Audone

The reason I ask is my Brullen catback muffler is shot, and I am looking for a replacement, but I cant find any decent rear sections (as only the muffler is bad) that are at least 2.5" diameter. I found a UUC VM2 for a great price, but not sure how much work it will take, and if its even worth while at all....
Or: would a 2.25" rear section affect performance THAT much?

Pat

I bought a used Milltek from an S4 and put it on my A4 1.8T. No big deal.

Mat Indukts Lambos

and it'd make sense to use a larger downpipe to mate up to it. Easy work for any competent exhaust shop.

JWG

Brullen's uses universal muffler/resonator parts on its exhaust kits, the only custom parts are the piping on both ends of mufflers.

Assuming you have a 2.8 and not a 1.8t, in any event, it will make almost zero difference to your power output if you are making less than 300 hp. The stock piping on all 2.8l A4's can easily handle up to 300 hp (even PES's supercharger makes 295 hp on a stock exhaust system). The cats restrict the air flow far more than the mufflers. Given the huge restriction "upstream" from the mufflers, the mufflers may at most make a 10 hp difference as they do between the A4 2.8 30v 190 hp with a "y" exhaust system from front to back and the A6 2.8 30v at 200 hp with a dual exhaust system front to back. I dare you to tell the difference between an A4's hp output and an A6's hp, as you will not see that 10 hp difference anywhere but near redline; thus, the overall power curves remain the same shape.

On the 12v Development Forum, we have scavanged and researched every possible method to squeeze out power from our tuner-abandoned motors. Our member tests have determined that going with a stock or cat-back tuner exhaust system does not materially change the hp, because (1) the piping has sufficient diameter to handle at least 300 hp, (2) the Audi mid-resonator is a masterpiece of efficiency and sound suppression that no aftermarket resonator can match, and (3) only the rear muffler may restrict hp a little on the high end but makes no significant difference on the low-end. Those of us who replace only the end muffler do so more for sound than for performance; with Magnaflow preferred on the cheap end and Remus preferred on the high end.

If you really want to squeeze some hp out of your normally aspirated car's exhaust system, replace the cats first, if you live in a place where you may do so.

S4 exhaust will fit an A4, but why would you want to do that? S4 is a turbo car with completely different exhaust needs than a normally aspirated car. A turbo car needs no backpressure in the exhaust because the turbo itself creates the backpressure; a normally aspirated car needs backpressure to perform properly and to make maximum horsepower.

The amount of backpressure is inversely proportional to the hp: a 6000 hp top fuel dragster needs none, a 850 hp racing engine needs a between 4 to 6 "inches of water" backpressure, a 200 hp V-6 needs at least 14 "inches of water" backpressure to function properly. Our 2.8l V-6's have about 24 inches of water backpressure from the factory. Now that's way more than it needs to function, but remember that backpressure can work for you when making torque and against you when making top-end horsepower. It's a balancing act. Too little backpressure, lot's of hp at the top end, but no torque on the low end. You will only know the perfect balance if you take your car to the drag strip and take her down dozen's of times with varying amounts of back pressure and check your various trap times, final 1/4 mile time, and top speed. In leiu of that, just rely on what those who have tinkered endlessly with exhausts before you know and have done.

You can even check the BMW boards, most of them say the same thing, 10 hp difference between stock and aftermarket exhaust systems. Why the same issues? Because, no intelligent tuner will stray too far from the stock backpressure on a normally aspirated street car that must have nice broad horsepower and torque curves to remain driveable on the street.

Have you ever wondered why the best drivers in the world manage to stall when going from a complete stop in the pits (or in F1 from their standing start)?--backpressure and intake resistance has a lot to do with it! The teams want max hp from the restricted engine size; so they put large intake piping on the car and run minimal backpressure. The result is simple: lots of hp on the top end, but miserable driveability on the low end. For this reason, many racing series have running starts to avoid the danger of someone stalling on the grid during a standing start.

My $0.02, go find a Brullen dealer and get a universal Brullen muffer, or go with any aftermarket rear muffler at either 2.25 or 2.5. It will change the torque and hp curves marginally at best.

By the way, doesn't Brullen have a lifetime guarantee on its exhaust systems?!?!?!

Audone

I have tried calling Brullen, and noone answered.
The first main problem with my muffler is that it is waaaay to loud, and second I had the DTM tips which I hated. So I was smart and cut them off, except it didnt turn out so well.
Since the rest of my exhaust is 2.5" (except 3" dp) I dont want aa smaller diameter muffler, and thats why I went looking at the S4 route, as the Borla rear for an S4 is 3" and still quite cheap (165 new). The other S4 exhasut I found was a VM2 which is for sale for 275, which I figured I could make it work for that price.

a_o_smith

...it would act like a 2.25 all the way through...only as large as its smallest section

JWG

The exhaust gas coming out of your tail pipe have cooled considerably since they came out of the combustion chamber. Thus, they take up less volume. Remember, these are gases and follow different physical rules than water hoses and/or other common items from which we derive our "common sense".

a_o_smith

didnt think about cooling, but a bottle neck is diffrent from ~1ft of smaller pipe