Skyfire

Manufacturers claim a 5-10% hp increase from just a muffler, but my gut feeling tells me otherwise. Plus some people actually stated power loss from a high-flow exhaust. I understand turbos are different from NA engines, so will an aftermarket exhaust help a NA engine as much?

Anybody with hard data on this topic? (ie, dyno graphs before/after)

JWG

An more free flowing exhaust gives the A6 2.8 30v 10 more hp than the A4 2.8 30v--EXACT SAME ENGINE!

I do not believe that just the muffler will give you 10hp, because the biggest restriction remains the cats. High flow cats will help free the exhaust flow far better than either the middle resonator or the end muffler.

A new exhaust system may help you free hp if you have more hp to free, i.e., you have modded your engine. Because the greater the hp, the less backpressure you need A 3000 hp dragster needs none while an 800 hp racing engine needs 5 "inches of water" backpressure; Audi gave my 12v has 24 inches of water backpressure, but modded it could do with as little as 15.

The 12v has a lot of small hp gain mods. I have not seen any minor engine mods for the 30v--only making it into a forced induction engine by adding a supercharger.

Loss of power/torque really occurs only when you either increase or decrease the backpressure from the optimal, because gases expand to fill the entire volume of whatever container you put them in. It's not like water that responds to gravity and uses only the bottom of the pipe when not at full volume. Gases require a certain amount of compression to move efficiently. You see the hp loss on this forum when people go too big, lose gas compression, and undo all of the benefits of their engine mods.

The ideal situation on a normally aspirated engine revolves around taking advantage of the pulse theory. Each combustion is a small explosion that gets focused into a pulse as that pulse goes down the exhaust pipe it creates a wake and a trailing vacuum. Ideally, you want to line up all of those pulses so that each one helps suck the next one down the pipe rather than each one pushing the prior one down the pipe. On street exhaust systems, this becomes hard to do, because we need cats to protect the environment and mufflers to protect our ears.

All this said, I would look into the Brullen full or cat-back exhaust system, as it wisely only increases the diameter by a quarter to a half inch so that the backpressure decrease remains moderate. Many people on the forum have it and really love it. Of all of the aftermarket exhaust systems, I have researched for the 2.8, it has received the most favorable reviews, because it has the best throaty (but not noisy) sound and it allows freer flow for better top end hp while suffering minimal, if any, loss of low-end torque.

You would need big bucks to custom design an exhaust system with an engineer who has the proper computer-aided-design programs to match an exhaust system to your engine's dyno graph. Big factory race teams spend an inordinate amount of money figuring out the details of designing the perfect exhaust for their racing engines, because finding the perfect balance that allows for optimal hp and torque requires a lot of hard technical science, experience, and much trial and error.