pipe size

[Grbmaverickmo]

Hi all, been awhile since I posted on here but read alot of your posts to learn about these motors. I`ve been gathering and gathering pieces for my 54 with the 390 and now looking at pipe size for exhaust. Does anybody run 3inch or is 2.5 big enough for a lets say 400 hp 390 not that mines making that now but one day it will so I don`t wanna do exhaust for the third time. It will be over the rear and out the back with a set of headers not sure yet if there gona be FPA`s or I try and make my own with the speedway FE header kit but it will have headers. As always Thanks for any help on this and can`t wait till next yrs FE race and reunion

[ScotiaFE]

2.5" mandrel bent is fine. The 3" sewer pipe looks great though.
If you have the stuff to make the rear over the axle pipes flanged to the front pipes for unbolting it will be
a huge time saver down the road.

[mlcraven]

3" can be a real PITA to fit when running over the rear end and out the back.  Your application may differ.

[fastback 427]

I would say there is more to be gained or lost with muffler selection than 2 1/2" to 3" pipe. If it was mine I'd buy the 3" mandrels or 3" doughnut from eBay with a set of hooker max flow or another good muffler and go 3" all the way. And I'd install an x pipe.

[ScotiaFE]

More on  the muffler.
You can cut the guts out of a cheap set of Canadian Tire mini's
and weld a 2.5" pipe in and have a pipe from the header back to the bumper.
In the concrete jungle it does tend to be a tad loud, but on the open road it seems fine to me.
Much more quite then open headers in town. 

[fe66comet]

Really it is relative to rpm, HP and length. 2 1/2 is ok in short length at the HP level you stated but if you run back past the rear tires 3" would serve you better. The smaller pipe will help in torque but at a cost of upper RPM HP, usually the header selection dictates collector size. I think in a full length exhaust at 400+ HP you would be restricting flow @ 2 1/2, usually even a stock vehicle can benefit from better flow.

[mlcraven]

I think in a full length exhaust at 400+ HP you would be restricting flow @ 2 1/2...

I'm going to disagree and suggest that, in street-driven cars at this power level, with similar header, muffler, and side-to-side balancing set-up (H-pipe, X-pipe, whatever), the only appreciable difference between 2.5- and 3-in pipes will be fitment aggravation and cost.  Of course, there is a purely stylistic reason to go with three inch pipe: it looks good poking out from under the car (a fact not lost on new auto makers, most of whom have climbed on the big diameter exhaust tip bandwagon)

[plovett]

I'm generally a proponent of big exhaust.  I think a lot people have too small of an exhaust.  That said, I think 2-1/2" pipes are plenty for a 400 hp engine, if it's a well designed system with good mufflers.  As has been said, mandrel bends would be beneficial, especially if the pipes are going over the rear axle and out the back.  Might as well get a cross-over of some type as well.

In very general terms you might say use 2-1/4" dual exhaust up to maybe 375 hp.  Use 2-1/2" up to about 425 hp.  Use 3" up to about 525 hp.  Use 3-1/2" above that.  I just made those numbers up, but they're in the ball park. 

I like to err on the large side.  It usually helps.  The downside (if too big) is increased weight and more difficult fitment.  I bet most people would add 25-50 hp to each of my recommendations above.

JMO,

paulie

[My427stang]

I like to err on the large side.  It usually helps.  The downside (if too big) is increased weight and more difficult fitment.  I bet most people would add 25-50 hp to each of my recommendations above.

JMO,

paulie

Not me!  I'd say "good job Paulie"

Primary pipe sizing can get too big, but after the collector, I like big guns....even my 270H, 8.5:1 396 in my truck woke up significantly with a change from 2.25 to 3 inch mandrel bent.  It was at best 340 hp and probably less, did it in preparation for the 445 and it loved it from idle on up. 

[plovett]

I like to err on the large side.  It usually helps.  The downside (if too big) is increased weight and more difficult fitment.  I bet most people would add 25-50 hp to each of my recommendations above.

JMO,

paulie

Not me!  I'd say "good job Paulie"

Primary pipe sizing can get too big, but after the collector, I like big guns....even my 270H, 8.5:1 396 in my truck woke up significantly with a change from 2.25 to 3 inch mandrel bent.  It was at best 340 hp and probably less, did it in preparation for the 445 and it loved it from idle on up.

Thanks Ross!  I tend to be a bit radical in my predilections compared to most people so I try to tone it down a bit.  Maybe I toned it down just right, or possibly a bit too much?  LOL!  Of course primary length is another factor.  More primary length mellows out large diameter primaries and vice-versa.

I totally agree regarding primary pipe diameter vs. the diameter further back.   But even then I've had good luck with bigger primary pipes compared to conventional wisdom. " Over-sizing" primary pipes is certainly more "dangerous" though.   I like to keep the pipe size constant after the collector for a longer distance than most.   If I downsize it, I like to do that after the crossover.   

JMO,

paulie

[Grbmaverickmo]

Wow ok thanks guys good posts to this thread! I know that I messed up the first time with my truck manifolds and dual 1 7/8 pipes out the back that's why this time its getting done rite I `m not sure if its gona get a h-pipe or not probably yes. I like x pipes and have done a few on my mavericks and 5.0L mustangs. Just with the 54 I`m trying to keep it period correct for a late fifties to mid sixties build so the Xpipe def does not have that sound. If it fits I think 3inch is the way I`m gona go cause my end goal would be to put a 6-71 huffer on the car way later in life.  Muffler wise I know some may hate them but true 26 inch case cherry bombs. Unless I can find NOS old school thrush coffee can mufflers at a swap meet or ebay. Loud yes but cool as all heck. Any way thanks for everything Chris

[fe66comet]

They still make the Smitty classics if you want the 50s stuff.