Its definitely not that big as I used to have a GP GXP with the LS4. I'm 99% sure its 3.25".

IMO there's no benefit to doing all 4" either on the LS4. That neck down in the intake manifold has got to be the equivalent of maybe 2.5" or 2.25" circular pipe, so thats always going to be the primary choke point in the intake tract.

 

For reference, above is a pic of what I'm talking about, and that's a ported intake (it shows the D shape better as the plastic is duller gray from porting).

Given the LS4 TB is a 75mm / 3", you can see how choked down the inlet is due to the opsu hump.

 

Ok good info, I had no reference so I kinda guessed. I kept the 4" suggestion throughout to keep it simple.

 

 

I would just buy everything in that size then. Did that filter you have come with reducers?

 

yes it did. I'm running the 2nd biggest reducer haha very technical I'm an idiot but I believe if I got a 3.5 or 3.25 inch flex hose I can make it happen. My dad had 4" dryer duct it was too big.