The logic in this...

[Kotta390]

After reviewing the size of our stock exhaust system's are, I am now trying to find out the point of going to a 4" or even a 5" exhaust system for performance. Now, sound is one thing, but performance, I don't understand the logic. 

 

 

It is simple, and maybe I am oversimplifying this (someone tell me otherwise) but if you have a 3" exhaust flange coming off of the stock HX35 or HY35 exhaust housing, how is putting a 4 or 5" down pipe work when the choke point of the exhaust is 3", then going to a 4" or 5" going to help you move exhaust gases better?

 

 

I come from drag racing, I always have and It has always been gas, not diesel. Most of the same principles apply in both worlds, but even Bernoulli's principle can't be justifiable in this scenario UNLESS the turbo exhaust flange is larger than 3" and the down pipe or whole exhaust system is smaller than the exhaust housing. IE: you have a Small block Mopar with 1 3/4" primary headers that come to a 3" collector. You can run UP to a 3" dual mandrel system for the highest flow. Now if you put on a 3.5" dual system, you gain no benefit but now just adding weight. The sound might be a tad different but you can't move air faster going from a small tube to a bigger tube. Now you can do the opposite to a degree but only under pressure which exhaust systems don't really have in the sense I am speaking of.

 

 

Please tell me I am missing something here, or do lost of people like running ginormous exhaust systems for the look? 

[MnTom]

There is no real need to increase the size of the exhaust unless you are quite high in HP. But, I can tell you that by going to a larger exhaust even with the smaller flange there is going to be a slight pressure difference with the larger pipe. Since the larger pipe has a lower pressure than the flange it actually can have a venturi effect there by assisting in drawing the exhaust into the pipe at a higher velocity.

[Kotta390]

There is no real need to increase the size of the exhaust unless you are quite high in HP. But, I can tell you that by going to a larger exhaust even with the smaller flange there is going to be a slight pressure difference with the larger pipe. Since the larger pipe has a lower pressure than the flange it actually can have a venturi effect there by assisting in drawing the exhaust into the pipe at a higher velocity.

 

See and that's why I brought up Bernoulli's principle. It can't actually act as a venturi because it is going the opposite direction. The only way to speed up the velocity of a fluid or gas under "constant" pressure is to decrease the size of the orifice it is traveling through http://upload.wikimedia.org/wikipedia/commons/5/54/Venturifixed2.PNG, like a carburetor's throttle venturies. So in it's case the pressure drops, velocity increase and temperature decreases. The exhaust system is actually working backwards from a carburetor in theory. By all means I am not arguing or calling you wrong, I just love having discussions like this. It's a good way to learn and I could be looking at this the wrong way, it's just been bugging me to no end LOL.

[rancherman]

EXHAUST   gasses   continue to  expand  all the way out to  the end of the  pipe,  from the  time   the  spark or  injection occurs.  Actually,   right up to the point they reach  ambient temp. (floating around  in atmosphere, several feet away from  tip)

So,   a  choke point  (where you  indicated)   is   handling  still    relatively  'compressed'  gasses,  and  the  area  they have to  travel through, well,  they  'fit'.    

Now,   7 feet   'down stream'   will   see  these  same  gasses   trying to  take up   more space,  just because they have  'expanded'   even more..    This  is  where  we'll  see   the muffler,  and   larger  tailpipe  configurations  in a lot of  vehicles.    

In   NA  rigs,   a   smaller   headpipe  is  for  velocity/cylinder scavenging,   and  they   progress  into  larger and larger  pipe  for   back pressure  management,  and   sound  reduction.    Small pipes  will have  speed,  which  is  momentum  that  can  help  'draw'    much like  a  chimney.  Chrysler  did  a lot  of  goofy looking    intakes  and exhausts  in the  50's and  60's    utilizing  this  phenom.

Edited April 22, 2014 by rancherman

[Kotta390]

EXHAUST   gasses   continue to  expand  all the way out to  the end of the  pipe,  from the  time   the  spark or  injection occurs.  Actually,   right up to the point they reach  ambient temp. (floating around  in atmosphere, several feet away from  tip)

So,   a  choke point  (where you  indicated)   is   handling  still    relatively  'compressed'  gasses,  and  the  area  they have to  travel through, well,  they  'fit'.    

Now,   7 feet   'down stream'   will   see  these  same  gasses   trying to  take up   more space,  just because they have  'expanded'   even more..    This  is  where  we'll  see   the muffler,  and   larger  tailpipe  configurations  in a lot of  vehicles.    

In   NA  rigs,   a   smaller   headpipe  is  for  velocity/cylinder scavenging,   and  they   progress  into  larger and larger  pipe  for   back pressure  management,  and   sound  reduction.

 

This was the only thing that I was wondering. If the expansion of hot gasses continue to expand then I can see the need for a larger pipe throughout the exhaust. In N/A vehicles this isn't really an issue because of the lower EGT typically. So I can see now how a larger exhaust system can be somewhat useful on these truck, of course it all depends on your modifications to the truck like stated before. I don't plan on changing my exhaust for quite some time but then again the only real thing in my future as far as performance goes is some small injectors.

 

Thanks for clarifying that up for me. As I stated before it was the only thing I could think of when it comes to thermal expansion and fluid dynamics. 

[rancherman]

cool.

like  Mntom  stated,   larger  systems  are for  higher  hp..   (which means   we are adding  a lot more air and  heat (fuel) to the  original system.)

 

So,    the  extra air  from   the  'now well-lit turbo'   and  the  heat from the extra fuel   HAS  to be able to  exit  fairly freely!...   hence the  4 or 5 inch  pipe.

