3" Downpipe + 4" Exhaust in A4 Jetta

diesel04

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Great mods FuB! If your gonna do anything in this life make sure you go big!

I have a 2004 Jetta TDI. Reading through all this and trying to figure out the performance mods for the TDI and being the muflerectemy being the cheapest to start out with. I would like to know if i just cut the mufler off and put straight pipe off the stock pipe would i gain any performance?

I drive 150 miles a day in the car and i am getting 525 - 550 out of the 13 gallon tank, i would think i could get 650mpg but not sure whats holding it back... maybe its my foot! lol
 

Fix_Until_Broke

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diesel04 said:
Great mods FuB! If your gonna do anything in this life make sure you go big!

I have a 2004 Jetta TDI. Reading through all this and trying to figure out the performance mods for the TDI and being the muflerectemy being the cheapest to start out with. I would like to know if i just cut the mufler off and put straight pipe off the stock pipe would i gain any performance?

I drive 150 miles a day in the car and i am getting 525 - 550 out of the 13 gallon tank, i would think i could get 650mpg but not sure whats holding it back... maybe its my foot! lol
diesel04 - yeah, no point in putting a 3" in when I thought 4" would fit :)

Cutting the muffler off on your car (if otherwise stock) won't likely make a measurable difference, but it wont hurt either. Head over to the Fuel Economy forum for info regarding your fuel economy - lots of reading over there
 

DSLFAN

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4" on a 1.9 is a waste of time, energy, money, and extra weight. I have to agree with the ricer comment, sorry! I drive and work on diesel trucks. There are more trucks with 5.9L, 6.0L, 6.6L engines and over 400HP with 4" exhaust than you can shake a stick at. And they are running over 50 psi of boost on the Dodges. A general rule of thumb is 4" is good to 450HP, but many run 4" to 550HP easily. My Dodge dynoed at 400HP on #2 only, with only the single charger. I have since gone to compound turbos and gained more power, and run 55psi boost pressures. I have a lot that I could do to make more power before worrying about bigger exhaust, i.e. head porting, cam, water/meth injection, etc. Also, DO NOT RUN PROPANE! It is fuel, and as such could start burning before fuel is injected. Again, diesel truck guys have ran it, and bent rods and burned motors have resulted many a time! Finally, if your pyro was reading 1200 post turbo, that is way to hot. 1250 PRE-TURBO is considered the safe limit for sustained runs. 1200 post-turbo could have been 1600pre very easily.
 

Fix_Until_Broke

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DSLFAN - Thanks for your feedback, I respect your opinion and viewpoint. Guess we won't see a 4" exhaust on your beetle anytime soon :)

Cost was incremetally more (I'd have to dig up the numbers, but I did the cost difference between 3" and 4" and it was less than 20% more)
Energy/Time - running 3" vs 4" all the way back is about the same. I wouldn't be less careful or likely be able to do it much faster with 3" vs 4", everything would still be centered and provide the most clearance.
Extra weight - it's 33% heavier than the same wall thickness 3" pipe, The muffler (now two) are definately heavier as well.

I don't know where the comment on propane came from - Thanks??

1650F pre turbo is the maximum recommended sustained temp for these turbos.

I need to get the downpipe flanges modified to accept pressure taps so I can evaluate post turbo exhaust back pressures of various setups along with the IMP/EMP measurements I did this winter. This will end the debate on weather or not there is an advantage to exhaust modifications and which ones provide which benefits, with some actual measurements instead of just speculation.
 

Fix_Until_Broke

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Edited 1st post - removed Aero Turbine muffler.

Also installed stiffer exhaust hanger over the axle to better support the big muffler and keep it from bouncing and hitting the axle when fully loaded.

[/IMG]

And here it is installed

[/IMG]

Still have not modified the spare downpipe flanges to measure exhaust back pressure, still on my list of things to do....
 

