How Smart are you? Exhaust Technical Information

[MustangChris]

Well ladies,

my older brother is a mechanical engineer, so he agreed to help me with my car by designing the exhaust for me. My uncle is a welder, so fabrication is not a problem... So, i figured why not take advantage of my resources?

I specified to my brother I wanted 3" collector on the headers (already purchased) and a 3" exhaust tip on both sides with a 3" muffler (for future use, ill never have to change headers or cat-back.)

so basically, we are simply fabricating the middle section of the exhaust to optomize back pressure until i go with a full 3" exhaust (years? down the line... decades maybe?)

He needs to know the "optimum mass flow rate" and "optimum exhaust pressure" at either the exhaust port on the head, or the port on the collector... does anyone know how i can find this out? or how to determine it? or where to research it?

Many thanks,
Chris

 

[Javi]

Get the car together and running and then worry about the exhaust.

 

[MustangChris]

ah. spend money on an exhaust i dont want.. .. . i like the way you think... ^.-?

 

[mygreengt]

U are never gonna get a serious answer when u make a thread. Lol

 

[MustangChris]

nope. didnt think so, but i figured Paul or Sxynerd might get lost and check this thread out and answer the question...

next step is designing the fuel system, which i probably wont even bother posting a thread for.. .... lol.

 

[OnyxCobra]

since u don't know when you'll even need 3" exhaust it would just make sense to buy a 2.5" catback that you like and just use it for a few years. Imo that makes a lot more sense then making a custom exhaust ass backwards just to change it down the road. It's not like exhausts are that expensive, ya know?

 

[MustangChris]

too late, already got the borals sitting in my garage. I was going with 3" all the way through, but then my uncle said we could customize it up, so i figured, why not....

brother does the math. uncle does the fabrication.... all i have to do is find these two pieces of information for my brother, which i knew was going to be harder than it sounds.

 

[94Pro-5.0]

why does he need to know this? exhaust isnt rocket science.

 

[Brahh]

I was intrigued so I searched the google for a bit :thumbsup: and stumbled on this. Hopefully it helps :angel4:

First, it should be intuitively obvious to the most casual observer that the amount of air that passes through the engine in will be equal to the engine displacement times the RPM divided by 2. For an engine of 3 liter displacement going at 3000 RPM, the amount of air pumped for minute will be 4500 liters.

That will approximately be the intake volume flow for an engine with the throttle wide open. If we assume that the throttle is only open 33%, the intake volume flow will still be 4500 liters, but the pressure will be one-third of an atmosphere. The equivalent mass of air will be the same as 1500 liters at one atm of pressure.

Neglecting the addition of the fuel mass, the mass of the exhaust gas will be the same as the mass of the intake gas. From the ideal gas law we know that the increase in volume of the exhaust gas will be proportional to the increase in absolute temperature. If we assume an intake temperature of 80 deg F, and an exhaust temperature of 1800 deg F (reasonable assumption, depends upon compression ratio), the absolute temperature will be 540 and 2260 deg Rankine, respectively. The volume increase will therefore be 2260/540, or 4.185.

For the hypothetical 3 liter engine running at 3000 RPM and full throttle, the exhaust gas volume will be about 4500*4.185, or 18,833 liters/min. At one third throttle the corresponding flow is 6277 liters/min. Since one cubic foot is equal to 28.3 liters, the respective CFM flows will be 665.4 and 221.8, respectively.

How about the contribution from combustion products? Assuming stoichometric combustion, there will be one pound of fuel burned for each 14.55 lbs of air. Air is 21% oxygen, so there is 3.05 lbs of oxygen available to burn each pound of gas.

A reasonable chemical approximation for gasoline is octane, which has a chemical of C8H18. The molecular weight is (12*8+18*1)= 114.

The combustion formula is C8H18 + 12.5 O2 ==> 8 CO2 + 9 H20. For each 114 grams of C8H18, there will be 12.5 moles of oxygen consumed, producing 8 moles of CO2 and 9 moles of H2O. For gas volume purposes, since equal moles of gas produce equal volume, the volume of exhaust gas replacing oxygen will be equal to 17/12.5 = 1.36.

The volume percentage of oxygen in air is about 21% (not exact, but work with me here). This volume will be removed, and replaced by exhaust gas with a "volume" of (21*1.36) = 28.56%. The resulting post combustion volume is (79% + 28.56% = 107.56%) of the pre- combustion volume -- assuming no temperature increase.

So what do we have? Combining the increase in volume from combustion reactions and thermal expansion, an engine with a 3 liter displacement running at 3000 rpm with the throttle wide open will have an exhaust volume (at 1800 deg F) of 665.4*1.0756 ~~ 715 cubic feet per minute. For the throttle one-third open, the exhaust flow will be 238.6 cfm.

