3" Downpipe + 4" Exhaust in A4 Jetta

dJValkenburg

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UFO said:
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.
Guess that answers my question as to whether there are any benefits. 1.1% increase from my 2 1/2....rofl...:rolleyes:
 

milehighassassin

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UFO said:
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.
Not sure I agree with you 100%.

I do hear what you are saying that there is a point that you won't see the gains.

Having had a 1.8t with around 210 HP wheel, there was a noticeable difference between 2.5" and 3". Yes, I doubt going to 4" would have done a lot.

But if you can make 4" fit, then why not? You certainly are not losing anything. Correct? ;)
 

majesty78

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I guess without remapping you can loose very much...
Some weeks ago i went for a ride with only the downpipe (2.5inch from Milltek) mounted, just to see what may happen.
First and worst: Very bad boost spiking issues up to 2.5bar, with exhaust i have stable 1.7 with very short initial spike to 1.9bar...
Seeing this already scared me a little, but i went for a ride on highway and felt a very very great power loss while trying to do one acceleration, felt like 50-60hp missing with way to high boost (over 2 bar)

I drove back gently then, not to do any harm to my turbo, and things where fine again.

So the least thing to do, i would suggest, is to get a remap and get the duty cycle for N75 fixed...
 

UFO

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milehighassassin said:
Not sure I agree with you 100%.

I do hear what you are saying that there is a point that you won't see the gains.

Having had a 1.8t with around 210 HP wheel, there was a noticeable difference between 2.5" and 3". Yes, I doubt going to 4" would have done a lot.

But if you can make 4" fit, then why not? You certainly are not losing anything. Correct? ;)
Correct. My only point is 4" exhaust is a lot of work for neglible gain.
 

majesty78

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Oh, i forgott about that! sound sustainely is much much more important than keeping things alive*g**g**g*

Why not take a straight piece of 4" pipe, and let it show out the front fender*g*
 
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dJValkenburg

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UFO said:
Correct. My only point is 4" exhaust is a lot of work for neglible gain.
Also, a lot more weight than the 2 1/2" I'm running. My point is that your not going to see a worth-while improvement going to 4" exhaust on a TDI and will probably see a decrease in performance due to that extra weight you now have from that exhaust. Just my opinion.
 

TDIJetta99

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03... Faster than yours =]
4" pipe isn't heavy at all.. it's usually a thinner gauge than most 3" pipe... the 3" I used for my system is 13 gauge... most 4" is 16 gauge... a 6 foot section of 13 gauge 3" is a lot heavier than the same length of 16 gauge 4"....

anything you put on will be lighter than the boat anchor of a muffler that VW puts on there..

you're right, there's no worthwhile improvement from 3-4, but the sound absolutely changes... personally I like the sound of bigger pipe.. plus I can build it, so why not?
 

dJValkenburg

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TDIJetta99 said:
4" pipe isn't heavy at all.. it's usually a thinner gauge than most 3" pipe... the 3" I used for my system is 13 gauge... most 4" is 16 gauge... a 6 foot section of 13 gauge 3" is a lot heavier than the same length of 16 gauge 4"....

anything you put on will be lighter than the boat anchor of a muffler that VW puts on there..

you're right, there's no worthwhile improvement from 3-4, but the sound absolutely changes... personally I like the sound of bigger pipe.. plus I can build it, so why not?
Your right, if it's free and doesn't cost anything then why not. I guess if the weight for the pipe your using isn't that much different and you prefer the sound, then why not. It's true though, that oem VW muffler was a huge chunk of metal. I removed my stock exhaust which must had been replaced very recently after I bought my car from the owner because it was in mint shape. I gave it to my bro because his exhaust was shot. I prefer the straight pipe with no restrictions. Sounds like a beast too! :)
 

milehighassassin

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dJValkenburg said:
Also, a lot more weight than the 2 1/2" I'm running. My point is that your not going to see a worth-while improvement going to 4" exhaust on a TDI and will probably see a decrease in performance due to that extra weight you now have from that exhaust. Just my opinion.
Bull****!

Weight is not measurable.
 

