Something to ponder when you consider exhaust pipe diameter...

[Forrest Nearing]

There is a pretty big debate on one of the Ford sites over 3. 5" or 4" downpipe. Most of the guys feel that using the 3. 5 downpipe keeps bottom end alive, where the 4" cause's lost bottom end with increased flow on top.

they also have to do firewall modifications to fit a 4" downpipe

 

[aaronisbad]

I put a Banks 4" monster exhaust on my 01" didnt have to modify my firewall in the least bit. I am pretty sure 5" would have plenty of room as well.

 

[Forrest Nearing]

sorry, I'm thinking about the earlier Fords!



Forrest

 

[slomodiesel]

length of pipe adds alot to the equasion. . while setting up my yard for sprinklers

i measured the gallons per min out of my faucet. 5/8 inch pipe faucet with a valve of approx 1/2 inch---26 gals per min. added 50' of 5/8 in hose and got

18 gals per min. with fluids (and gasses) a pipe with a small restriction expanding to larger diameter for a given length will flow a lot more than the same length of pipe the diameter as the restriction. that is why an exhaust system with a smaller down pipe than the rest of the system won't hurt flow as much as one with the smaller pipe all the way through

Slomo

 

[Gary - K7GLD]

"that is why an exhaust system with a smaller down pipe than the rest of the system won't hurt flow as much as one with the smaller pipe all the way through"



Which is also why I was not quite as concerned about the relatively short 3. 5 inch turbo exhaust elbow on my new 4 inch system as I was the longer 3. 5 inch downpipe that system would have normally used - I figured that while that elbow WAS a restriction, it wasn't quite as big an issue as the much longer total downpipe length. Probably even both combined left at 3. 5 on an otherwise 4 inch system MIGHT not make much actual power difference - but I'm far happier with the SHORTER restriction going into a 4 inch downpipe, than I woulda been with the longer one! Oo. :-laf

 

[RJPotts]

Has anyone accounted for unburnt fuel exiting the exhaust, I would have to imagine, especially on a modded truck/car/anything, especially diesel, that our clouds of black smoke take up more room than "clean" exhaust gasses.

I'd say for the average turbo diesel especially, but for anything with a turbo go as big as possible/economical since back pressure is the enemy, and remember that exhaust will only flow as much as the smallest bend allows.

-R. J.

 

[JCleary]

Can I ask a question of you guys that have the ability to figure these things out?

OK, here goes.

If I seldom ever run my RPMs over 2300,and average travel RPMs are 1900, and boost never exceeds factory settings of 22, would I see any benefit in a 4" over the stock system on my 04? I think it is 3. 5"

I tow constantly, if that makes any difference. I also put on about 80,000 mi a year. I would like to know if the benefit of 4"would outweigh the cost. Givin the cost of fuel, I need all the feasable edge I can get. TIA

I've been told that if you go from the stock downpipe to a 4" on the 555's you can actually realize a decrease in performance.



This is apparently due to the turbo being able to spool quicker, raising boost too fast for the ECM, so the ECM defuels a bit.



I have no proof of this, and my truck did not seem to suffer from a bigger exhaust except for a decrease in mileage. This is due to my right foot being guided by my ears, though :-laf

 

[Hohn]

length of pipe adds alot to the equasion. . while setting up my yard for sprinklers

i measured the gallons per min out of my faucet. 5/8 inch pipe faucet with a valve of approx 1/2 inch---26 gals per min. added 50' of 5/8 in hose and got

18 gals per min. with fluids (and gasses) a pipe with a small restriction expanding to larger diameter for a given length will flow a lot more than the same length of pipe the diameter as the restriction. that is why an exhaust system with a smaller down pipe than the rest of the system won't hurt flow as much as one with the smaller pipe all the way through

Slomo

This is not necessarily an accurate comparison to exhaust.



When fluids flow throw a conduit, there is a "skin friction" that the flow column experiences. Imagine that you have a cross-sectional slice of the water flowing though the hose. In the center of this slice, you have the highest velocity in the whole flowstream. At the edge of the slice, the velocity is ZERO.



There is a gradient (like a ramp) of velocities, starting in the center with the fastest velocity and ramping down to the slowest velocity at the boundary where the water meets the hose.



Now, this gradient is NOT linear, it's more like a steep exponential decay in velocity as you approach the outermost edge.



I'll refer to this area as the "boundary layer", where velocity is somewhere above zero, but not as fast as the main flowstream.



How Thick is this boundary layer? This is important because it will determine how much skin friction we will have. If we have a boundary layer 1/2" thick, then a 1. 5" hose doesn't have much "free area" in the middle, and you will have more friction loss per foot.





Now, if you have a 5" diameter hose, that same 1/2" boundary layer causes proportionally much LESS restriction.



Before we come back to the topic of AIR flowing in a conduit like exhaust pipe, there are a number of things that contribute to the thickness of the boundary layer-- fluid viscosity, surface texture of the conduit, and velocity.



Viscosity is a BIG player in the boundary layer thickness, and THIS is why water flowing though an exhaust pipe would have more loss per foot than air would. The boundary layer is VERY thin-- probably less than 1/4". It's thinner for air than for water because air is less viscous.



Surface texture is important, as a turbulent boundary layer helps to reduce drag. Exhibit A: a dimpled golf ball. Exhibit B: the tiny holes on top of certain aircraft wings that are energized with bleed air to create boundary layer turbulence.