 

[MustangChris]

why does he need to know this? exhaust isnt rocket science.

Well im glad you're an expert... how much 2.5" pipe should I used on my 2004 cobra motor after my 1 3/4"x 1 7/8" long tube headers? keep in mind the collector is 3", so we are downsizing. also, how long or how many cats should i use? how far back from the cross over should i return to 3" plumbing?

my mods are as follows:
CAI, underdrive pulley on blower, overdrive pulley on crank, custom port job on blower, long tube headers. Please keep in mind i want it to be emissions legal.



.... thats why he needs to know this stuff....

I was intrigued so I searched the google for a bit :thumbsup: and stumbled on this. Hopefully it helps :angel4:

First, it should be intuitively obvious to the most casual observer that the amount of air that passes through the engine in will be equal to the engine displacement times the RPM divided by 2. For an engine of 3 liter displacement going at 3000 RPM, the amount of air pumped for minute will be 4500 liters.

That will approximately be the intake volume flow for an engine with the throttle wide open. If we assume that the throttle is only open 33%, the intake volume flow will still be 4500 liters, but the pressure will be one-third of an atmosphere. The equivalent mass of air will be the same as 1500 liters at one atm of pressure.

Neglecting the addition of the fuel mass, the mass of the exhaust gas will be the same as the mass of the intake gas. From the ideal gas law we know that the increase in volume of the exhaust gas will be proportional to the increase in absolute temperature. If we assume an intake temperature of 80 deg F, and an exhaust temperature of 1800 deg F (reasonable assumption, depends upon compression ratio), the absolute temperature will be 540 and 2260 deg Rankine, respectively. The volume increase will therefore be 2260/540, or 4.185.

For the hypothetical 3 liter engine running at 3000 RPM and full throttle, the exhaust gas volume will be about 4500*4.185, or 18,833 liters/min. At one third throttle the corresponding flow is 6277 liters/min. Since one cubic foot is equal to 28.3 liters, the respective CFM flows will be 665.4 and 221.8, respectively.

How about the contribution from combustion products? Assuming stoichometric combustion, there will be one pound of fuel burned for each 14.55 lbs of air. Air is 21% oxygen, so there is 3.05 lbs of oxygen available to burn each pound of gas.

A reasonable chemical approximation for gasoline is octane, which has a chemical of C8H18. The molecular weight is (12*8+18*1)= 114.

The combustion formula is C8H18 + 12.5 O2 ==> 8 CO2 + 9 H20. For each 114 grams of C8H18, there will be 12.5 moles of oxygen consumed, producing 8 moles of CO2 and 9 moles of H2O. For gas volume purposes, since equal moles of gas produce equal volume, the volume of exhaust gas replacing oxygen will be equal to 17/12.5 = 1.36.

The volume percentage of oxygen in air is about 21% (not exact, but work with me here). This volume will be removed, and replaced by exhaust gas with a "volume" of (21*1.36) = 28.56%. The resulting post combustion volume is (79% + 28.56% = 107.56%) of the pre- combustion volume -- assuming no temperature increase.

So what do we have? Combining the increase in volume from combustion reactions and thermal expansion, an engine with a 3 liter displacement running at 3000 rpm with the throttle wide open will have an exhaust volume (at 1800 deg F) of 665.4*1.0756 ~~ 715 cubic feet per minute. For the throttle one-third open, the exhaust flow will be 238.6 cfm.

That's great! thank you. but i wonder how much this gets changed once there is a blower placed on the engine... so the engine is no longer sucking air, gets getting force fed air. i will show this to my brother and see if he has any ideas. Thank you for the help!

 

[94-427]

I have run both 2.5 and 3 the three did make more power but with more noize

 

[MustangChris]

noize is good.

what kind of mods did you have at the time?
im only looking at 550(high expectation) HP, so 3" is probably going to be a bit much.

 

[Twista]

Umm you want 3.5 or 4 inch tips.

3 inches are puny and stock looking IMO.

 

[MustangChris]

4"?????? lol. i could jsut steal some off a toyota tundra

 

[All_Ford_Fun]

Wow , all that for an exhaust system. Im all about more ponies under the hood but im pretty sure NASA would blush if they read that formula. But on a searous note , I I've seen someone eles try to fan there own exhaust like this on an 04 termi and basically came down to to hard to get a true formula due to the fact there is almost no data stating how dense the air really is from the blower and to many engine variables to consider. If you do come up with a good formula let me know plz , the guy is really a know it all. I would live the pleaser to beable to rub it in his face lol