Interesting aside to the new exhaust system...

[VAP]

with the installation of the new exhaust last night I was getting some lean conditions showing on my fuel/air monitor on my drive home. Nothing anywhere near deadly lean but leaner than I've been running, especially at WOT. So I keep turning the dials up on my Split Second ARC2 to dial in a little more richness. Turns out by the time I was home I was set at all zeros across the board on low, mid, high and acceleration modes on the ARC2. Which means I was leaning a bit too much with my RC Engineering injectors running at 3.5 bar on my adjustable FPR. So this morning I decide rather than bump my fuel pressure regulator up on pressure I'll try re-installing a stock 4 bar non-adjustable regulator and see what happens. PERFECT fuel/air ratios all across the board!! All ARC2 ***** set to zero. Which in effect means it's doing nothing at all to the MAF/ECU communication stream. Translation: with all stage 3 induction mods, a better flowing exhaust system you should be able to run 19lb (190cc/min) injectors with near-perfect fuel/air ratios and stock 4 bar FPR with no modifications or adjustability whatsoever. Of course the adjustable FPR is very nice and I wouldn't be without one but the ARC2 is absolutely unneeded with this state of tune. It's a perfectly balanced system now. I'll leave the stock FPR in awhile and report on it's long-term use as well as higher temps than I had last night or this morning.

[Black Sunshine]

[JWG]

You could have put it closer to the engine, why did you choose to put it so close to the catalytic converter and "Y" joint?

I feel somewhat surprised that it had such a significant impact so close to the joint. I would have guessed otherwise. Well, that's why experience from research and testing kicks theory's butt. I need a little more background info on balance pipes and their function.

Query: on the stock A4 system, if we placed a balance pipe before the "Y" connector but after the cats, would it have the same effect of smoothing the flow of the two pipes into the one? Or, do we need to place the balance pipe before the catalytic converters to receive a benefit?

Additional Query: on the stock A6 system with dual pipes front to back that never join in a "Y", would a balance pipe have any function? Allegedly it does, but I cannot understand why one would want to mix the exhaust pulses from two pipes if they never met later in the system. It's not like one side of the engine would produce measurably more exhaust gases or pressure than the other, so why mix?

[VAP]

is determined by painting a long, narrow strip of paint on the downpipes into the cat. The spot where the paint bubbles, blisters and sloughs off first is the "sweet spot" for the balance pipe location. When we tested for this spot it actually occurs towards the outlet end of the new flex joints. Well you can imagine the nightmare of trying to install a balance pipe into the flex joints so I did the next best thing: located it 1.5"-2" aft of the ideal spot rather than 6" ahead of it. And since the downpipes are actually cut flush with the lower transmission casing there's no way I could've located it any closer to the exhaust manifold and that would've run counter-productive to the "paint strip test" method for locating the ideal spot. Also the most broadly accepted size for a balance pipe is 70% of the pipes it connects diameter. Since mine are 2.25" pipes the ideal size would've been 1.575". But since there is no "off the rack" hole saw available in that diameter I chose the next standard larger size hole saw, ie; 1.625" (1 5/8"). Then used 1.75" pipe to join the two pipes and enclose the drilled holes. However I don't believe that a 1.5" hole would've necessarily shown significantly less (or any at all benefit/gain. In point of fact 1.5" may have proved better. I went on hunch, gut instinct, "bigger is better" and nothing more. Even if I'm wrong I'm only .125" wrong.

I "think" best results would be obtained in any system with a balance pipe before the cat(s) for best flow and velocity thru the cat. To do it after the cat may simply be a case of the "Emperor's New Balance Pipe." I also believe that is somewhat borne out in the benefit associated with equal length headers. And even our stock dowpipes are not equal length nor are they even the mirror image of each other. They are different length and different configuration.

I don't believe connecting a balance pipe aft of the cats would be very effective as it's purpose is to balance flow rates from two banks. And I believe that balance should occur as soon as possible in the system for maximum results (remember the paint test).

As far as the A6 and what I know today I would think there is benefit and perhaps even substantially so. Two weeks ago I would not have thought so.

But my biggest nagging question is the balance pipe and a pair of unequal length headers. Could someone reap some, much or most of the benefit associated with equal length headers using the FAR less expensive unequal length ones with the addition of a balance pipe somewhere aft of the header collectors?

[JWG]

Just on the A6 dual exhaust system: You essentially have two separate three cylinder engines each with a different exhaust system, but both connected to the same crankshaft.

If you correctly surmise that an "X" pipe or balance pipe would benefit a dual exhaust system that otherwise would never join, then, it logically and necessarily follows that each engine bank does not create the same basic exhaust pressures, volumes, etc. Otherwise, there would exist no reason to balance the two separate exhaust systems--by definition, balancing would only become necessary if there existed an inequity in the system. By balancing the exhaust system, you equalize the back pressure on both banks, but why would there ever exist a difference to begin with?!?!?!?!?!? Does the dual cat set-up have an inherently more restrictive side that requires balancing???

