I am looking into my future exhaust setup for my new 406. Right now, I have stock manifolds into true dual exhaust with small tubing, so it all has to go. I would like to go from headers to x-pipe to 2.5" mufflers and out to 2.5" tail pipes.

I am looking at the x-pipes and wondering what would be the best size to use.

If the header collector is 3" in dia. Would it be best to use a 3" x-pipe to match the collector dia? Or use a reducer cone from 3" to 2.5" and a 2.5" x-pipe? Will there be a lose in scavenging by necking down to a 2.5" pipe after the 3" collector? Other draw backs / benefits?

Thanks - John

Depends on what you wanna use the 406 for. If it's for torque, it would be best to use a 2-1/2" collector and 2-1/2" pipe all the way back (so I've been told).

But if it's for HP, I'd go 3" collector, 3" x-pipe, and 2-1/2" tails.

But if you already have a 3" collector you can use those long cone reducers to help the transition to 2-1/2" as well. I've never tried them, but I've heard they work well.

John,
I read somewhere (can't remember exactly where) that it's best to match the output of your header collector (in your case, 3"), & then make your reduction on the output side of the X-pipe. I know Dr. Gas makes a 3" input/2.5" ouput. I bought one, just haven't had the chance to install it yet.

I'm building some headers (well....waiting for a mill to be free so I can make flanges) for a Buick 340 in a Jeepster.

Here is a good site for research
Header design parameters (http://www.headerdesign.com/extras/design.asp)

Thom

Dr Gas recommends keeping the X inlet the same size as your collector diameter.

Thanks guys,

I'll keep the x-pipe at 3" then. Here is what I am looking doing then with p/n's from Summit. I was looking at pypesexhaust. Very nice setup but I needed to order other stuff from summit and their shipping killed it.

Use a 3" x 3" header reducing (okay not reducing but gives me something to bolt onto the headers and weld/clamp to the x-pipe). p/n sum-g4752.
Summit 3" x-pipe kit. p/n sum-642130
Flowmaster 8" long reducing cones to from 3" to 2.5" on the back end of the X-pipe. p/n flo-r3025
Summit 2.5" turbo mufflers. p/n sum-630225
flowmasters 2.5" tail pipes. p/n flo-15802

- John

Sounds like a plan!