Fix_Until_Broke

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Dankcorey22 said:
did u costom make that or buy that downpipe ?
Custom made from a few mandrel bends, a reducer, the existing turbo flange and multiple iterations of fit, mark, tack, refit, remark, retack, etc to get it to fit just right.

There's a reason that no one offers a 3" TDI downpipe for an A4 that you can just buy and bolt in. That extra 1/4 inch in all directions makes a big difference - particularly in the tunnel/steering rack area.
 

nicklockard

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milehighassassin said:
It might suffer on the low end, but with the torque we have that is not a big deal.

I don't understand this. Can you explain it?

I also have a basic question of FUB and the other knowledgeable folks out there:

If diameter of exhaust post-turbo is increased, the velocity goes down, and pressure goes up for a given mass flow rate.

What is more important in exhaust design: maintaining high exit velocities throughout the entire system, or maintaining the same mass flow rate at lower velocities and higher pressures?

So if a larger post-turbo exhaust runs at higher pressures and lower velocities, why are we putting larger exhausts on? What are we trying to accomplish?
 

Fix_Until_Broke

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Nick - it's late but, I'll give it a shot quick. I may edit it tomorrow when I re-read it as it's been a long day...

Yes, the velocity goes down and the pressure goes up for the same mass flow rate at a single point in the system all other things being equal, BUT...that does not come for free.

Those well versed in the thermodynamics of compressible flow will cringe, but, here's how I can best explain it.

There are various forms of energy we can have in a particular system (exhaust in this case): Pressure energy, Inertial (kenetic) energy, Thermal energy being the main ones. There's a trade off between these various forms of energy but after the turbo, there is no energy added or subtracted from the system so a change in velocity will result in a change in pressure, and for the sake of explination, we'll say it's a 1:1 tradeoff. If you want to decrease the pressure at a point, you need to increase the velocity at that point for example. To increase the velocity, you must accelerate the mass of the gas and once accelerated, you must keep it at that velocity since things like bends/mufflers/cats/wall friction/turbulance want to slow it down - similar to the aerodynamic effects of driving your car.

The lowest pressure we can practically hope for in the post turbo exhaust system is atmospheric - less than that and it won't flow out the pipe :). If it's lower than atmospheric at any point, we're accelerating the gasses needlessly at some point in the system.

In order to achieve high velocity like you describe (and therefore low pressure), the gasses need to get accelerated to that velocity in the first place - this takes a differential pressure to accelerate the gasses. You can get the pressure at a point in the system to be a significant vacuum (think carburetor venturi), but you need to have a sufficient enough pressure upstream of the restriction to accelerate the gasses fast enough at the restriction to lower the pressure at that point.

You have a low pressure at one point in the system but it is a trade off (and a loss) due to having to have a higher pressure upstream to get it. For example the turbo itself is like this. The pressure right at the turbine exit is likely very low at high mass flow rates. The VNT passages and exhaust wheel create the restriction (smallest area=highest velocity), the pistons create the upstream pressure in the exhaust manifold. If the turbo vanes/wheel wasn't there (and there was an equivalent mass flow rate) the pressure at the same distance from the engine will likely be higher but the pressure upstream of this will be lower. I really need a graph of absolute pressure vs position in the exhaust system here - I'll see if I can put something together (Note: Figure 3.3.28 on Page 3-56 of the 10th edition of Marks Standard Handbook for Mechanical Engineers is just what I'm looking for). Lurker Mike and I had some discussions about measuring the pressure right at the turbo outlet and trying to get an accurate measurement due to the high velocity and rotating gasses. Measure at the outer perimiter of the pipe, the center radially or axially, pitot etc. Empirical data will be most useful here and end the theoretical debate (that I just contributed to above) - just need to take the time to gather it, I have everything I need but time :(

The pressure in the pipe needs to be higher than the atmospheric pressure outide the pipe for it to flow out of it. Any restrictions are more/less additive (bends, mufflers, cats, pipe itself, etc) If you keep the pipe diameter small throughout the exhaust, yes the velocity is high and the pressure at the very end of the pipe may be less than atmospheric due to the sudden expansion, but the pressure back at the turbo exit will be higher due to having to accelerate the gasses to maintain that velocity throughout the length of the pipe along with all the wall friction from that high velocity.