UFO

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dJValkenburg said:
Also, a lot more weight than the 2 1/2" I'm running. My point is that your not going to see a worth-while improvement going to 4" exhaust on a TDI and will probably see a decrease in performance due to that extra weight you now have from that exhaust. Just my opinion.
Yes, that is correct. I built my 3" system. 3" exhaust pipe is very heavy, about 2 lbs per foot, and the old 2-12" is about 2/3 lb per foot, significantly lighter gauge. Maybe one can make a 3" or 4" exhaust from titanium that weighs the same as stock....;)
 

IndigoBlueWagon

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FWIW, what I've learned is the quality of the bends matter, too. I saw a noticeable change in power (measured on the dyno) and a noise reduction when I went from traditional bent pipe to mandrel bends. The 2" exhaust on my A3 flows significantly better (8 HP better) than the 2.25" OE DP and 2.25" muffler shop bent exhaust that was on the car before. Not inexpensive, but the stainless systems I have on 3 cars will be the last exhaust they'll ever need.
 

dJValkenburg

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milehighassassin said:
Bull****!

Weight is not measurable.
Your right, weight is not measurable. If you slam a stock TDI engine into a MKI or MKII and race a MKIV or MKV TDI all at stock, who's going to win? :rolleyes:

My point is that if your after performance, making your pipe bigger than 2 1/2" isn't going to help you at all because the extra weight will just slow you down.

That's interesting about the Stainless Steel exhaust flowing better. I had no idea that it would. I just didn't want to pay that extra money for it if my car wasn't a show car and the exhaust was never going to be seen anyways. ;)
 

greenskeeper

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If you go too big, wouldn't the exhaust gases expand and slow down hurting performance? no?
 

milehighassassin

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dJValkenburg said:
Your right, weight is not measurable. If you slam a stock TDI engine into a MKI or MKII and race a MKIV or MKV TDI all at stock, who's going to win? :rolleyes:

My point is that if your after performance, making your pipe bigger than 2 1/2" isn't going to help you at all because the extra weight will just slow you down.

That's interesting about the Stainless Steel exhaust flowing better. I had no idea that it would. I just didn't want to pay that extra money for it if my car wasn't a show car and the exhaust was never going to be seen anyways. ;)
I am not talking weight in general, I am talking about the difference in weight between a 2.5" exhaust and a 3" exhaust. There is going to be very little difference and it certainly won't hurt your performance. The difference in having a full tank of fuel and a 1/4 of fuel will effect more, even that is a stretch.
 

milehighassassin

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greenskeeper said:
If you go too big, wouldn't the exhaust gases expand and slow down hurting performance? no?
It shouldn't be an issue. The exhaust is not long enough.

Heat = expansion

Heat=spead

When you expand you gain velocity. The gasses cooling down would be more of an issue.
 

UFO

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milehighassassin said:
I am not talking weight in general, I am talking about the difference in weight between a 2.5" exhaust and a 3" exhaust. There is going to be very little difference and it certainly won't hurt your performance. The difference in having a full tank of fuel and a 1/4 of fuel will effect more, even that is a stretch.
Absolutely. The difference in weight between 5' of the 18 gauge 2-1/2" and the 16 gauge 3" is 5 pounds. 4"? Ok, at most 10 pounds...:rolleyes:
 

dJValkenburg

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UFO said:
Absolutely. The difference in weight between 5' of the 18 gauge 2-1/2" and the 16 gauge 3" is 5 pounds. 4"? Ok, at most 10 pounds...:rolleyes:
Apparently the extra weight doesn't matter, maybe we should just throw sand bags in our trunk so they got a chance to keep up! ;)
 

Mach1

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Be more concerned about thin wall Vs. thick wall, I used some thick wall fence post, it is heavy--will last forever but its heavy!!
 

Fix_Until_Broke

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Bump - Made a video today since it was nice enough to be outside but not crazy windy like it typically is in the spring.

This is with the latest setup below.

03 Jetta, 3" downpipe, 4" exhaust, CC Vent, Boost & EGT, PD Lift Pump, EGR Delete, PP502, RC6, Keystone,BB Dual Diaphram Clutch, 3" Turbo Intake Tube, VNT 17/22, 3 Bar MAP, VR6 MAF, Vented Fender Liner


Sounds like crap from the outside, especially at idle, kind of like an annyoing air leak, but it's nice inside the car and not really attention getting in normal driving. It comes to life though when you really get after it :)

I might go out tonight and get some incar video so everyone can hear how it sounds inside the car.
 