 

In  gassers,   dual exhaust  usually does the trick.    (but,   remember  in gassers,   I  'shift'  at   6,500  rpm)    and   the amount  of  cfm  going  downpipe  is   at least  double  of  what  factory  ever   expected!)

[Kotta390]

cool.

like  Mntom  stated,   larger  systems  are for  higher  hp..   (which means   we are adding  a lot more air and  heat (fuel) to the  original system.)

 

So,    the  extra air  from   the  'now well-lit turbo'   and  the  heat from the extra fuel   HAS  to be able to  exit  fairly freely!...   hence the  4 or 5 inch  pipe.

 

In  gassers,   dual exhaust  usually does the trick.    (but,   remember  in gassers,   I  'shift'  at   6,500  rpm)    and   the amount  of  cfm  going  downpipe  is   at least  double  of  what  factory  ever   expected!)

 

 

Lol yeah very true. Well at least that answers my question. 

[MnTom]

You also need to remember that on a N/A or non turbo'd engine (most gassers) the size of the exhaust is in itself a bit of engineering to keep the proper velocity for scavenging purposes, but once you throw a turbo in the mix it changes everything. On a gasser if you go to large you run into the problem of the exhaust cooling in the pipe causing turbulence since cooler exhaust is harder to push than warmer exhaust. A turbo pretty much eliminates that by combining the pulses that normally travel down the exhaust. If you have time to kill here is some technical reading: http://www.mech.kuleuven.be/en/doctorates/persoons.pdf

[Kotta390]

You also need to remember that on a N/A or non turbo'd engine (most gassers) the size of the exhaust is in itself a bit of engineering to keep the proper velocity for scavenging purposes, but once you throw a turbo in the mix it changes everything. On a gasser if you go to large you run into the problem of the exhaust cooling in the pipe causing turbulence since cooler exhaust is harder to push than warmer exhaust. A turbo pretty much eliminates that by combining the pulses that normally travel down the exhaust. If you have time to kill here is some technical reading: http://www.mech.kuleuven.be/en/doctorates/persoons.pdf

 

I can agree with this 100%

[hex0rz]

...well, I may not have as  much as a technical comment here, but can agree. Because I have read numerous times about how people put on a larger exhaust and really see no performance gains, except for lower EGT's. I plan on going to a 4" for just this reason.

[rancherman]

...well, I may not have as  much as a technical comment here, but can agree. Because I have read numerous times about how people put on a larger exhaust and really see no performance gains, except for lower EGT's. I plan on going to a 4" for just this reason.

 

X2!    I'd  like to  run 5 inch,  but geez,   the  cost difference  is  substantial!

probably go  with  a  mbrp 4  system here..     maybe a  5 or 6 inch  tail

[mopartechnician]

4" will handle lots of hp. I bet close to 1000. I hate the way 5" sounds espcially towing. Plus there is no added benifit. I always say mbrp 4" turbo back with muffler.

[rancherman]

since I'll be in the market for a system here shortly..

 

are all  these aftermarket  pretty much the same  gage?   I've heard  some   drone more than others..   is  this  the  thinness of the  tube..  or just lack of a muffler? 

Would a  heat wrap  help out?

[mopartechnician]

I do believe its lack of muffler

[Wild and Free]

The 4 inch systems have been dyno tested up to 1000 hp without any negative effects, 4 inch is the only real exhaust needed never saw any need for 5 inch as it adds weight and all rigs I see that actually get used off pavement can't keep exhaust hangers in one piece, tail pipes always hanging due to broke hangers.

 

One needs to remember the thing that gets restrictive in a regular exh system is the bends, this is where manderel bent tubing is better, it stays a constant diameter throughout the bend and not kinked and squashed and have ripples in the bends that cause a restriction more than smaller pipe does in some cases.

[Mopar1973Man]

As for daily drivers even 3" has no effect to MPG or daily driver performance because typically your at very low boost pressure and even drive pressures so the 3" exhaust has very little effect. But 4" is optimal but once your boost/drive pressures are on average very low.  Now for full on racing I can see possibly that 4" exhaust doing better than 3" because the extremely high boost pressure and the drive pressure of the exhaust system. Depends on where you at and what your doing of what is best for exhaust system. But the turbine housing is the biggest choke point than the exhaust elbow.

[Gamble]

I will be the 1st to state I bought 5" just because I wanted the sound.  My truck is fully undercoated and is remarkably quiet in the cab.  I am a fan of the deep sound and spend most of my drive time with my passenger window down a little.  I took my time installing my exhaust and kept it up as tight as possible up inside the frame rails and rear 1/4 panel.  I don't think I would drag or hit my 5" any faster than someone would on a factory 3" maybe less of a chance since I don't have the ginormous muffler up there either.  Absolutely no HP gain from going 5", However the happiness factor was a great return.

[Wild and Free]

I am not talking about hitting or dragging it, it is the extra weight of the 5 inch system that is the problem and bouncing down rough potholed or washboard roads daily causer the hangers to break off from the extra weight especially if going stainless over aluminized.

[Mopar1973Man]

I am not talking about hitting or dragging it, it is the extra weight of the 5 inch system that is the problem and bouncing down rough potholed or washboard roads daily causer the hangers to break off from the extra weight especially if going stainless over aluminized.

 

Good point! I forgot to consider that myself.

[rancherman]

yep,  stainless  isn't quite as   'vibration proof'.    It'll  crack  a little  quicker than  mild/aluminized

 

Just a tad more brittle,   and  it  work-hardens 

Edited April 23, 2014 by rancherman