Looking to do up exhaust for a 440 that is mildly built up (about 400 hp). Whats your opinions, 3" or 2.5"?

You don't need a 3" system to support 400hp.
Big pipes also drone more often than smaller.

...that said, I always thought a 2.75" system might be just right for so many applications.

3" will work real good.

A well executed 3" with an x-pipe AND tailpipes w/ muffler of your choice will allow room to grow when you want more than 400 hp....and you know you will want more!

My brother had 2.5" and went with 3" when he still had the 350 in his olds. It ran in the low 14's - high 13's most of the time. While the change didn't provide a huge power increase, the engine did seem to rev more freely, and it stopped blowing collector gaskets out where the tubing size transitioned from the 3" collector to the 2.5" exhaust pipe.

And yes 3" equals more noise vs 2.5" pipe. Depending on where you live the cops might take an interest in the sound making capacity of your ride.

IMO 3" isn't needed, unless you plan to up-power the motor at some point.

I think anytime a header collector reducer is constricting the flow that power is being left on the table. A 3" system as said is much louder & correct muffs, an X pipe and resonators downstream would be mandatory.

Had a 3" TTI H-pipe system and it was too freeking loud (as in drone). Changed to a 2.5" X-pipe system and it seems to have more low end torque as well.

Aren't the F.A.S.T. cars using 2.5"? Pretty sure and they are in the 10's. If that's quick enough for you, stay with the 2.5".

Thanks.

I think that 2.5" is plenty for a 400 hp engine. It should be quieter, have less drone and package better than 3". Also the extra velocity and heat in the exhaust gas should help keep the exhaust from rusting out from condensation, without being too restrictive. And many mufflers that have 3" in/outlets actually are 2.5" cores with the tubes swedged out, and don't really flow any more.

Hmmm...

Ideally, you would run open exhaust with optimized length collectors. Generally speaking, the less restrictive the system, the better.

Sometimes Low-end torque is slightly improved by a more restrictive system. There are systems I have read about the purportedly produce more power with X-Pipes and optimized, tuned mufflers, etc.

Still, less restrictive is typically the better way to go.

I own a '79 Jeep CJ5 with AMC 304 V8. I run 3-inch pipes and Dynomax Mufflers on it. It's not quiet, but not overly loud either.

I also own a '71 Charger. When I upgraded from 2 1/2-inch to 3 1/2-inch exhaust on the 440 in it, 1/4-mile ET performance improved by 7/10-second.

I'm happy with mine.

Here is a page from Magnaflows web site. One thing to consider also is the difference in mandrel bends and the typical exhaust shop bends that have the wrinkles in the bends. A 3" system that has kinks and wrinkles in the bends will not outflow a good mandrel bent 2.5" system.

On my 512 challenger motor I have 2" TTI headers and 3" pipe into flowmaster 40's that dump just before the rearend. My neighbor complained saying that when I idle my car it vibrates stuff off of his walls in his house 75 feet away. If he wasn't such a jerk I would care but I think he needs to insulate his house better. My other 4 neighbors say they can hear it but it doesn't bother them they love the way it sounds. I haven't been pulled over for it yet but I know I will after all I live in Idaho. In Idaho if it's fun or free it's illegal.

I'm gonna go out on a limb here and specutate that you are making more than the 400 HP that the original poster asked about

I'd just hate to mess up this engine by over heating valves and so on. You guys are right about the horse power tho, I haven't upgraded the intake, carb or the heads so I do have room to grow. However, it's a street car and I'd like to build it to be durable.