Originally Posted by My5ABaby

Racing Beat sells a true dual header.

Originally Posted by BzR5150

I went a full replacement twins setup, for 2 reasons
1.the look & style
2. for the extra 0.316425 % horsepower gain i could not look past point #1

Originally Posted by Roen

From what I know,

the true duals all the way back are good for stock or mild streetports.

A custom-made tuned collected header is better for the larger ports.

Originally Posted by collink

I've been told that the stock style dual exhaust (Y-pipe I believe?) is somewhat less than desirable for performance because of the face that where the pipe splits, turbulence is caused.

I've also been told that you want a very unrestrictive exhaust with a rotary...at least as far as performance goes.

Assuming that the previous statements are true, would there be any performance value in taking...say a stock FC header, and cutting it off before the two pipes meet, then running completely separate piping for each. Seems to me like it'd flow a lot better. And I don't believe rotaries need back pressure....do they?

Anyway...it was just an idea I had...I always loved the look of the dual exhaust on these...but I don't want that turbulence in the exhaust system.

Thanks in advance,
Collin

Originally Posted by collink

I've been told that the stock style dual exhaust (Y-pipe I believe?) is somewhat less than desirable for performance because of the face that where the pipe splits, turbulence is caused.

Originally Posted by RotaMan99

I hate it when people say you need backpressure to make power. Exhaust velocity is more important.

Originally Posted by NZConvertible

Backpressure (really it's just "pressure") in an exhaust system is intrinsically linked with the gas velocity. If velocity goes up, so does pressure. If velocity goes down, so does pressure. The pressure is an indication of how hard the engine has to push to get the exhaust gases through the system, so the higher the velocity, the higher the pressure and the higher the power the engine is applying to do that work. Power used to push exhaust gases is power not going to the wheels.

While low exhaust velocity can have a negative effect on an NA engine's perfoemnce, so does too much backpressure. Like a lot of things, there's compromise that needs to be reached. The S4's 6PI system complicates things, as low backpressure will cause a performance drop if there's not enough pressure to open the auxiliary ports at the right time.

Originally Posted by RotaryGod

If you look at the tuning of the stock manifold vs different headers, the effective length of the stock exhaust pipes in the block and the manifold is only about 2". It's pretty short. That nevertheless does do something to the powerband. Useful in some parts, not so much in others. It should have peak efficiency at around 7000 rpm or so but also other certain spots in the powerband. There are actually several different lengths that can tune there based on the order of reflections in the system. When you have a system tuned to one spot, it is also tuned to other spots as well. In between these it is worse than with different lengths. If you had a header that was 30" long or so, it would have peak efficency around 5000-6000 rpm but possibly less than the stock manifold immediately above and below these points then make more power again off idle. The effects of scavenging in addition to the tuned length could possibly make up for the losses in some spots but again, hurt in others.

A stock manifold can make decent power in spots and so can a header. A short primary header will do it over a narrower range than a long primary. A dual is different still. You probably spent most of your time where the header wasn't much of an advantage but was more noise than anything. The Pacesetter doesn't give as nice a powerband as an equal length unit does. My big issue is that people so often only focus on peak horsepower numbers when these are the absolute least important numbers to focus on. Alot of people completely ignore everywhere else even though they all spend most of their time driving there. Headers can have a huge impact on your powerband. It is all dependent on length. Some header lengths aren't as friendly to power as others. The problem is that alot of people don't recognize the differences between them and just randomly spout off how they "don't make more power". Some headers can make much more peak power, some can make much more average power, and if you do it wrong, some can make less peak or average power. A nice set will definitaly make more power than a stock manifold and do it over a very wide range.

Originally Posted by RotaryGod

In general the more port overlap you have, the less reason you have to use a true dual. A large overlap engine such as a bridge or a peripheral port needs to have a collected system to help scavenge the other side. This is very important with these port styles. A true dual does not scavenge. You have to collect to scavenge. A true dual does tune though so don't think it can't work. A large overlap engine is much more sensitive to exhaust backpressure than a stock or streetported engine is. A true dual does have more backpressure than a collected system as it has no scavenging ability which pulls gasses down the opposing pipe. The benefits of the true dual on a less agreesively ported engine outweigh the negatives though. If you use a true dual on a bridge or peripheral ported engine, it will fall flat on it's face on the top end. I have a dyno chart of an unnamed person who has a peripheral port running a true dual system with 2 Walker turbo mufflers. The owner claims the car makes 310 hp apparently by virtue that it is a p-port but the dyno chart suggests otherwise as it falls on it's face at 7500 rpm.

Different lengths in a dual system will tune for a different powerband just as with a collected. A longer system will give you more low end and a shorter one will give you more top end. Don't assume that because a system is long and runs to the back of the car that it will only have low end and no top end. Not true. In fact it is a very nice length as it makes your average powerband real nice. A short true dual may ultimately peak higher but the average will be lower and the peak may be after redline anyways so it would be no good there.

A collected system is also length dependent. There are many things that come into play with a collected system that can help or hurt it. Obviously we have distance from the engine to the collector. This is an important thing when deciding how long to make the primaries. This distance can affect your powerband. So can the diameter of the tubing. The size of the collector can also affect the powerband independent of the primary lengths. As with anything if it gives a gain in one area it will be a loss in another of the powerband. The true duals suffer the same fate. you can't have everything. The key is designing a system that makes the best power where you need it an loses in the leas important areas. For a street car a wider powerband that may lose a couple of peak power is the best setup. If a collected system fell on it's face at one point on a certain engine, it just means that it wasn't the best designed collected system for that application. Changing something about it could remedy the issue so don't think that all collected systems have the same problems in the same spot. They don't. Neither do duals.

