3" Downpipe + 4" Exhaust in A4 Jetta

[Fix_Until_Broke]

It does, but I don't understand the sentence of yours above that I bolded for emphasis. Why is it beneficial?

I edited post 99 with additional info/examples. I've a couple charts I'll try to scan in tomorrow and post which will help clear things up I hope.

 

[Fix_Until_Broke]

Why would you lose low end torque? I don't understand this.




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.

Nick - are you flowing from left to right (small to large area) or right to left (large to small area) in your diagram above?

P1 (average pressure across the crossection) will be less than P2 if the mass flow is from right to left. It has to be otherwise the flow is going the other way.
Yes V1 will be higher than V2 for the same mass flow rate regardless of flow direction

 

[Fix_Until_Broke]

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.

I'll need to write these out as I'm having trouble sorting through all the exponents . What is the upper case S notation? Just a typo?

 

[nicklockard]

I'll need to write these out as I'm having trouble sorting through all the exponents . What is the upper case S notation? Just a typo?

yeah, just seconds (typo.)

Also, thanks for this clarification Matt:

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.

That does make some sense.

Okay, to explain the context of my questions: I'm not trying to dog you or anyone or say that a larger pipe isn't the best. At this point I really don't know and just want to have a better grasp. What I can say is that I've just installed a 2.5" DP, 2.5" Magnaflow cat, and all 2.5" pipe to the tailpipe last Saturday. Without a doubt, it has kicked my Tdi in the balls. It lost all of its low end power and some on top. There is massive turbo lag now when there was ZERO before. I have not retuned for it. I have removed everything aft of the aftermarket downpipe without positive effect. So, I'm trying to figure out if this is the inherent consequence of "going big" or if there's something wrong with my DP or its installation. {See posts 24, 30, and 33 here: http://forums.tdiclub.com/showthread.php?t=218662 }

Did your power band shift when you installed the big piping? Can others chime in on this too? What were your observations when installing a larger exhaust system on a stock VNT15 (especially those with a similar setup to mine?)

 

[Scott02]

I put a 2.5" straight pipe on my stock mk3, 1ft after the turbo, back to the bumper, with no adverse effects. It acted and felt just as i thought it should, it felt "opened up to breathe" a little bit more than with the stock piping. Then i got rc3 for it, and it felt pretty damn strong, comparing it to my mk4 rc3 and pp520's, and stock exhaust.

Also recently took the back half of my mk4's exhaust off, (with a turndown for now, wouldn't recommend this drone's like a mofo) and the "clutch-slipability" has gone way up. (stock clutch still). Slippage used to be temperature related, now i can slip it whenever at will, irregardless of gear choice. Soooo, something happened, and it didn't loose torque.

 

[Fix_Until_Broke]

Nick - No worries about questioning what I've done. If I can't explain and justify what I've done, shame on me.

It sounds like you've got something amiss with your installation - gasket not lined up properly or something. Before you switch downpipes back to OEM, can you get a pressure measurement before the turbo and then again after the downpipe is swapped? Just for a sanity check?. By the sounds of your results there should be an obvious difference in EMP's.

 

[SBAtdijetta]

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..

Great! Thanks! This is what I wanted to hear. Having the about the same issue and waiting on a 3" to help my EGT situation .

 

[TDIJetta99]

I never really did a test with the stock pipes and a muffler/cat delete.. This was a comparison between a completely factory system and 3"... My downpipe has the stock elbow, an adapter up to a 2.5" flex, then 3" from there to the back bumper.. I made the adapters and everything... I think I'll try to make some sort of a 3" downpipe similar to what FUB made... It probably won't make too much of a difference compared to my current setup, but I figure if I CAN build it, why not?

 

[milehighassassin]

Okay, to explain the context of my questions: I'm not trying to dog you or anyone or say that a larger pipe isn't the best. At this point I really don't know and just want to have a better grasp. What I can say is that I've just installed a 2.5" DP, 2.5" Magnaflow cat, and all 2.5" pipe to the tailpipe last Saturday. Without a doubt, it has kicked my Tdi in the balls. It lost all of its low end power and some on top. There is massive turbo lag now when there was ZERO before. I have not retuned for it. I have removed everything aft of the aftermarket downpipe without positive effect. So, I'm trying to figure out if this is the inherent consequence of "going big" or if there's something wrong with my DP or its installation. {See posts 24, 30, and 33 here: http://forums.tdiclub.com/showthread.php?t=218662 }

Did your power band shift when you installed the big piping? Can others chime in on this too? What were your observations when installing a larger exhaust system on a stock VNT15 (especially those with a similar setup to mine?)

Could it be the Magnaflow cat that is causing the problem?

VAG-COM??? Any codes from your 02?

Edit...
From your other linked thread:

One thing I noticed is that the gasket that came with the downpipe did not match the hole. I wonder if that could be the source alone.

I think this could be your problem.

 

[Mach1]

SB, you see where tdi99 has the stock collar and 2.5 then to 3"(sound familiar, this is how I did mine)..

Steve, I bet your EGTS are through the roof, trying to flow 200hp through the stock exhaust, you are hitting 1800 degrees!!!

