Milltek exhaust

[The Spanish]

To continue the previous thread by pisobiker about the SuperSprint system, I also mailed milltek to find out if they had a downpipe and cat assembly.
They sent me two pictures, but I don't know prices yet. (See images bellow)

To make this a standard fit to any S2 they have a downpipe with a single cat an then the cat exit divides into two sections so anyone is able to connect it to the OEM central silencer. For those like me who have the milltek system it will result in a downpipe, cat, then divided in two, then the 2 into 1 section, central silencer and back box.
I am sure this is the best cost effective options even without knowing prices but for the experts what are the downsides of having the cat exit divided in two and inmediately after the two into one section?
One is the weight for sure but performance wise?

Does anybody think they can make a straigt tube front the cat exit to the central silencer?

[The Spanish]

Cat picture

[Markus]

I've been wondering about this 1-2-1 section myself. The milltek-website says "This downpipe and catalyst combination can either be fitted to the standard Audi system or a Milltek Sport performance system.", but I'm not sure what that means. Does a full Milltek turbo-back system have this 1-2-1 section after the cat? Or are there two different versions available, one with the 1-2 section after the cat (for use with a stock exhaust) and another without the split (for use with the Milltek cat-back)?

Does anybody have any price information on the Milltek system? And what about dimensions?

[The Spanish]

The full milltek system does have the 1-2-1 split as they allways provide the 2 into 1 section as far as I know. Somebody correct me if wrong please.

I am sure they can build a 1-1-1 exhaust as they only need a single in-out cat and a straight tube to the center silencer. It should be even cheaper than their standard version.

I have mailed them for prices and width and had no response yet.
Will post somethins soon though

[urq20v]

The Milltek S2 downpipe is 236.25 pounds and the sports cat is 438.75 pounds(retail).I have just finished fitting the downpipe and cat to the Milltek cat back system I had installed some time ago to my ABY S2.
Rex

[The Spanish]

I have been quoted by Milltek £793.13 inc VAT + carriage wich at today exchange rates is 1127.64 Euros. Considering the value of the Euro is very high now this is a tad on the expensive side also.
Supersprint downpipe with metal cat is 1149 Euros here in Madrid and in my opinion looks better made. The only problem is the custom adaptation needed.
Milltek donwpipe is 70mm and cat exits are 58mm each.
Still don't know any technical disadvantages of a 1-2-1 split exhaust.
Can anybody enlight me please.
Cheers,
Pic of the Supersprint

[Dr H]

I would have thought the more you split and then rejoin pipes the less easily the gases will flow. A single pipe must give better flow than a 1-2-1

[The Spanish]

Thats exactly what I think. A single pipe must have better flow.
I don't understand Milltek.
If the quote around 1100 pounds for the complete system (downpip3, cat, 2 into 1 section, center silencer, back box) why the don't have and alternate solution with a straight pipe from the Cat exit to the center silencer?
Cheers,
Rafael

[Markus]

Every bend, split, exit, flange, reduction etc. will increase flow resistance.

Flow resistance in a straight pipe is however dependant of the flow velocity which in its turn is dependant of the cross sectional area and volume of flow. The easiest way to reduce flow resistance in a straight pipe is to increase the diameter. That's why performance exhausts often come in bigger diameters than stock ones:

Bigger diameter -> larger cross-sectional area -> reduction in flow velocity for a given volume of flow -> reduction in flow resistance -> lower pressure drop

Another way to increase the cross-sectional area is to use multiple, parallel pipes. In this case I would however guess that the length of the paralell piping is so short that the benefits due to increased area are outweighed by the additional resistance caused by the split and joint. I can't however say how big this negative impact from the 1-2-1 section really is.

From a engineering viewpoint the 1-2-1 solution seems really daft due to higher materials' and manufacturing costs (+added weight). The only advantage is that the product palette can be kept smaller.

[Pisobiker]

For sure a single pipe will have less flow restrictions compared to a twin pipe (both of the correct sizes, of course). In the best hypostesis (perfectly matched twin diameters compared to the single one, and perfectly shaped collector and divider) there will be same flow as the single pipe, but for sure not better.
In athmospherical engines (where pressure pulses are used to increase scavenging effects on the exhaust) collectors, lenght of runners and layout (4 in 1, 4 in 2 in 1 or whatever) and ducts diameters fix pulses frequency and intensity which will make the system resonate (thus increasing flow performances) at precisely fixed rpm.
This system will work even on a supercharged engine, where exhaust pulses are not disrupted by the supercharging device.
Doesn't work instead on a turbocharged engine for more than one reason:

- Turbine receives pressure pulses from each cylinder but turbine disrupts flow and discharges in a much less pulsating form, thus dramatically reducing the usable resonant effects;

- Turbine converts part of the exhaust kinetic and pressure energy into mechanical energy, thus flow energy on the outlet will be sensibly reduced;

- Turbine efficiency is proportional to the pressure difference between inlet and outlet of it, so the least backpressure on the downpipe will maximize turbine efficiency.

Latter statement clearly shows that looking for max performances on a turbocharged engine, there is no practical advantage in making a 2 in 1 layout, and even less in making a 1 in 2 then going again 2 in 1.
Only reason Audi might have chosen to split into two pipes is for using two smaller and higher flow rate catalitic converters instead of a big and presumibly more restrictive one, which will also have had clearance problems underneath the car.


Marco