4" to 3" exhaust

[brando4905]

Had to replace the last 4' of exhaust pipe while on the road(fumes inside coach). Only shop I could find had 3". Now that I am home, I am having trouble finding someone that can do 4". This is a 4905 with 8v71. Do you folks think this 4' section of 3" at the very end of the exhaust will hurt anything?

Thanks,
Brandon

[kyle4501]

The 3" flows 1/2 what the 4" does which will increase backpressure & I can't see any good coming from higher backpressure.

Can you install an additional 3" pipe? A good muffler shop should be able to make the adapter to tie in an additional pipe.

[zubzub]

I'm guessing here but once the exhaust has gotten to the end of the pipe it has cooled significantly and has less volume.  As to how much less volume  perhaps someone remembers this from high school.  What I'm saying is there is a possibility that a you can get away with the 3", otherwise grab some from a scrapped truck.

[Len Silva]

Take a look  http://store.airflo.com/4310-448.html

[kyle4501]

A little math for just pumping air:
71 ci * 8 = 568 ci per revolution * 2000 rpm = 1,136,000 ci/min

air speed in pipe = volume/ cross section area
- a 4" pipe = 12.566 sq inches
1,136,000/ 12.566 = 90402 in/min = 7533 ft/min = 85.6 mph


- a 3" pipe = 7.068 sq inches
1,136,000/ 7.068 = 160,724 in/min = 13,394 ft/min = 152 mph


While the engine isn't 100% volumetric efficient, adding fuel & burning it will increase the temperature, pressure & volume sufficiently enough to make the above numbers conservative. . . . .

This is the reason 4" was put there originally when 3" would have been cheaper & easier. 5" or 6" would even be better.


On my muscle cars, bigger exhaust pipes always allowed more power  . It's almost impossible to fit anything larger than 3" under a car & not rub or scrape. 

[TomC]

My transit with 8V-71 and V730 has 5" all the way.  If you go anyway, go up-5" is easy to obtain since it is the standard of the industry for big rig trucks.  Good Luck, TomC

[bottomacher]

I can't advise, but I have about 14 feet of 304 stainless 4" pipe in Atlanta if you need some.

[brando4905]

Thanks All, had a hell of a time finding 4" stock in my area, but I got it. And would'nt ya know, the existing pipe is not even 4, it's somewhere between 3 and 4! Hope the clamp will cinch it on down.

[Don4107]

Did you notice any change in power with the smaller pipe?