Liquids and gases are both referred to as fluids in Fluid Mechanics. Similar engineering and design principles apply to both liquid fluids and gaseous fluids, when it comes to sizing the delivery piping, tubing, ducting for the various rates of fluid flowing down those conduits.

I'm not arguing that a 4" exhaust may not be large enough. It may be for the 7.3L PS engine, no matter how hawged-out and modified. But that determination can not be reached solely by determining the size of the turbo outlet and then concluding that drop & exhaust pipe larger in diameter than the turbo outlet provides no additional benefits to engine performance. That should be apparent to most now, since the accepted conclusion in this thread is that larger-than-turbo-outlet diameter drop and exhaust pipes are the standard/norm for modded 7.3s.

The reason the higher HP 7.3s benefit from the larger drop and exhaust pipes is that they produce substantially higher exhaust flow rates (measured in cubic feet per minute) at WOT than factory 7.3s. Those higher exhaust flow rates will produce excessive back pressure which will compromise peak engine performance, if they aren't exhausted through sufficiently large exhaust tubing.

 

This thread is great and what makes it better is that I know it got started with me and Tugly bickering back and forth on this thread https://www.ford-trucks.com/forums/1...t-rules-2.html
I do have to admit that Tugly your garden hose idea sounds good, but lets do this and throw in turbo flow or CFM to complicate matters even more for everyone, so far no offense but CSIPSD and I have about the closest opinions on this topic this is fun keep it going
Ziggy/Cory did the math for us awhile back stock turbo flows about 838 CFM if I recall right

 

Wow. I never would have guessed that exhaust gasses would have the same flow properties as a liquid. I guess I'll be quiet now...

 

No so much the same flow properties, But same principles of pipe size and restrictions.

 

I'm only an amateur diesel mechanic and a total newb to the forum, but as an electrical engineer and firefighter I'm really enjoying this thread. The analogies to power lost in electrical wiring as a function of distance and to the pressure lost in various garden hoses were excellent. Real world examples of engineering theory are always exciting (geek!).

Here's my analogy for any firefighters reading the thread: The exhaust system back pressure is similar to friction loss in a fire hose. In pump school we are taught that pressure lost depends on volume of water flowing, hose diameter and hose length. Life is good as long as the hose diameter is large enough to deliver a volume of water the required distance at sufficient pressure. A larger hose will give less loss, but isn't required to get the job done, and is more work to pick up. A larger exhaust will give less loss, but no measurable HP gain in most engines, and is more costly.

 

Would it be better to take a 5" Bend it upward through the hood ? 3 foot long (one bend)

I see this at truck pulls. Some,Most trucks are under 1000 horsepower . I hardly see a truck pull with a 4" all the way out the back. I do see tons of of stacks!. Go tell them that it is all you need is a 4 inch bending around under the truck.

 

I have just a minute to post right now, but I bear gifts - a respectable reference. I can express my thoughts (if needed) later.

 

I'm home now. According to another reputable source (I can link you to the giant PDF if you have doubts), figure horsepower times 2.5 to get exhaust CFM on a 4-stroke turbodiesel. 300 HP is the red line on the graph, 400 HP is the yellow line, and 680 HP is the green line. Each one of those lines represents the point at which back pressure starts to impede exhaust flow.

Anybody who has gone from 3.5" exhaust to 4" exhaust and saw a jump in performance knows that red line at 300 HP is asking a lot out of that 3.5". Using this same flow restriction reference, 400 HP is demanding a lot out of a 4" exhaust. Only a mega-modder would get past 320, because it takes different injectors, more tuning, and a lot more intake air to get above that point ( I refer to cleatus12r). I know many members have surpassed 400 HP and some are shooting for 500 HP. I would ask if any of them made it that far with the 4" single exhaust as the final volley against my case.

I don't profess to know these engines, but I know a thing or two about engineering for flow.

 

516hp before I blew up the last motor... 4" turbo back...

Blowby's 1000hp truck...

F250 7.3 Diesel Blowby 1000 + Hp - YouTube

Powersmokers 1000k run... hood stack but the outlet of the turbo is 4"...

Dyno Pull at Rudy's Dyno Day: 1069 rwhp, 1768 rwtq - YouTube

Rudys 6.4... Wait... another 4" down pipe... hummmm

6.4L Getting it done Rudys Diesel 1216 H.P - YouTube

The only reason anyone goes to a stack is for looks, sound, or to get the smoke off the track. It has nothing to do with performance.

 

400 on a bone stock intake, turbocharger, and exhaust (straight-through stock 95-97 muffler). The only real limiting factor now is my high pressure oil pump, but that's not what this discussion is about.

 

This is all anyone really needs to know.

 

This is a great thread........ Tugly

Now let me throw this in the mix too..... intercooler piping is 3" I think, smaller than the original 3.5" exhaust, the turbo spools forces air threw the system and out the turbo.... the air that goes in is the air that has to come out right... so the only real benifit to going bigger on the exhaust side of things would come from making all the intake piping bigger

Correct me if Im wrong but being a pipe fitter/millwright and all you learn a few things along the way

 

Stock injectors and tune?

I had it in my head (but I didn't say it straight out) that we were talking about a street-configuration 5" exhaust. I didn't see any of the mega-modded trucks with a 4" pipe go the full street configuration all the way behind the passenger wheel. Why not?

I'm just looking to learn something here.

 

Pull up a chair I think we all are

 

Great point JOHN2001... but there is one more factor to this - expanding gasses from ignited fuel. I get tangle by this same thing when I think about it, but I found a math formula.

EGT (F) + 460
Divided by 540
times Intake CFM
equal Exhaust CFM.