There's alot to condiser when it comes to exhaust design. A true dual system removes alot of complexity. If you start to second guess yourself when it comes to the best overall system, you'll end up confusing yourself to death and get nowhere. On a stock or a streetport system, if you can live with the exhaust tone, you'll probably be very happy with a true dual setup.

It's hard to speculate how much power your car will make. You are more limited by ecu than you are by port size. A properly tuned standalone ecu on a stock port will actually get you more power than a streetport on a stock ecu so it's really hard to tell. Just know that your exhaust won't be the issue as long as the stock manifold is gone.

Originally Posted by RotaryGod

This isn't a dual exhaust. A typical header is an example of a short primary system. It's still a tuned length that benefits from both scavenging and acoustic pipe resonance. A long primary is really nothing more than a really long header. Obviously this will tune differently. A dual is a system that never collects, or from an effective standpoint, collects at infinity as the pulses eventuallly interact somewhere.

There are many things that affect where the powerband will be. Among these are the length of the pipes before they collect, if they do at all, the diameter of the pipes, the collector size, and even the length and diamter of the pipe after the collector. The muffler style also affects tuning. A chambered muffler such as a Flowmaster (don't use one!) will not be "seen" by the engine. Effectively the engine thinks the exhaust ended at the beginning of these types of mufflers. The area increase inside the mufflers gives it this effect. A straight through muffler is like adding a longer pipe to the system. It increases the effective length of the system. Remember we have actual length and effective length. They aren't always the same thing.

There are some generalizations that many people make about the powerband of each system but these are not rules that are set in stone. They can all be altered and in different setups, one type can be made to out do another while redesigning the system can make that result reverse and give the other system the advantage. This is only true to a point though.

When deciding which system is best suited for your particular application, you need to first decide what the car will be primarily used for. Your style of porting will also affect what system works well for you. Typically the smaller the port overlap, the better a non collected system will perform. Again, this always has to be quantified. A true dual system does not scavenge. I don't care what anyone else says or who wants to argue that. They're wrong. End of story. It tunes according to a principle of pipe organ resonance and that's where the benefit comes from. This is using the acoustic soundwaves that bounce back and forth inside the pipe. The goal is to get a low pressure trough of sound energy to arive back at the port at the right time. This only works over a narrow rpm range though. The longer the pipes, the more of these waves there will be in the pipe to have an effect on anything. Each wave helps the other waves in the pipe as well. A collected system, long or short, will also add the scavenging benefit. Scavenging is when the gasses of one pipe combine with another to use their velocity to help pull the gasses of the other pipe along. You can not have that happen if they never collect.

A long primary may or may not tune to a lower peak rpm than a short primary depending on how they were each designed. It will have a wider affect on power though. If you shift at 7000 rpm and your short primary is tuned to 7500 rpm, that's worthless. If your long primary was tuned at 6500 rpm, that's perfect. You might make a couple of horsepower less at peak power but you'll have more average and that makes you faster. At the end of the day which would you rather have, a higher number on paper, or a faster car on the street? What matters more? This is just an example to get a point across. It's usable power that counts. If you have a peripheral port engine that wants to be driven between 7500-9500 rpm, a long exhaust tuned to 6500 rpm isn't going to cut it. You'd want a short system that was designed to tune you at around 8500 rpm so your average was high. A true dual would have to be short to tune up high but then you run into other issues with high overlap and lack of scavenging that I'm not going to get into right now.

As with any setup, there will always be spots where there is a negative impact on power. You can't avoid it on any system. The key is to try to place this at an rpm that you don't care about. A longer pipe will lower and widen the powerband while a short one raises and narrows it. The higher your needed powerband, the shorter it needs to be. A long true dual system adds a nice margin or power over the full useable powerband when used on stock to midly ported engines. The greater the port overlap, the smaller their benefit becomes and once you get up to a peripheral port engine, you'll stop making power right where the engine really wants to take off.

Understand that the stock box exhaust manifold is terrible. It's effective runner length is only about 2" including the engine runners. This tunes it to about 7000 rpm so it still makes decent peak power but doesn't make the average power that a good set of headers make. The outlet is only 2" on it as well. It cracks me up that people worry about getting larger exhaust pipes when they still have a stock exhaust manifold. For good power, use only as large a pipe as you need and no larger. Smaller is better until you max it out.

There is one thing that can be said that does always apply to exhausts. The farther back the pipes collect, the harder it is to muffle it effectively with a true dual being the hardest to quiet down. As the pulses interact, much of their soundwaves can cancel each other out to some extent. The longer they are collected, the greater this is. Another thing that happens is that the farther back the exhaust collects, the lower the sound of the engine. Don't get this confused with the effect that different sized mufflers have on the system. To me a true dual at 8000 rpm sounds like it is actually at 4000 rpm in frequency. This is because we have half of the number of pulses (waves) in each pipe which means the acoustic frequency goes down. Different frequencies have different wavelengths. As you move a collector farther forward for a shorter collection point, you give the collected sound waves more room to interact. Lower frequencies are longer so the longer the primaries the more these get highlighted. As you get the primaries shortened down, more of the high frequencies get highlighted. Changing the length does alot.

Originally Posted by lax-rotor

Not to say that what you have stated is inaccurate, however the forumla is PV=nRT, such that if velocity decreses then pressure increases, if velocity increases Pressure decresses. :