The sooner you can slow the gasses to the point where their velocity is low enough such that the pressure is ~atmospheric, the better. The longer the high velocity is maintained from the exit of the turbo the more frictional losses are incurred and the more differential pressure it takes to maintain this velocity = higher pressures upstream of the turbo.

More later as the screen keeps going in/out of focus due to lack of sleep. Hope that makes some sense for now at least.
 
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vwmikel

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nicklockard said:
I don't understand this. Can you explain it?

I also have a basic question of FUB and the other knowledgeable folks out there:

If diameter of exhaust post-turbo is increased, the velocity goes down, and pressure goes up for a given mass flow rate.

What is more important in exhaust design: maintaining high exit velocities throughout the entire system, or maintaining the same mass flow rate at lower velocities and higher pressures?

So if a larger post-turbo exhaust runs at higher pressures and lower velocities, why are we putting larger exhausts on? What are we trying to accomplish?
The general idea with the larger exhaust is to increase the pressure differential across the turbine wheel to improve spool and efficiency.

I think his concern relates to the decresed velocity reducing the scavenging effect.


edit-It looks like I took too long to answer
 

nicklockard

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vwmikel said:
The general idea with the larger exhaust is to increase the pressure differential across the turbine wheel to improve spool and efficiency.

I think his concern relates to the decresed velocity reducing the scavenging effect.


edit-It looks like I took too long to answer
Yes, exactly, but unless I remember the thumb rule incorrectly, a larger pipe flows at lower velocity and higher pressure, so putting a larger diameter pipe on the down wind side would seem to be the exact wrong thing to do, as it does not maximize dP but does the opposite. Somehow I'm still not getting it, but probably because I'm tired too. I'll read what FUB has to say when he's fresh tomorrow :)
 

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nicklockard said:
I don't understand this. Can you explain it?

I also have a basic question of FUB and the other knowledgeable folks out there:

If diameter of exhaust post-turbo is increased, the velocity goes down, and pressure goes up for a given mass flow rate.

What is more important in exhaust design: maintaining high exit velocities throughout the entire system, or maintaining the same mass flow rate at lower velocities and higher pressures?

So if a larger post-turbo exhaust runs at higher pressures and lower velocities, why are we putting larger exhausts on? What are we trying to accomplish?
I am saying that you might actually lose a little torque on the low end of the RPM range if you have too large of an exhaust, but you will make up for it later on. You don't need any more back pressure like a normally aspirated motor does, your turbo sitting on the exhaust manifold provides MORE than enough.


Yes with a larger exhaust your pressure decreases, but look at it this way. A large turbo provides more air to the motor on less PSI. Why? Because it is flowing more air. The same is true with the exhaust. It is like trying to empty a swimming pool with a straw at 25 PSI, or trying to empty the same pool with a fire hose at 5 PSI, the fire hose is moving the water slower, but it has more volume.
 

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milehighassassin said:
I am saying that you might actually lose a little torque on the low end of the RPM range if you have too large of an exhaust, but you will make up for it later on.
Why would you lose low end torque? I don't understand this.

MHA said:
Yes with a larger exhaust your pressure decreases, but look at it this way. A large turbo provides more air to the motor on less PSI. Why? Because it is flowing more air. The same is true with the exhaust. It is like trying to empty a swimming pool with a straw at 25 PSI, or trying to empty the same pool with a fire hose at 5 PSI, the fire hose is moving the water slower, but it has more volume.

If you have a pipe like this:



Then the pressure in section 1 (P1) will be less than that in section 2.

