so as long as everything else is working fine you should be ok.

I would recomend porting you wastegate either way tho. just to be safe. Its 3 hours work or so and its worth while when compare to a blown engine which is what will happen if you lean out from over boosting which happens far to easy in these cars. you will also need a FCD if you intend to go over 8psi

 

go for a 3". better off for turbo. leave the 2.5" for the NA guys

 

I'm running 4"

 

How is this a question.... 3" all the way no cats!!!

-Ben

 

really depends on your set up. Sounds like you got stock turbo. Think about this, if you have a more open exhuast, you're boost will go up.
Can your turbo handle 12 + lbs?
how old is the turbo? IT may need a rebuild.

I run a 3" dp and 2.75 exhuast. i am still having boost issues in 4-5th gear and I have a ported wastegate.

You really need to figure out what you are doing with this car. fast spooling? late powerband? your question is too vague.

 

I just bought a hybrid knightsport s5 untouched, just getting ideas on what kind of set up i should use.

 

Thunderfab 3" dp to the Y. I see 12lbs easily and spikes to 14 while rolling. I have fuel upgrades to support it. Untuned less than 11.5afr, 206 rwhp 10 psi easy at 6k.

 

So a full 3" won't be over kill for the engine as long as i port the wastegate

 

that is may create LOTS of boost in high gears. you should have an external wastegate