Im going to be getting the HKS Hi- Power exhaust for my baby (GS400) and am planning on going full custom piping. What size piping should i go with???

 

cat back 3" to the split 2.5 after

 

I do not recomend the 3" from the catback toward the split because of back pressure and air resonance. I would do 2.75" with a x-pipe and then go 2.5" from the split all the way towards the muffler.

 

The pre-fabricated HKS Hi-Power for the GS400 are 60mm inlets. If you go with larger, you'll be creating a bottle neck (which may be ok because it'll keep some of the backpressure you'll be getting rid of with the large piping).

 

i agree.....I have the HKS on my 400...Im going to be doing the same....

 

3" is a bit large for an N/A car.

 

Read my posts above... It's been tried and tested. No need to go any larger than 2.75" if you're not running any FI.