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? 

Gotcha.  I would like to weigh a system and see the difference, but I don't think there is nearly as big of a difference as most think in the weight department if your talking new straight vs old with muffler, unless of course someone does stainless.  I will agree it doesn't take a drastic increase in weight to have a negative effect on a hanger, but I would also say it will be an individual circumstance situation. i.e., how fast does one drive down said pothole, washed out roads.  I have a  2" section of 5" if someone else has a 2" section of 3" we can get at least a comparison that way.  I went aluminized as I keep my undercarriage pretty clean so it should last a long time.

Another thing to consider when discussing "fluid dynamics", for any gas or vapor, a larger exhaust pipe will create less back pressure on the engine. Think of trying to blow through a coffee stiring straw versus blowing trough a McDonald's straw...... After all, internal combustion engines are just reciprocating compressors and the less back pressure on a compressor the more efficient the compressor is. Or think of it as overall compression ratio, the larger the exhaust system, typically you will have lower drive pressures, thus a smaller compression ratio across the engine.

You are correct that there are small choke points in the system but larger piping in the system allows for higher flows with lower pressure drop through the system.

So why isn't people increasing the size of all the intake piping and intercooler? (Just to make people think a bit).

So why isn't people increasing the size of all the intake piping and intercooler? (Just to make people think a bit).

Because believe it or not, for the most part the engineers got it right! The stock intake (except for the intake on the head) is good for about 500HP if I remember right.

I seen stock intake horn on a 750hp pickup. I always say stock intake and horn till 500 than add intake horn

So why isn't people increasing the size of all the intake piping and intercooler? (Just to make people think a bit).

Honestly I feel it is just as important to address the suction side as well as the discharge. Heck, that is one main reason to port the intake and exhaust valve passages in the head.

Another thing to consider when discussing "fluid dynamics", for any gas or vapor, a larger exhaust pipe will create less back pressure on the engine. Think of trying to blow through a coffee stiring straw versus blowing trough a McDonald's straw...... After all, internal combustion engines are just reciprocating compressors and the less back pressure on a compressor the more efficient the compressor is. Or think of it as overall compression ratio, the larger the exhaust system, typically you will have lower drive pressures, thus a smaller compression ratio across the engine.

You are correct that there are small choke points in the system but larger piping in the system allows for higher flows with lower pressure drop through the system.

You are right in one sense. Now we discussed thermal expansion of hot gasses but you are now comparing that too ambient air and if you blow through a coffee stir straw (which is the choke point per say) and then cut off half of that straw and add a mcdonalds straw to the end of it, you didn't do anything or increase the flow of air if the amount of air is constant. You are right in if you replace the WHOLE straw with a bigger one, that definitely make a difference.

I have to put my .02 in here.  #1 The exhaust can only flow as much as the engine puts out regardless.  The manifold is not 5",4", or even 3" for that matter.  Then we have the same in regards to the turbo itself.  The HX35 only has a 3" exhaust outlet, but can the turbo flow more than the exhaust outlet can handle or vice versa. obviously through engineering it is known that what is used is sufficient to meet needs however, what I'm getting at is there are a lot of choke points in an exhaust system, but at any point you open it up (the sooner the better) I feel it relieves pressure all the way down the line. (prior to the choke point.)  The sooner the pressure is released and allowed to expand, the easier it is on the whole system. Humor me in proof of this using straws.  Take a regular straw.  cut 2" off the end of the straw.  Now crease 1-3/4" of that straw so you have 1/4" of normal straw then atmosphere.  Now take the rest of the straw and crease the enter length of it.  Blow through the 2 different straws and you will quickly notice the short straw in substantially easier to push air through.  The only thing done is opening the choke point to larger and then to atmosphere. Obviously in this case your body is the engine, but its hard to deny its easier on your body just like it would be less strain on an engine. Not to be bias, I asked my wife to perform this science experiment with no knowledge of what it was for, she immediately new the answer.  

 

This is why most daily drivers I talk to see little to no difference in upgrading exhaust. There was a person a while back that dyno'ed 3" and then upgrade to 4" and dyno'ed again and no difference was noticed. Now the ultimate way of fixing the exhaust issues is talk to TH Racing here on the site he's got a Cummins header.

 

 

now that is what you call an equal length header that is designed to flow as much as you can put through the cylinder. Making the restriction the turbine housing...

That would be called a "tuned" header, right?

Edited April 25, 201411 yr by hex0rz

I have to put my .02 in here.  #1 The exhaust can only flow as much as the engine puts out regardless.  The manifold is not 5",4", or even 3" for that matter.  Then we have the same in regards to the turbo itself.  The HX35 only has a 3" exhaust outlet, but can the turbo flow more than the exhaust outlet can handle or vice versa. obviously through engineering it is known that what is used is sufficient to meet needs however, what I'm getting at is there are a lot of choke points in an exhaust system, but at any point you open it up (the sooner the better) I feel it relieves pressure all the way down the line. (prior to the choke point.)  The sooner the pressure is released and allowed to expand, the easier it is on the whole system. Humor me in proof of this using straws.  Take a regular straw.  cut 2" off the end of the straw.  Now crease 1-3/4" of that straw so you have 1/4" of normal straw then atmosphere.  Now take the rest of the straw and crease the enter length of it.  Blow through the 2 different straws and you will quickly notice the short straw in substantially easier to push air through.  The only thing done is opening the choke point to larger and then to atmosphere. Obviously in this case your body is the engine, but its hard to deny its easier on your body just like it would be less strain on an engine. Not to be bias, I asked my wife to perform this science experiment with no knowledge of what it was for, she immediately new the answer.  