Yet, in order to satisfy the law of conservation of mass, velocity in section 1 (V1) must be higher than in section 2. These two factual statements are in the picture above.

I understand perfectly about the larger turbo flowing more mass of air at a lower pressure. That's not at issue remotely. The question concerns post-turbo exhaust diameter. The question has no relation to the turbo itself whatsoever, just the pipe after it.

I'm still confused.
 

milehighassassin

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I don't see how the pressure in P1 would be less than P2. It should be the opposite.

You lose low end torque because your entire power band is shifted slightly higher because of the lack of backpressue.

You see many newer cars/exhausts that have a spring or electronic operated exhaust flap that stays shut at low RPM's and opens at higher RPM's to compensate for this. The R32 is an example of this.

I'll try to find something that explains this better.
http://www.amazon.com/Maximum-Boost-Turbocharger-Engineering-Performance/dp/0837601606/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1216048115&sr=8-1
 

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milehighassassin said:
I don't see how the pressure in P1 would be less than P2. It should be the opposite.
Agreed, Higher velocity through a smaller pipe (which acts like a smaller orfice) will cause MORE pressure drop.

Nick, think of the exhaust system in reverse... Pressure drops in the exhaust subtract from the EMP and what we're left with is the dP across the turbo. Yes we have some temperature deviations too, but lets ignore those for now.

dP across the turbo = EMP - piping losses

We know that at the end of the exhaust pipe we have atmospheric pressure, or "0" boost psi. So the LEAST amount of pressure drops in our piping we can get, MAXIMIZES the pressure drop across the turbo.
 
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milehighassassin

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It seems to be easier to explain liquid flow over air flow, although they are similar (I am by no means an engineer or know a lot about flow dynamics, but I understand the basics...).

Take a 3/4 water hose flowing say 15 PSI. Put a reducer on it that takes the opening down to 1/4"

What will the water pressure on the other side of that reducer do? Rise or drop?

Rise of course, because you are trying to move the same amount of water through a smaller opening. Much like when you stick your thumb on the end of the hose to squirt water further, you create more pressure.


The link I posted about is a book call Maximum Boost. It was wrote by Corky Bell who is pretty much the "Guy" when it comes to forced induction. It is a GREAt and VERY techinical read but even newbs can read it and understand most things.

The bottom line:

On a turbo charged motor "no exhaust is the best exhaust".



The reason you get the lower end torque drop is because the backpressure is inadequate. Yes you can size an exhaust properly to fix that, but then ultimately you would lose power in the midrange and a lot of power on the top. With a turbo charger you typically already have GREAT low end torque, so it really won't be missed.
 

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FUB, is that a maroon/silver SD in the back ground, If so, I have one just like it..Most excellent vehicle..

I am thinking about doing my PD100 in 4", yours sounds awesome!!

Thanks for the Tech stuff as well..
 

nicklockard

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Yes, I own the book called Maximum Boost. In fact I was reading it just after MHA posted what you did, but believe it or not, it does not say anything about exhaust pipe diameter w/r/t this pressure or differential pressure. It simply says that to make 200 hp, all one needs is 2.25" exhaust diameter (there's a graph.)

The question remains and hasn't been addressed properly yet (no offense, I just want to fully understand it.)

P1 in section 1 is LOWER than P2. Pick up any engineering 101 handbook and you will see I am correct.
 

nicklockard

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milehighassassin said:
The reason you get the lower end torque drop is because the backpressure is inadequate. Yes you can size an exhaust properly to fix that, but then ultimately you would lose power in the midrange and a lot of power on the top. With a turbo charger you typically already have GREAT low end torque, so it really won't be missed.
I believe this is wrong.