 

[SBAtdijetta]

SB, you see where tdi99 has the stock collar and 2.5 then to 3"(sound familiar, this is how I did mine)..

Steve, I bet your EGTS are through the roof, trying to flow 200hp through the stock exhaust, you are hitting 1800 degrees!!!

My EGT's are high right now... but mostly due to the remap and more fuel from stock injectors, and the timing advance may be causing them to still be injecting after TDC? I may back the timing back to 0 rather than +2 tonight and see if it makes a difference. 1550-1600f at 110 mph on a WOT run.

EDIT: Just remembered my Vag-com (mini-can) is not with me..., awaiting a new one from Ross-tech (Hex-usb+can). Doh! Anyone near Houston? Lol I have already used the vag list, no luck so far.

 

[Snot Rocket tdi]

where did u find a 3" dp..... i have been looking for one for a while or did u custom make it

 

[Fix_Until_Broke]

I mak 3 dp custm myslf

 

[SBAtdijetta]

I mak 3 dp custm myslf

Its 5 O clock somewhere... lol J/K with you. But that had me ROTFL Thanks, because I had a crappy day and needed that!

Sorry done being way off topic.

 

[Fix_Until_Broke]

Well - there was a failure where the downpipe connects to turbo flange

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A bad motor mount was allowing the engine to move too much combined with not enough flex in the exhaust. The result fatigued the reducer I made and cracked it right at the flange. It lasted (need to look this up ~20 months and 50K miles guessing?).

The motor mount was replaced and the downpipe design was changed. I decided to eliminate the reducer and go with 3" right off of the turbo flange. The 6" long flex section was replaced with an 8" long flex section as well to provide more vibration isolation in all directions.

Much more detail to follow - tomorrow...

 

[dJValkenburg]

I really don't understand the purpose of going with an exhaust bigger than 2 1/2" esspecially when all your using is a stock turbo. Your outlet hole isn't over 2" anyways so whats the point of going 3"? I mean it's your exhaust go nuts, I'm just curious as to why you would need a 3" or even a 4" exhaust for running a stock turbo with only a moderate tune and some nozzels?

 

[danielhf]

I really don't understand the purpose of going with an exhaust bigger than 2 1/2" esspecially when all your using is a stock turbo. Your outlet hole isn't over 2" anyways so whats the point of going 3"? I mean it's your exhaust go nuts, I'm just curious as to why you would need a 3" or even a 4" exhaust for running a stock turbo with only a moderate tune and some nozzels?

Did you read his signature? He's running a 17/22, RC6 and PP502s (the only bigger nozzles are R520 5 and 7 holes, and DSS Ultimate). So if RC6 is a "moderate" tune, what is a "crazy" tune. The larger the exhaust, the more reduction in back pressure, right? How could a large exhaust be a bad thing?

 

[dJValkenburg]

lol....I only looked at the first page or so of this and noticed something about a stock turbo and stage 2 chip...lol...well then..My bad!..It's not so much that the larger exhaust is a bad thing just more expensive. Performance wise, I don't see how a 4" exhaust is going to be any better than my 2 1/2" exhaust considering the hole coming off the turbo is still only 2" or so. It's not bad to have a bigger exhaust, and I guess if money isn't an issue or the material is free why not go bigger. I'm just curious as to why bother going that big anyways?

Did you read his signature? He's running a 17/22, RC6 and PP502s (the only bigger nozzles are R520 5 and 7 holes, and DSS Ultimate). So if RC6 is a "moderate" tune, what is a "crazy" tune. The larger the exhaust, the more reduction in back pressure, right? How could a large exhaust be a bad thing?

 

[UFO]

Did you read his signature? He's running a 17/22, RC6 and PP502s (the only bigger nozzles are R520 5 and 7 holes, and DSS Ultimate). So if RC6 is a "moderate" tune, what is a "crazy" tune. The larger the exhaust, the more reduction in back pressure, right? How could a large exhaust be a bad thing?

A large exhaust is not a bad thing, but for 200hp you will not see any improvement from 2-1/2" or 3". I run a 2-1/2" down pipe and 3" exhaust on my 350hp gasser, it was a small improvement over all 2-1/2".

 

[dJValkenburg]

A large exhaust is not a bad thing, but for 200hp you will not see any improvement from 2-1/2" or 3". I run a 2-1/2" down pipe and 3" exhaust on my 350hp gasser, it was a small improvement over all 2-1/2".

Ahhh icic. I could understand an improvement on a gasser having 350 hp. But we all know that he doesn't have more than 200hp, so is it really going to benefit him at all? That's all I'm trying to get at. Not to be an ass, just to find out if there really is any benefits to going this big?

 

[UFO]

Ahhh icic. I could understand an improvement on a gasser having 350 hp. But we all know that he doesn't have more than 200hp, so is it really going to benefit him at all? That's all I'm trying to get at. Not to be an ass, just to find out if there really is any benefits to going this big?

Yeah, I doubt a 200hp diesel will see more than 0.5% hp (1hp) improvement (it might see that) -- I was trying to make the point that 3" supports 300+ hp without significant restriction, even on a diesel with greater exhaust flow per hp.

