1. Myth 1
: The Obsession Over Exhaust Sound Quality: "What Makes A Good Sounding Exhaust?" AND "It Sounds Loud. So It Must Make a Lot of Power!"
The Exhaust Noise is the most common sound source of engine noises, and is usually 10 to 15dB higher than the overall noise level of the engine. The exhaust is of high temperature (800 to l000íŠ) and high pressure (3 to 4 barometric pressures). The exhaust process is divided into two stages: free exhaust and forced exhaust. The exhaust gas spews out of the exhaust valve and enters into the muffler along the exhaust manifold before draining into the atmosphere from the tail pipe. This process yields wide band exhaust noise.
The exhaust noise contains complex noise elements, including the exhaust noise with a base frequency measured in the number of exhausts in unit time, the resonance noise of the gas column in the pipe, the gas stream blowing noise at the exhaust manifold, the exhaust gas jetting and impact noise, the Helmholtz resonance noise of the cylinder, the Karman eddy noise and the turbulent noise inside the exhaust system.
Key factors deciding the exhaust noise of the engine includes the cylinder pressure, the exhaust valve diameter, the discharge capacity of the engine and the opening characteristic of the exhaust valve. For one same engine, the rotation speed and the loading of the engine are among the most key factors that contribute to the exhaust noise.
Loudness does not equate to power gain...loudness AND SOUND QUALITY depends on these:
- muffler length and size (volume or displacement: a bigger can is quieter),
- having a resonator pipe (no resonator = coffee can or bee hive and loud),
- length of the resonator pipe (longer is quieter),
- the type of sound absorption material in the muffler (glasspacks suck, they melt),
- whether the pipe inside the muffler has louvers or holes (holes are quieter and flow better),
- exhaust tip size/length (big tip is loud),
- the exhaust's design (3 types as described above).
So when you shop around, compare and ask about these features that affect sound quality. The more features, the better the sound.
Straight-through designs with a resonator, or a chambered design, or a twin-pass design are quieter than a straight-through design without a silencer cone or resonator. Having no resonator ensures a coffee can sound. Straight-through resonators that have the same ID as the rest of the exhaust tubing is better for performance. The number of passes through the muffler, like in the quieter 2-pass Mugen or Hy-Tech exhausts, determine how quiet an exhaust is.
If you want a non-coffee can quiet throaty sound, look for the exhaust design characteristics I have listed..a longer muffler and having a resonator are good starting points. Power depends on how the exhaust works with the header collector size and catalytic converter size, to help maintain a high exhaust gas velocity compared to the amount of fresh air you are dumping into the engine...most experts agree that the exhaust flow should be at least 70-85% of the intake flow (if it's more than this...even better). So for exhausts as related to power?:
remember, please pay attention to diameter, diameter, diameter that will suit your hp goal.
A big newbie misconception: My exhaust is loud so it must be great!
2. Myth 2:
Big huge diameter tips are better.
You design the tip size to fascilitate where you want the bulk of your power to be along the rpm band. Bigger tips tend to push the peak hp up but at some cost to lower rpm power.Changing tip size affects the pitch of the exhaust note. Bigger tips have a lower tone. Don't make the exhaust tip, even a resonated one, your focus of attention. It plays a minimal role in your system's performance gains.
3. Myth 3
: I Need A Little Bit of Backpressure For Midrange Power
THE MIGHTY BACKPRESSURE MYTH
You want zero backpressure not some backpressure as you may sometimes hear from a salesman or an old timer V8 hot rodder.
Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.
In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.
This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.
So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.
Regarding the backpressure issue
Many people use backpressure to get midrange driveability at the sacrifice of lower power potential at the upper powerband rpms. Using back pressure is the wrong way to build a high performance exhaust system. The exhaust system should extract the exhaust from the header, to minimize parasitic pumping pressures.
The proper way to make an exhaust system that will act as an extractor is to properly size the tubing so that the the exhaust gas' flow velocity creates a "vacuum" behind the header.
Also, you have to realize that making a sytem which provides the best performance at all throttle positions and all powerband rpm ranges is next to impossible. There's always going to be a compromise and giving up some optimal power potential in one area of the rpm range.
You must tune the exhaust size/length for the throttle positions and rpm ranges where you want the most performance knowing that you'll sacrifice performance at the other end of the rpm range.
If the exhaust has the design characteristics you want and is cheaper, get it. Please try not to be hooked by a sales pitch or brand name hype. There's not much separating exhausts these days in terms of performance and design features for 2.25 to 2 3/8 in. straight-through designs. They are all pretty much the same.
In summary, plan where you want your peak torque will be and how wide your power band will be along the rpm range. Then choose a header-cat-exhaust system with the design characteristics that facilitates that goal.
You may get more midrange power but give something up at the top rpms.
Or the opposite, you can plan that you want more power in the upper rpms with some compromise losses at the midrange rpms.