Exhaust question for mild engine build

[travis]

2.0" vs 2.5" dual exhaust with small tube headers. Other than sound control, and room to grow...is there any advantage one way or the other on a very mild (basically an enhanced stocker) 350-360 inch sbc or sbf? This thing will spend 99% of it's life below 3500 rpm's, and fuel economy is important.

[F-BIRD'88]

You can road test that yourself by running it "uncapped" and with a 2" collector extension, then a 2½" collector extension .

[F-BIRD'88]

If you can apply a heavy load (say a boat trailer or car trailer)(up a steep hill) to it, the tested difference will be more apparent.

[PackardV8]

In the application you describe, short answer, no.

[BOOT]

Exhaust size is often rated for PEAK HP, how often will you be at peak? I generally size mine a bit small, it's the muffler flow that matters more in my opinion. Why not split the diff n get dual 2-1/4"

[PackardV8]

FWIW, In the 1960s, Chevrolet Engineering had all the money in the world to play with and not enough to do, so they spent time picking the flyshite out of the pepper. On the 300hp 327" stick shift builds, they used 2.25" exhaust manifolds and headpipes. On Powerglides, they used 2", just because it worked better, fit better, cost less and lasted longer. In the pre-aluminized exhaust pipe days, the resonator system Chevy used at the very rear, if used for short trips, would rust out in a couple of years. The 2" ran higher velocity and hotter, thus lasted a year longer.

[71vette]

One aspect I have experienced w/ engines like you have described in a 2" vs. 2-1\2" is exhaust leaks are much more prevalent with the 2" dia dual exhaust. If one ever allows any exhaust fastener just to become snug with the 2" exhaust an exhaust leak will quickly occur. On a few applications where 2" dia was being used then a straight thru best flowing muffler helped alleviate the leaks but the blat seems to occur at a much lower rpm than with 2-1\2" dia exhaust. Also I have never any performance detriment w/ the
2-1\2" exhaust at any point during normal driving.

[Tom68]

2.5'' then when you decide to put a 383 in you're right to go.

[PackardV8]

One aspect I have experienced w/ engines like you have described in a 2" vs. 2-1\2" is exhaust leaks are much more prevalent with the 2" dia dual exhaust. If one ever allows any exhaust fastener just to become snug with the 2" exhaust an exhaust leak will quickly occur.

Trying to get on the same page with you here. What, specifically, will cause 2" exhaust to be more leak-prone than 2.5"?

[71vette]

PackardV8 I don't have any before and after test data such as doing an exhaust back pressure check with a gauge 2" vs 2-1\2". But on several "evolving" projects thru the years I've noticed this said phenomenon. Several small block powered vehicles w/ cheap headers (flowtech, blackjack, etc) 2" duals, aftermarket "turbo" mufflers (not super turbos w/ the flow deflectors). If I swap out the turbo mufflers for a glass pack (perforated tube not a louvered) performance improved and leaks became less prominent. Same vehicles refitted with a 2-1\2" dual exhaust system and magnaflow mufflers (an H or X pipe to abate pulse blatting) would go years w/o any header gasket leaks. A few times during preventative maintenance I would check header bolts, find a few a little loose, but no leaks w/ the bigger exhaust. I have also seen many many vehicles go years w/o exhaust leaks w/ 2" exhaust pipes.