IndigoBlueWagon

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FUB, I'm not sure I'm ready to go with a 3 or 4" throughout, but I'm thinking of ways to keep back pressure as low as possible on my Golf. One option is a 3.5" spun CAT from Magnaflow. My club rules say that if the car came with a cat it has to have one for the track. I've seen on my wagon that the CAT does create some back pressure and power reduction at high RPMs (over 5000). Given that I'm wondering if the spun cat in a larger diameter would flow better. Something like this: http://www.car-sound.com/02product/universal/600d.asp

I'd probably bring it back to 2.5" because even that size clearing the rear axle when the car's low enough for the track is a challenge. But I was thinking about a 3" or 4" muffler and turn down tip at the end. Even though it's a track car I drive it there and quiet is better IMO.

What do you think? Worth it to use the bigger CAT and muffler or a waste of effort? The rest of the system will be mandrel bent stainless.
 
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Fix_Until_Broke

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Peter - Yes, I think it is worth the effort over the OEM CAT. Since they all seem to be the same overall size (just different inlet/outlet sizes), I'd go with a 4" mostly because 3.5" is an oddball size to find adapters for. However, if you run 3" all the way back, you can probably expand the 3" pipe out to match the 3.5" diameter on the cat and weld it all together.

My only concern is that if it's actually a functional CAT and you have too low of temps in the CAT itself, it might start to clog, but given what you're doing with it, if it starts to plug up a bit on the way to the track, it'll be cleaned out after a couple laps :) and you probably won't be too smokey anyway.

I'm the first to admit that 4" is probably no improvement over 3" (and heavier) but I would bet that 3" shows some measurable improvements over 2.5" particularly at high RPM's and over the length of the car that the exhaust runs.

You should still be able to get 3" over the rear axle even on a lowered car. The point where the exhaust goes over the axle is close to the pivot point on the frame - at this point the axle moves something like 1/5 the distance of the wheels so a 2" drop reduces your clearance by ~0.4" for the exhaust.

If you want quiet with no resonances, put the muffler in the OEM location Mine is a 4" ID with an 8" OD (the oem heat shield for the OEM muffler gets recycled - weight reduction :)). The 4" ID tube is perforated with luvers pointing back - kind of like a cheese grater and the gasses flow across it the "easy" way. Given the fact that I can stick my entire arm through it, the flow restriction is negligable, however it does wonders for noise/resonances/drones. I assume that 3" would be the same

If you're really concerned with weight (and you're going to think I was dropped on my head when I was younger......), after the flex pipe/bellows, you could probably get away with running aluminum pipe - just use the stainless flat band clamps to make connections to the stainless parts and hold everything together. Put the CAT back at the OEM muffler location and figure out which tip/turndown combination works for you for drones/resonances.
 

TDIinTexas

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Fix_Until_Broke said:
If you're really concerned with weight (and you're going to think I was dropped on my head when I was younger......), after the flex pipe/bellows, you could probably get away with running aluminum pipe - just use the stainless flat band clamps to make connections to the stainless parts and hold everything together. Put the CAT back at the OEM muffler location and figure out which tip/turndown combination works for you for drones/resonances.
I also have contemplated this idea. 6061 melts at 1100*, just need to find out at what point the exhaust gases are below that.
 

IndigoBlueWagon

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Great ideas, thanks. I am still thinking of a 2.5" stainless DP to the CAT, a 3.5" cat, I'm betting I could come out of the cat, go to 3" or even back to 2.5", and then within a foot or so sleeve clamp on aluminum pipe for the rest of the run. I really like that idea. If I go 2.5" I probably won't use a muffler at all: My wagon is quiet without one, only starts a bit of a drone at 90+.

I know my 2.5" hits the bolt heads for the rear bar if it's not adjusted just right, and I had to swap the bolts so the heads are on the top to reduce the amount of space they take up. The bar's coming out of the car anyway so that won't be a problem, and maybe I can run a 3" all the way back.
 

Fix_Until_Broke

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I bet right out of the turbo it'd probably melt on the outside of the first bend, but once you're past the downpipe/flexpipe, there's enough heat dissipation where I'd be pretty confident it would last short of an extended (multiple minutes) run at WOT, but even then it would probably only need and additional foot of pipe or so to cool it off enough.

I'd definately be easier on the mounts and isolating vibrations
 
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