I tried to do a search, but, it doesn't recognize 2.5" or 3"; so, all it searches for is inch. I'm planning on a system close to the Magnaflow system, just running pipe back to a Flowmaster or Magnaflow muffler. With the only mods being 5/16" spacer, Z tube, K&N and Megan HR style Y-Pipe what would be better. 2.5" or 3". I don't plan on doing any mods later on.

 

2.5 is just fine

 

For a single, you won't need anything over 2.5in unless you are going for max power.

A 2.5 is fine, the Megan Y pipe merges to a 2.5 I believe.

 

The Y-pipe has 2.25 inlets and a 3" outlet

 

You should stick with 3" after the outlet if you're going with the Megan Y-pipe. You'll gain a slight amount of power and it will sound much better. You don't wanna ever go from 3" to 2.5".

 

Exactly. You want a similar diameter all the through. 3 -> 2.5 will create rasp.

I think mixed up the stock diameter of 2.5.

The stock cats are 2.25in. It's exactly the reason I have a 2.25in dual exhaust

A lot of people go for 2.5in test pipes/ HFCs, when in fact they should be using the same diameter as their y pipe or x pipe in my case. Then they wonder where the massive amount of rasp came from.

 

OEM Cat exit 2.25
OEM Y exit 2.3"
OEM HR-Y exit 2.5"
OEM Mid inlet 2.5"
Megan HR-Y 2.5 inlet to 3" exit.

 

2.5 should be good

 

Why? His Megan Y-Pipe is at 3" exit, leaving the 2.5" mid will create turbulence at the merge. It would be best to swap to 3" mid + axle back and swap to HFC. It will shift the power curve higher in the rpm's, but a tune will help create some power in the lower rpm. if he wants to stay at 2.5" then ye should sell the Megan Y and get an OEM HR-Y.