First, we don't have any valve overlap in a diesel, thus no scavenging effects where some back pressures can be helpful, especially to a naturally aspirated gasser. Second, we have a turbo, which provides all the backpressure (and then some) that the engine could ever hope to see. The turbine cross sectional area where the exhaust gases first enter the blade area from the snail is the ultimate restriction point. A turbo engine, to my knowledge, does NOT benefit from further additional back pressure aft of the turbine, no matter the degree of valve overlap or scavenging effect.
 

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Nick, is that for real, 2.25" wil take you to 200HP..

Thanks..
 

nicklockard

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Scott02 said:
Agreed, Higher velocity through a smaller pipe (which acts like a smaller orfice) will cause MORE pressure drop.

Nick, think of the exhaust system in reverse... Pressure drops in the exhaust subtract from the EMP and what we're left with is the dP across the turbo. Yes we have some temperature deviations too, but lets ignore those for now.

dP across the turbo = EMP - piping losses

We know that at the end of the exhaust pipe we have atmospheric pressure, or "0" boost psi. So the LEAST amount of pressure drops in our piping we can get, MAXIMIZES the pressure drop across the turbo.
Yes, but do we want to maintain constant, high velocity flow at the lowest pressure (like OEM,) or is it more important to maintain the highest mass flow rate at a higher pressure (like aftermarket?)

Flow and pressure are NOT the same thing.

Law of conservation of mass states that the product of pressure*velocity in any section of a pipe must equal that of every other section.

Unit analysis:

force = mass * acceleration = Kg*m*s^(-2)

pressure = force/area = Kg*m*S^(-2)* m^(-2) = Kg*m^(-1)*S^(-2)

Pressure * velocity= Kg*m^(-1)*S^(-2)*m*s^(-1) = Kg*s^(-3)

Thus, we can see that the product of pressure * velocity of the gases is (1/S^2)* mass flow rate.

Of course a larger diameter pipe will flow more mass of air per unit of time than a smaller pipe. This is not contested. The question relates to the nature/type of flow that is best; or is simply the highest mass flow rate best under all conditions? I just want to understand. Thanks all.

Please correct me if I've gotten anything wrong.
 
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nicklockard

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Mach1 said:
Nick, is that for real, 2.25" wil take you to 200HP..

Thanks..
According to (Bentley Publishers) page 133, figure 11-4 of Maximum Boost by Corky Bell it is.

Actually, looking even closer at the graph, it only takes ~2.175" for a single pipe to get to 200 hp.
 

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nicklockard said:
Yes, but do we want to maintain constant, high velocity flow at the lowest pressure (like OEM,) or is it more important to maintain the highest mass flow rate at a higher pressure (like aftermarket?)
I think i'm getting jumbled in your question / description...
When you say "highest pressure" where do you mean ?
Lets say exhaust pressure can be measured in 3 places, as nothing else really matters...
1. EMP 2. immediately post turbo 3. End of exhaust pipe = Atmos

EMP - Post Turbo = dP across turbo
Atmos - Post turbo = Pressure losses in exhaust piping


Ok, lets pose some open questions for discussion...

1. Why would we want high velocity in our exhaust system?
To get heat away from the turbo? To keep soot off of back of car?
Any other reasons ?

2. Why do we want high pressure in our exhaust system? (post turbo measurement)
I really don't have a reason... Anyone else ?

3. If we have 35psi of EMP, and exhaust flow Z, and we take OFF the turbo and exhaust, we still have flow Z (neglecting the fact we don't have boost anymore, hehe) How much EMP do we have ?

Now i love number crunching as much as anyone else, but here i think there is something that you aren't realizing here... (unless i'm not understanding what you're after)

We are talking pressures and flows of "exhaust system A" and how they compare to "exhaust system B"
They are two different systems, soooo, your little pipe going into the larger pipe, really doesn't apply to what i think is your initial question.
Perhaps i'm wrong though... getting late, bedtime.
 
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milehighassassin

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nicklockard said:
According to (Bentley Publishers) page 133, figure 11-4 of Maximum Boost by Corky Bell it is.

