Skyfire

Espeically when it comes to bigger, hi-flow exhaust systems on a NA engine like the 2.8 V6. I heard if it's too non-restrictive (like 3" diameter) I'll lose back pressure and therefore lose power. So how does this all work and how do I know what exhaust system will give me more performance instead of just make loud noises?

ryoung

Any pressure in the headers just results in more work that the pistons have to do during the exhaust stroke, subtracting from HP at the crankshaft.

JWG

A 4000 hp top fuel dragster needs none.

A 600 hp racing engine usually runs a back pressure of about 4-6 "inches of water" (the standard unit of measure).

A 200 hp n/a V-6 like ours needs about 14 inches of water.

Our OEM exhaust system has a back pressure of approximately 24 inches of water.

Thus, ideally, you remove about 10 inches of water of backpressure to reach the ideal amount.

Since our exhaust system flows pretty well out of the box, we can get pretty close to ideal by reducing the biggest restrictors: #1 the cats and #2 the muffler. So, get high flow cats and a low restriction muffler and bring your backpressure down dramatically.

You do not need to go to 3" pipes, 2.5" pipes will prove more than adequate for our hp needs, because you do not want to cool down the exhaust too much--cooler air is less dense takes up less space and is harder for the engine to push down the exhaust system. Designing exhaust systems is a tricky balancing act: too big you lose too much back pressure and slow the air down as it cools and cannot fill the volume of the larger pipe; too small and you get too much back pressure. You need to find the sweet spot.

Additionally, a balance pipe between the L & R exhaust down pipes prior to the catalytic converters will smooth your exhaust flow by neutralizing the exhaust pulses and creating a non-pulsed exhaust flow through the cats. As I recently learned, a pulsed exhaust theory only works with normally aspirated race cars that require no cats, mufflers, or other types of restrictive devices.

If you have a 12v 2.8, get headers; the OEM exhaust manifold also is a big restrictor. Unfortunately, if you have a 30v, you will likely never have room for headers due to the space constraints between the big DOHC heads and the frame of the car.

JWG

would just run pipes straight out of the block into the atmosphere. However, they do not, because you need to make big hp (like a top fuel dragster) to run with zero back pressure. Read my post to below if you have time. Otherwise, remember this one rule: requisite back pressure is hp dependent. For optimal power, it's an inverse relationship: the greater the hp, the less backpressure you need.

P.S. How does your S8 like those 20" wheels? Can the S8 suspension handle 20" without any problems?

Skyfire

I recently put on a custom exhaust with a magnaflow center resonator and exhaust. While the sounds is more "deep throat", I can't feel the power increase, instead it felt like the car is slower (although I don't have any evidence to support that except my "butt-o-meter"). Sometimes, especially when I ease the throttle to change gears, there is a "pop" sound coming from the muffler, almost like a hickup. Does this mean I don't have enough backpressure? If so, how can I solve this problem?

ryoung

unless rules on noise level make a muffler necessary. You won't find any exhaust system restrictions in anything from our 150 hp 2L FC race motor to 900 hp V10 F1 motors.

The simple part ends when you try to tune the length and diameter of the primary headers and the collector. The optimum is highly dependent on engine configuration, valve timing, desired peak HP rpm, etc etc. Whenever possible, you try to use scavenging to take advantage of valve overlap while minimizing reversion. As you mentioned above, this becomes more difficult with a V6 configuration. In a V8 without a flat crank, primary headers need to cross over to the collector on the other side for optimum tuning (I love this pic). I'll stick with my original statement, though, that backpressure is always bad.

<img src="http://www.lambolounge.com/Building_Notes/Countach/KenImhoff/images/P1150035.jpg">

ryoung

The S8 suspension can definitely use the 20" wheel without problems. With the tire's short sidewall, handling is predictable, sharp, and responds well to steering with the throttle.

In my "spare" time, I'm crew chief for a Formula Continental. My driver ran 1:50 lap times in the S8 at Mid-Ohio last year, in his first time ever driving any Audi. [Actually, he drove the S8 about 2 miles that morning from breakfast to the track.] He was very surprised at how well the car handled and felt that with more seat time, he could trim another 5 sec. The only problem is that the S-03's quickly overheat when pushed that hard, and after about 3 laps, the greasy tires force you to slow down. I had the same problem at Road America last week. During the afternoon of the 2nd day, I overheated the rear tires only and ended up with an S8 that was oversteering badly.

<img src="http://pictureposter.audiworld.com/17157/mid-ohio_t8.jpg">

<img src="http://pictureposter.audiworld.com/17157/grattan_t7.jpg">

<img src="http://pictureposter.audiworld.com/17157/hre_atspeed.jpg">

Audi2ptzero

If you live in the high rpms 99% of the time then yes you want as little back presssure as you can get. But when driving around town at 2500 rpms you will end up with no tq to move you forward any faster without having to down shift if you have no exhaust.

ryoung

at any rpm?

Just to break even, you have to gain as much as the extra work done by the pistons when they have to force exhaust gas out of the cylinders against back pressure.

JAFO

i have to agree with ryoung! you can move the rpm at which maximum scavenging takes place by changing the lenth and/or diameter of your exaust pipes. some systems i have seen for V8 and inline 6's have different lenth pipes on some cyl. to smooth the torque curve. these are generally popular on trucks.