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Last Updated At: 5/10/2003 10:00:15 AM This article is provided "as is" without any representations or warranties, express or implied. Team-Integra makes no representations or warranties in relation to this website or the information and materials provided on this website. |
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GOALS OF AN ENTIRE EXHAUST SYSTEM
The 2 goals of a header-cat-exhaust system (notice I didn't just say exhaust only) is to:
a) to efficiently remove as much of the combusted inert exhaust gases out of the cylinder.
Remember that burnt exhaust gas is inert or does not combust twice (EGR & fuel economy is another story) and therefore cannot make power if it is in the cylinder...it takes up space in the cylinder and prevents fresh air and fuel from coming into the combustion chamber to make power.
b) to keep the velocity or speed of the exhaust gas leaving very high.
When high exhaust gas speeds are reached, a wake is created from an exhaust pulse leaving the cylinder head (see SurferX's exhaust article here for some nice pics of this wake or pulse). Following behind this wake is a low pressure pulse that acts like a vacuum is created. This vacuum literally sucks in more fresh air and fuel at cam overlap, when the intake valve is just starting to open and the exhaust valve is almost about to close. Since both the intake & exhaust valves are partially open at this time of cam overlap, the header is actually "connected" to the intake manifold & intake port for a brief period. The exiting exhaust gas helps pull in the next fresh intake air & fuel. This is called scavenging. And scavenging is what helps draw in more oxygen and fuel for combustion.
More fresh air and fuel coming in, with less inert burnt exhaust gases occupying combustion chamber volume, makes more power.
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EXHAUST DESIGN
Generically speaking, there are 3 exhaust designs:
1. straight-through
2. chambered
3. 2 pass or sound cancellation design .
The straight-through design is when you look through the tip opening, you can see straight through to the other side of the muffler...no bends , or staggered tubing, or chambers.
The majority of popular 60 mm diameter aftermarket exhausts are straight-through designs like the Apex N1, Tanabe Racing Medallion or G-Power, Greddy SP or EVO, 5 Zigen Fireball, HKS Drager or HP or SE, etc.
A twin- pass or sound cancellation exhaust has a second pipe beside the muffler tip that loops back to the front of the muffler. Mugen and HyTech make very good twin-pass exhausts.
Most stock exhausts (like the ITR exhaust) are chambered exhausts. In a chambered exhaust, the muffler pipe goes into separate chambers and an outlet pipe is staggered and not inline with the inlet pipe into the muffler.Straight-through and twin-pass designs flow much better than chambered designs.
A. Design Characteristics to Look For When Shopping for an Exhaust
We may want to look at what exhaust characteristic makes power:
1. Diameter is King:
The most important factor about an exhaust is the B pipe and inlet muffler tube diameters needed for your flywheel hp goal. These outer diameter sizes are suggested by SMSP. The suggested diameters assumes that the thickness of the exhaust tube is 16 gauge steel:
Most Integra owners have 1.8L engines with "bolt-ons only". You may want to look for a 2 1/4 in. diameter exhaust to start off with. If you begin to think about big lift-long duration cams, bigger 2.5 in. collector headers, headwork, or boring out to get 2 L displacement, then go up to the next levels in diameter as determined by your power goal.
2-1/4" up to 210HP @ the flywheel (about 180-185 whp)
2-3/8" (60mm) up to 235HP @ the flywheel (about 200-207 whp)
2-1/2" up to 265HP @ the flywheel (about 225-235 whp)
2-3/4" up to 325HP @ the flywheel (about 275-285 whp)
3" big for big HP (Forced Induction: > 275 whp)
The general rule here for sizing an exhaust is: for every 60 flywheel hp, you need 1 square inch of exhaust cross-sectional area (i.e. 60hp/square in.), in order to achieve the exhaust flow speed required for adequate exhaust gas scavenging out of the cylinder.
All aftermarket exhausts are mandrel bent these days and so this isn't as important an issue when you compare exhausts. However, many of you prefer to have custom exhausts made for yourselves. To show you how important diameter is, a 2.8 in. crush bent exhaust will outperform a 2.25 in. mandrel bent exhaust, if the header collector and cat are 2.5 in. diameter as well and you are aiming for power above 210 whp. Don't get me wrong, if you can get a 2.5 in. mandrel bent system, it's the way to go. This is just to show the importance of diameter and sizing for the entire exhaust system together.
[ Aside: If you don't know what cross sectional area is, I have defined it in the article entitled "Ideas: flow velocity, flow capacity, and flow quality" in the Performance, Engine External section . If you can't be bothered to look at that thread, then use these outer diameters as a guideline for selecting the proper exhaust for you. ]
Please be careful: The diameter just behind the exhaust flange that connects the Bpipe to the catalytic converter on many aftermarket exhausts bottlenecks down to a smaller diameter, compared to the rest of the B-pipe. So check that out when you look at exhaust diameters before purchasing an exhaust. (Please see the my article on how to remove this and get better exhaust flow performance in the Performance, Engine External section).
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Other exhaust characteristics you may want to look at (but aren't as critical as diameter) are:
2. Insulator Material in the Muffler:
Pay attention to what is used as a sound absorbing insulator in the muffler. Stay away from fiberglass or what's called a "glass pack". Fiberglass melts with heat over time and guess what? you become loud as shit and it's not due to more power..it's due to a failed muffler.
3. The Rust Factor: Stainless Steel versus Aluminized Steel versus Mild Steel
Pay attention to materials, if you live in a snowbelt area that uses road salt: get aluminised mild steel or better yet, stainless steel, so you don't rust out your exhaust.
4. Gauge Thickness of the Tubing:
Pay attention to the gauge thickness of steel. A 16 gauge steel thickness (0.65 in.) is thicker than 18 gauge steel. The 18 gauge steels saves you about 20 lb. on an exhaust which is great for racing but can dent easily and is not as durable for the street as 16 gauge.
The same goes for titanium exhausts over stainless steel or aluminised steel. Titanium is lighter but less durable.
5. Hardware Provided
Pay attention to fitment and hardware provided like gaskets and areas for the hangers to hook on to.
Please don't obsess over an exhaust because it makes around 2-4 whp in "bolt-on only" engine combinations. If you plan to make over 185 whp then the size of the exhaust becomes very important.
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B. SOME EXHAUST MYTHS TO DEBUNK FROM BEGINNERS
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!"
quote:
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 l000¡æ) 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 larger can will be quieter),
- having a resonator pipe (no resonator means coffee can 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),
- exhaust tip size/length (big tip is louder) ,
- 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.
A straight-through design 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 cam 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 above..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 oldtimer domestic 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 gas 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.
C. SUMMARY
If the exhaust has the design characteristics you want and is cheaper, get it. Please try not to be hooked by a salespitch 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 identical.
The trick these days is finding a 2.5 in. diameter exhaust for the longer hybrid 4-2-1 and traditional 4-1 headers with bigger JDM-style 2.5 in. collectors.
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.
Remember, if you get more midrange with some exhaust backpressure (the old backpressure myth) in a "bolt-ons only" engine package, ask yourself: what is the loss in hp at the upper rpms with more backpressure? And will this loss in power up top be acceptable to you?
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Posted 2/16/2002 6:33:17 PM
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