Actually, looking even closer at the graph, it only takes ~2.175" for a single pipe to get to 200 hp.
I don't have my book with me, but what is the size of that motor?


Also, the way I read that is, 2.175" pipe limits out at 200 HP, that does not mean that if you went with say a 3" pipe you would not gain MORE power.
 

nicklockard

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Fix_Until_Broke said:
Nick - it's late but, I'll give it a shot quick. I may edit it tomorrow when I re-read it as it's been a long day...

Yes, the velocity goes down and the pressure goes up for the same mass flow rate at a single point in the system all other things being equal, BUT...that does not come for free.

Those well versed in the thermodynamics of compressible flow will cringe, but, here's how I can best explain it.

There are various forms of energy we can have in a particular system (exhaust in this case): Pressure energy, Inertial (kenetic) energy, Thermal energy being the main ones. There's a trade off between these various forms of energy, and for the sake of explination, we'll say it's a 1:1 tradeoff. If you want to decrease the pressure at a point, you need to increase the velocity at that point for example.

The lowest pressure we can practically hope for in the post turbo exhaust system is atmospheric - less than that and it won't flow out the pipe :). If it's lower than atmospheric at any point, we're accelerating the gasses needlessly at some point in the system.

In order to achieve high velocity like you describe (and therefore low pressure), the gasses need to get accelerated to that velocity in the first place - this takes a differential pressure to accelerate the gasses. You can get the pressure at a point in the system to be a significant vacuum (think carburetor venturi), but you need to have a sufficient enough pressure upstream of the restriction to accelerate the gasses fast enough at the restriction to lower the pressure at that point.

You have a low pressure at one point in the system but it is a trade off (and a loss) due to having to have a higher pressure upstream to get it.

The pressure in the pipe needs to be higher than the atmospheric pressure outide the pipe for it to flow out of it. Any restrictions are more/less additive (bends, mufflers, cats, pipe itself, etc) If you keep the pipe diameter small throughout the exhaust, yes the velocity is high and the pressure at the very end of the pipe may be less than atmospheric, but the pressure back at the turbo exit will be higher due to having to accelerate the gasses to maintain that velocity throughout the length of the pipe along with all the wall friction from that high velocity

The sooner you can slow the gasses to the point where their velocity is low enough such that the pressure is ~atmospheric, the better.

More later as the screen keeps going in/out of focus due to lack of sleep. Hope that makes some sense for now at least.
It does, but I don't understand the sentence of yours above that I bolded for emphasis. Why is it beneficial?
 

nicklockard

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milehighassassin said:
I don't have my book with me, but what is the size of that motor?


Also, the way I read that is, 2.175" pipe limits out at 200 HP, that does not mean that if you went with say a 3" pipe you would not gain MORE power.
I don't think engine displacement has anything to do with it. I think the statement is a simple one regarding power-versus-ex. pipe dia, for any IC engine....however he does have a strong gasser slant to everything, so maybe diesels are a little different.

You may be right.
 

SBAtdijetta

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nicklockard said:
According to (Bentley Publishers) page 133, figure 11-4 of Maximum Boost by Corky Bell it is.

Actually, looking even closer at the graph, it only takes ~2.175" for a single pipe to get to 200 hp.
My question is what would your EGTs be with that small of a pipe vs. say a 2.5" or 3" open pipe? Also what would the EMP differences be?
 

nicklockard

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SBAtdijetta said:
My question is what would your EGTs be with that small of a pipe vs. say a 2.5" or 3" open pipe? Also what would the EMP differences be?

My EGT's with the stock pipe and what's in my signature peak out at 862 C.
 

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My WOT EGT went down a LOT with the 3" exhaust.. Cruising EGT was a little bit lower. I can still peak at 1600F, but it takes much longer to get it up there.. Almost a full mile to get to 1600 from a standing start where before with the stock exhaust I could hit 1600 before 5th gear..
 
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