O.K., let me throw my 2 cents in. The smaller the "hose",less volume more pressure or velocity. The bigger the hose, less pressure, more volume.

 

So your calling me a liar? If I'm not telling the truth I must be huh? I really wish I had more time to go into this tonight but I don't. But do me a favor, don't call me a liar.

Really? So tell me, why is it that Flowmaster, Blackjack, Headmann, etc all have some kind of backpressure? Don't you think with the millions they spend in setting up an exhaust system that if there was no need for ANY backpressure that they would have figured out how to do that?

 

It seems like every exhaust thread turns into a back pressure debate which I'm going to leave my opinion out because the term doesn't always get used right . (I may not even use it completely right sometimes.)

If you are going to have headers I think dual exhaust would be more beneficial with 2.25" pipe. If you are set on single then probably the largest pipe diameter you would want is 3" max. 4" is just making a place for small animals to sleep at night . IMO 4" won't give you any benefit.

On my truck I have headers into 2.5" pipe into mufflers and 2.25" tailpipes. If I were to redo it I would do it in all 2.25".

I do work in a shop were I do custom exhaust and some of the best sounding systems were 2.25" over any other diameter when it came to trucks. I think some seemed to be more free flowing then some 2.5" systems that I have done. I don't have any scientific info to back that up though. All of this is mostly just my opinion from what I have done/seen. I agree with UTFball about 2.5" being about the biggest you should go with stock/slighlty mod engines. For a stock 302, etc I would venture to say even just 2.25". 3" would be good for highly modified engines.

Sorry for rambling.

 

the appropriate sized pipe will keep restriction at a minimum and keep velocity speeds up.

the less restriction will of course let the engine breathe out as quickly as it needs. the velocity of the air will have the exhaust gasses actually pulling air into the combustion chamber during the time when exhaust and intake valves are opened at the same time. this increases volumetric efficiency just like adding boost through a supercharger or turbo, but from the other end of the engine.

its actually pulling air into the engine downstream instead of pushing air into it from upstream like a super/turbocharger i guess is a way to put it.

nascar (yuck) engines have exhaust systems that scavenge through the exhaust like crazy.

 

I remember reading an old performance book by David Vizard, they talked about back pressure-and basically it came down to anything with more than 5psi of pressure at wide open throttle, is too much.
They welded a bung to the exhaust pipe, with a length of steel tube(so the rubber hose wont burn), then hose to a low pressure guage to test.
He also stated that any backpressure will result in power and economy loss. If a supercharger is used, the exhaust system, from a full on setup, to zoomies is just there to direct the exhaust away.
I know the 460 powered motorhomes warp and break manifold bolts alot-due to heavy work loads, and restrictive catalytic converters, which can melt out-just makes too much heat, that it can't get rid of fast enough. Solution was to either remove the cat, if allowed, or run free flowing ones.
My dad had a mid 70's car with a 302, that would struggle to do 55mph, he cut the cat. out, and disconnected the air pump and thats it-it got better mileage and would do 75mph + way easier.
The new mustangs have a very nice exhaust, flow wise-not much can be done to improve it, usually need to replace everything for a few ponies.
From what I have learnt and read, a mandrel bent 2.5" dual system should be good to up to 400 hp.
Also keep in mind, most of us aren't gonna have the happy pedal holding high rpms for extended periods of time.
For us pickup owners, an H pipe may be a good thing, increase the torq. a bit, and control the blatting sound. Dyno tests showed scavenging improvements with a large U shaped H pipe, about 8' long, still helped.
Exhaust is odd, and almost an art form for high efficiency-there's a ton of tricks to do, for more power, some are easy, and some just aren't feaseable.

I myself currently run stock 2wd 460 manifolds(they were free and fit), and a 2.5" non mandrel bent exhaust with a couple of walker dynomax mufflers. Works ok, and sounds good enough. I do want to run headers again, with an H pipe, and mebbe some good flow through glasspacks.
If you change your exhaust, there is a good chance you will need to retune your engine.
Some thoughts!

 

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