Velocity is a player in the same way that pipe diameter is. For a given restriction, you can pump a more viscous fluid at a lower flow rate to "see" the same overal restriction. As velocity goes up, it has an effect similar to increasing the viscosity, and makes the pipe seem smaller than it is.



So, water flowing through a hose is not the same as air flowing though exhaust pipe. The "skin friction" losses on exhaust pipe are negligible when comparing the lengths you would see on a truck. If you had a 50ft pipe, then things might be different.



So Gary, you don't need to worry about the restriction of a 3. 5" elbow compared to a 3. 5" downpipe, as the difference between the two is TINY.



JMHO

Justin

 

[Huff N Puff]

Thankyou Captain,as usual a very enlightening thread that gets us thinking

Something else to ponder as you increase the volume threw a set diameter pipe you will increase the pressure in the pipe. If you moved 2000cfm of air thru a 4in pipe and 2000cfm thru a 5in pipe wouldn't the pressure in the 5in pipe be less. All things being equal wouldn't a turbocharger spool faster with the 5in pipe especially if you took in account the length of the pipe.

 

[Hohn]

Something else to ponder as you increase the volume threw a set diameter pipe you will increase the pressure in the pipe. If you moved 2000cfm of air thru a 4in pipe and 2000cfm thru a 5in pipe wouldn't the pressure in the 5in pipe be less. All things being equal wouldn't a turbocharger spool faster with the 5in pipe especially if you took in account the length of the pipe.

Yes, the 5" should have less pressure, imo.



But if you reduced the flow down to 100cfm, I doubt the difference between 4" and 5" would even be measurable with normal equipment.



jlh

 

[RKerner]

Yes, the 5" should have less pressure, imo.



But if you reduced the flow down to 100cfm, I doubt the difference between 4" and 5" would even be measurable with normal equipment.



jlh

Static?

Dynamic?

Stagnation?

or total?



What's normal equipment? Is a water manometer normal? Magnahelic in inches H2O?







One misnomer sometimes is that a larger flow potential increases turbo performance. If you talk to ferd guys, they might mention a slight loss in spooling due to a slower exhaust velocity after switching to a 4" exhaust. Food for thought.

 

[Hohn]

Normal equip-- the cheapie gauges sold at auto parts stores and such. A manometer should show it... .



When it comes to exhaust, the "Goldilocks theory" is what I follow-- not too big or too small, you want "just right".



jlh

 

[RustyJC]

Depending on the configuration and location of your pitot tubes, you also have the "velocity pressure" versus "static pressure" variations to contend with.



(Just yanking your technical chains, guys!!) :-laf



Rusty

 

[JLehman]

In regards to the 3 1/2" elbow, how bout the venturi effect?? I know in the toxic gas industry we used venturis to effect some amazing results. Just occoured to me, any idea??

 

[Gary - K7GLD]

In regards to the 3 1/2" elbow, how bout the venturi effect?? I know in the toxic gas industry we used venturis to effect some amazing results. Just occoured to me, any idea??

"Venturi effect" involves the ability to create a relatively concentrated high pressure/velocity air or fluid flow that pulls SURROUNDING OUTSIDE air or fluid into it's stream of flow - since the typical exhaust system is a CLOSED system, the venturi effect doesn't apply. Also, the requirement for restriction needed to create the high pressure/velocity is exactly the reverse of the free, unrestricted exhaust flow from the engine we want to begin with.

 

[Luke Warmwater]

Oh come on now that's how the aero turbine thingamgiggy works. By the time you have 10 of them babies installed inline you don't even need a turbo anymore because the ATs are sucking air through your motor so hard, you are at warp speed anyway.



A low pressure area created by increased fluid velocity doesn't "suck" but rather the ambient atmoshpere "blows" lol



Let's ponder the difference between sucking and blowing... on second thought, let's not.

 

[Forrest]

Let's ponder the difference between sucking and blowing... on second thought, let's not.

It's called the Monica Effect.

 

[JLehman]

Thanks for the reply sounds like you are rite.

 

[Hohn]

Well, not so fast...



True, the exhaust is a closed system, BUT there is still something to consider about the acceleration caused by a narrowing of the pipe.



In most high-end cylinder heads for racing engines, there is a narrowed portion of the intake port just above the valve seats. Why? It's done to accelerate the intake charge just before it enters the cylinder. It's proven to make HP... .



Now, what about the other end? Since the exhaust has mass, it has inertia. What if the outlet of the turbo had a slight narrowing to it that forced the exhaust to accelerate as it left the turbine? Might we see more flow?



Probably not. In the intake example, the inertial effects are helpful because of piston dwell at BDC and valve overlap. On the exhaust, you have nothing that benefits from the increased restriction, as the acceleration of the exhaust charge is offset by the reduced pressure differential across the turbine.



JLH

 

[Gary - K7GLD]

"In most high-end cylinder heads for racing engines, there is a narrowed portion of the intake port just above the valve seats. Why? It's done to accelerate the intake charge just before it enters the cylinder. It's proven to make HP... . "



I'm not sure whether that was done so much for charge accelleration, as it was to enhance cylinder turbulence for better/more complete cylinder charging and fuel distribution - SB Chevy heads had a similar "bump" towards the center of the combustion chamber for the same reason...