As I said before, a balance pipe before a "Y" connector makes a lot of sense, but a balance pipe should make no difference on an equally matched dual exhaust set-up. If a balance pipe has an effect it means that there exists no such thing as an perfectly equal dual exhaust.

I want an answer--someone help!!!!

P.S. I have unequal length headers that I will hopefully install in the near future; however, I need to wait a while before I have the need and the cash to upgrade the exhaust system. Do you see why I want an answer? I want to better plan my future system. :-P

[JWG]

Balance pipe and "X" pipe have similar purposes, but the "X" pipe goes to an extreme by fully combining the two exhaust flows.

Basically, there do exist inequities in engine banks based simply on compression values. Further, a dual exhaust system also has different flow rates even using the same hardware.

However, the real magic comes from eliminating the engine pulses. Instead of using exhaust pulse theory like race cars do, street legal cars do the opposite by eliminating exhaust pulses completely. By balancing the pressures between the pipes, the postive and negative exhaust system pressures created by the leading and trailing edges of the pulses get cancelled out as they push and suck air through the balance pipe.

Why would anyone want to do that? Well, supposedly this allows a nice smooth flow of exhaust gases through cats, resonators, and mufflers. While a race car with no cats or mufflers can use equal length headers to have a nice orderly flow of exhaust pulses where the prior pulse pulls the next behind it in its vacuum, the unequal length exhaust manifolds, cats, resonators, and mufflers on a street car disrupt the pulses making them useless and the entire system less efficient.

Thus, adding a balance or X pipe to an exhaust system (after the hottest part of the system per Mance's posts) allows for the cancellation of exhaust pulses and smoothing of the exhaust so that there exists a continuous pressure and even flow throughout the remainder of the system.

[VAP]

The ideal intake on any engine would provide for a central distribution point for it's intake plenum. ie; equi-distant from every intake valve. On a V-configuration engine thats best accomplished by a throttle body location squarely centered in the intake manifold not at all unlike the V8 muscle cars of yester-year. But todays cars are bult around so many other criteria that the centrally located intake plenum is designed around engine fitment with crumple zones, center of gravity, low hood lines (fuel economy) that simple engine efficiency has been compromised to a greater/lesser degree in favor of the "green" approach.

The next best layout for a V-engine would be intake plenum up front so that air distribution is further assisted/supplemented by air slamming into the front of the car as found on Corvettes and a relatively few others. This assures some measure of forced or at the very least "assisted" intake air that is cooler, denser and closer to the source of ambient supply.

The least efficient for engine breathing is the one we have to deal with, ie; front intake/rear engine distribution. The very worst for trying to overcome all the problems with engine heat, restricted flow and and a interrupted and convoluted flow path leading up to a 180 degree rotation of incoming air before it gets into the cylinders. In fact so much of an exaggerated flow path that our charge air travels over 6 feet of diagonal left, down, up, diagonal right, down, sharp right, over/under, up and around the intake only to return to within 4" of it's original intake inlet by the time it reaches cylinder #1. If it weren't so serious it'd be a comical study in inefficiency. But given the remarkable design of this system it functions incredibly well in spite of itself. But velocity, flow and volume have to be tuckered out and waning towards cylinders 1 & 6 and to a lesser extent cylinders 2 & 4. Which leaves cylinders 3 & 5 with the lions share of most easily available, coolest, most dense air as that air doesn't get any cooler, more dense traveling thru the intake manifold forward to the front of the engine, less than six inches away from where it entered the intake tract to begin with.

Now take the lowly longtitudinally mounted 4 cylinder. It's got everything going for it with a cross-flow head (intake on one side, exhaust on the other). It's got relatively even intake distribution and exhaust evacuation and very few turns to make as it's intake faces the front of the car and exhaust faces the rear. Great efficiency!

On our exhaust side we're blessed with an exhaust that has to make at least a 90 degree turn to get it going in the correct direction out the rear of the car. Then there's the downward turn and another 90 degree or similar once it's cleared the belly pan.

So it'd probably be a little much to presume we can have much of any semblence at all of a balanced exhaust when there's so much imbalance dialed in from the factory. And we havent even touched on valve carbonization, different length and configuration downpipes, compression ratio comparisons from cylinder-to-cylinder, individual valve seat-to-valve face and cylinder-to-cylinder leakdowns. But even with a new car right off the showroom floor there's still a ton of obstacles that are designed-in that would prevent/preclude prefectly balanced left/right exhaust bank velocities, flow rates and/or volumes.

But the REALLY astounding fact remains that we can undo SOOOOOO many wrongs with a little 4" long piece of 1.75" pipe and a couple 1.5" holes drilled in the downpipes on both sides of it, connect them and not only balance the exhaust flow, volume and velocities but also create a more balanced system at the intake valves as well. The balance pipe is much farther reaching than just exhaust as it's balancing dynamic is affecting before and behind it's installed location... all the way to those number 1 & 6 intake valves. Not bad a bad days work for a thirty cent piece of tubing!!