Basically what I was saying before. I agree with this. 

 

This is why most daily drivers I talk to see little to no difference in upgrading exhaust. There was a person a while back that dyno'ed 3" and then upgrade to 4" and dyno'ed again and no difference was noticed. Now the ultimate way of fixing the exhaust issues is talk to TH Racing here on the site he's got a Cummins header.

 

With equal length header primaries, you will gain some performance but Idk if it will be as much on a turbocharged engine compared to an NA engine because of the scavenging effect the exhaust pulses create. I Imagine it will create the same effect just larger pulses. 

 

 

BTW that is a gorgeous looking turbo manifold

That would be called a "tuned" header, right?

 

Calling it "tuned" is relative per application. In some sense a equal length header can be considered tuned but just because it is tuned doesn't necessarily make it equal length. Now the science in a real tuned header will be different for every combo. A "tuned header" usually describes a header that has equal length (or nearly equal length) primary tubes. We all know that primary tube diameter, tube length and collector design all go into the recipe for a tuned header. Now, the chore here is to get the header "tuned" to the RPM at which the engine makes the most horsepower. A "tuned" header is actually in "tune" during a very short RPM band. (This RPM band is even shorter in a high RPM Pro Stock of Competition Eliminator class car.)

 

 A "tuned" header is not at all necessary in a street or street/strip application. This is because a street/strip vehicle needs to produce HP and torque throughout a wide RPM band, therefore the header would be in "tune" for a very minimum amount of time.

 

 

well,  here we go.

 

a  'pump'    (turbo)   is  only as  efficient  as  it's     plumbing.          Just  because  there  is  a  heck of  a   force   (please  don't  consider a  piston coming up  a   good  reason  to move  gasses,  otherwise   too  much   energy  'gained'   in the flywheel  from the  powerstroke  would be  needed to  push  this   gas out)    Sure,  the  piston  does  help,   but if  we  rely  on the  piston to provide  the  power to spin the turbo,  it'd be   like  thinking  we could  add  a  generator  to  the  front axle,  just to drive  the rear axle motor, for free... uh, no.

 

anyone  ever  wonder  what the  cylinder pressure is  JUST  BEFORE  EXAUST VALVE OPENS???        well,   the  'pop'   sure isn't  the piston pushing it out!

 

Back to  first  paragraph!..    the  PRESSURE  DIFFERENTIAL  ON  BOTH SIDES OF THE  TURBO  is  the  only  way to   get  WORK  done.    HIGH  on  one side,  LOW   the  other.   There is  a  reason  the  temp differential in exhaust  is  about 150 -250 degrees  cooler on the  downside of the  turbo.     It's  gone  from    high pressure to  relatively low pressure..     and  has  EXPANDED  considerably!  It  needs  way more room  too.   Ever  wonder  why  air tools   get cold  when they  run?   Same principle going on here.. 

Moparman,  This  is  the  exact opposite  reason  why your  question on intake mods  earlier    is  answered here.    'Intercooler'.

Imagine  our  systems  without the cooler...  and  trying to stuff  200 degree  compressed air  into our engine.   There simply isn't  enough  pipe  capacity 'as designed'    to    get the  job done.   (not enough  moles  of   O2  in   that  hot  charge).   But,   cool it  down to  more  manageable  temps..  ~100-125*   (half  the  temp, doubles  the  density)   then  we see   some  results!

Intercooler  mods  would  be needed  if  it  isn't large enough to cool sufficiently.  More surface area, more cores, etc). An  I'd  think   until we start  winging our  engines  to  3200 + and up  rpm,  the  capacity of our   intake system  works pretty well.  Then  of  course,  larger pipe  going in,  and  going out.

 

Dang it,  I got a little  ahead of myself on the  piston  driving the  turbo..  

Nothing is  'free'.     we are   taking the  energy still available   in the  very hot  energy-packed  exhaust  gas,  to  do work for us..   We  still want  a happy medium of  relatively  packed  HOT gas  (pre turbo)  and   extract this  energy  as much as  possible  in the  turbo, without  degradation of  cylinder scavenging.       Keeping  the   gas hot  and  still  'packed'   before the  turbo is  the key.   Getting  the  'spent'  gas   away from the turbo is  just as important to  maintain  amount of  'work' done inside the  turbo.

Edited April 25, 201411 yr by rancherman

Moparman,  This  is  the  exact opposite  reason  why your  question on intake mods  earlier    is  answered here.    'Intercooler'.

Imagine  our  systems  without the cooler...  and  trying to stuff  200 degree  compressed air  into our engine.   There simply isn't  enough  pipe  capacity 'as designed'    to    get the  job done.   (not enough  moles  of   O2  in   that  hot  charge).   But,   cool it  down to  more  manageable  temps..  ~100-125*   (half  the  temp, doubles  the  density)   then  we see   some  results!

Intercooler  mods  would  be needed  if  it  isn't large enough to cool sufficiently.  More surface area, more cores, etc). An  I'd  think   until we start  winging our  engines  to  3200 + and up  rpm,  the  capacity of our   intake system  works pretty well.  Then  of  course,  larger pipe  going in,  and  going out.

 

Yeah I know very well... (Results are roughly speaking)

http://www.stealth316.com/2-turbotemp.htm

 

 

 

nice calculator Mike.    thanks  for posting!

For a laymans terms, it reminds me of a high pressure low volume paint gun... lol.

This is the reason that the high HP guys look to use things like water/meth to help the cooling and expansion of the compressed air we put into the cylinder.