I found this rule of thumb on a gasser site. "Vizard's rules of thumb: A pipe will flow 115cfm per square inch of cross-sectional area. You need 2.2cfm of flow capacity per horsepower." A 3" pipe will support 350 hp for a gasser.

 

[milehighassassin]

Yeah, I doubt a 200hp diesel will see more than 0.5% hp (1hp) improvement (it might see that) -- I was trying to make the point that 3" supports 300+ hp without significant restriction, even on a diesel with greater exhaust flow per hp.

I found this rule of thumb on a gasser site. "Vizard's rules of thumb: A pipe will flow 115cfm per square inch of cross-sectional area. You need 2.2cfm of flow capacity per horsepower." A 3" pipe will support 350 hp for a gasser.

Good info but what works for a gasser does not always work for a diesel and vice versa.

 

[UFO]

Good info but what works for a gasser does not always work for a diesel and vice versa.

Only with respect to a greater exhaust flow on a diesel, but at full acceleration with visible smoke your air/fuel ratio is similar to a gasser under the same conditions.

 

[dJValkenburg]

Good info but what works for a gasser does not always work for a diesel and vice versa.

Yeah those are my thoughts as well.

 

[Fix_Until_Broke]

Someday I'll take the data to prove/disprove the benefits of an exhaust larger than 2.5", but for now it's what I've got.

Anyway....

I figured that I'd take a multiple solution approach to the problem. First I added a longer flex section, 8" of true flexible length.

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Second, I fixed the sloppy motor mount which really helped a lot of things including a shutter that I've been chasing for a year.

Third, if all else fails - go bigger

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It's not the prettiest thing and if I was starting from scratch I'd do things cleaner, but it's functional and a good enough prototype. I'd probably put some sort of better transition from the turbo outlet to the flange as well.

The two main advantages of this that I see are more material which will have more strength to resist future failures and it provides an extra degree of freedom of movement with the band clamp. The clamp is not fully tight which allows the engine to rock forward/backward and the exhaust will pivot there which reduces the ammount of compliance that the exhaust system has to deal with as the motor moves about.

Still a few minor things to work through - need a better gasket between the flange halves (just used a piece of copper wire that I annealed and formed to shape). It really doesn't leak due to the closed nature of the band clamp and low pressure in the area to start with. After driving it 50 miles with the downpipe completely off of the turbo when it broke off (I could see the exhaust wheel) how quiet it was and how little soot and zero heat damage there was, I'm not concerned with a small little gap, but want something that's easier to assemble.

I'll also probably put a spring on the band clamp to make it's swivel easier to adjust and make it less sensitive to wear.

I've got a few hundred miles on it and so far so good. Things keep changing a little as the mount wears in, the bellows loosens up, etc.

So far I really like it - it's definately a lot easier to manufacture and fit to a car with that extra degree of freedom with the band clamp. Also super easy to install/remove. This might be a solution for those who want a larger downpipe without all of the custom fit issues that go along with a fixed one like I originally built.

We'll see how it lasts

 

[TDIJetta99]

that's purdy... I'm going to be trying to build a 3" downpipe as soon as my turbo shows up.. should be here wednesday or thursday.. is there an easily available source for the band clamp pieces? If I can get ahold of something like that for Thursday I'll try it like you have it there.. I really like that idea..

edit... looks like I can get the clamps easily, but the pipe ends I'm having trouble finding..

 

[Fix_Until_Broke]

An internet search should return many sources for weld on flanges, band clamps, mandrel bends, bellows, etc. I think the one I used was intended for GM 6.5 downpipes, but it's a standard 3" size, nothing special, off the shelf stuff. I got mine from the shop where I do all my work which has all of this stuff on hand.

 

[TDIJetta99]

yeah I've been trying to find a part number or something for the guys at Napa.. I miss working at a custom exhaust shop.. they had all that stuff pretty much on hand...

EDIT- these guys have all sorts of cool stuff... reasonable pricing too... http://www.siliconeintakes.com/

 

[milehighassassin]

Only with respect to a greater exhaust flow on a diesel, but at full acceleration with visible smoke your air/fuel ratio is similar to a gasser under the same conditions.

similar, and after that there is little in common.

No ignition, no throttle body, etc.


On any turbo charged motor, no exhaust is the best exhaust. You don't need anymore back pressure than the turbo already provides.

Read Corky Bell's book Maximum Boost.

 

[UFO]

similar, and after that there is little in common.

No ignition, no throttle body, etc.


On any turbo charged motor, no exhaust is the best exhaust. You don't need anymore back pressure than the turbo already provides.

Read Corky Bell's book Maximum Boost.

I've read it from cover to cover multiple times. Flow is flow. My point that similar power levels flow similar exhausts (within 20%) still stands. I understand turbos prefer less restriction, but for 200hp, you will not be able to document the difference between 2-1/2" or 3" (or 4" or 5"). There are diminishing returns, for instance if you gained 10% from 2" to 2-1/2", you will only gain 1% more to 3", and 0.1% more to 4". Run the numbers on cfm and restriction imposed.