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Header-Exhaust Design Effects on Engine Power

Posted 02-10-2002 at 12:00 AM by MichaelDelaney

Some common questions I often get on Honda forums are:

a) Should I get a 4-2-1 (tri-Y) header or should I get a 4-1?


b) Which exhaust has a throaty deep sound and the best for my setup?

You may want to understand some basics about header and exhaust design, before making your decision about purchasing one, so that your choice will be an informed one.

You have to remember that it is called an exhaust SYSTEM for a reason.

Getting a header without considering how the header fits in with the cat and exhaust could prove to be a mistake you will regret in the future, if the header turns out to be incompatible with the other system parts.

The same can be said about an exhaust...it must be chosen with consideration for how the exhaust fits in with the header and cat design. You may not want to buy these parts without thinking about the other parts of the system.

They work together as a unit to extract as much exhaust flow speed as possible.

In this 5 page article, I'll try to show various aspects of both header and exhaust design and how these design characteristics affect your car's power output. This fairly long article is split and shortened into more specific sections elsewhere in the Articles Performance Engine Externals, if you are only interested in info specifically related to the header only or exhaust only.


For specific information on various header brands for the Bseries, please go to this link of my other header article at:


It is bandwidth intensive with many jpeg's and so please be patient and allow it load if you have a 56k modem.

The most common thing you hear is 4-2-1 makes more midrange power at the sacrifice of peak hp and 4-1 makes more peak power at the sacrifice of midrange power.

Is this always true?

Not these days, when we have long hybrid 4-2-1's which combine the extra length of a traditional 4-1 and the layout of a traditional 4-2-1 (or tri-Y as they are sometimes called) into one header. Why is this new and different?

The header's primary tubes, secondary tubes, and collectors length & diameter affect WHERE power is created along the rpm range.

To understand how header design affects power and powerband location, you have to first understand how the ACTUAL or REAL torque curve is shaped relative to the IDEAL shape of the torque curve when there is maximal cylinder filling, as shown in the graph here below:

Explanation of the above graph:

Exhaust gas flow velocity (or flow speed) determines where peak torque occurs along the rpm range. As a general rule, when exhaust flow velocity reaches the mean value of 240-260 ft/sec., peak torque is achieved.

Peak torque also marks when your engine has achieved it's highest volumetric efficiency (or maximal cylinder filling ability). Control how fast you can get up to a mean flow velocity of 240-260 ft/sec by looking for certain header-ex characteristics or design and you control WHERE peak torque occurs.

Notice that even if you can get optimal cylinder filling to achieve optimal volumetric efficiency, other factors, like how much exhaust gas you can remove from the cylinder after combustion, affects the maximal "actual" torque you can squeeze out of your engine package.



The 2 goals of a header-cat-exhaust system 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 still left 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 wave that acts like a vacuum. This vacuum 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, 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.



There are several aspects of header and exhaust tubing that affect when a mean exhaust flow velocity of 240 ft/sec. is achieved:

1. Diameter (or header tube cross-sectional area) :

Bigger diameter shifts peak torque to a higher rpm compared to a smaller diameter.

The bigger the diameter, the more cross-sectional area. Exhaust flow must overcome this extra tube cross-sectional area and therefore the flow travels slower . It takes the rpms to climb to a higher rpm before the speed of 240 ft/sec (and therefore, peak torque) is reached. So increasing diameter shifts when 240 ft/sec and peak torque is achieved to a higher or later rpm, because it takes lon for the air flow speed to reach 240 ft/sec.

In addition, a bigger diameter will increase the actual peak torque number (i.e. not only does diameter change the location, it also increases torque) .

You can also vary diameter, as well, along the length of the header tube: This is called "stepping" the header. A "stepped" header will have along it's length the diameters gradually increasing as it moves towards the muffler end and away from the engine. Stepping a header will prevent exhaust flow from travelling backwards to the engine (called reversion). Stepped headers therefore have anti-reversion characteristics, as well as achieving a broader powerband.

Figure 1. Here is a pic of stepped diameters on a Toda header where the diameters start at 45mm near the flange, then gradually increases to 50 mm further down at the secondaries, and 60mm just before the collector. Several good aftermarket headers are stepped.

Some people port the JDM ITR or DC JDM 4-1 flange ports to a little larger diameter than the cylinder head exhaust port diameter to get this stepped effect early. Some people also port the JDM ITR 4-1 collector. Here's how much to dremel port the JDM ITR flange ports:

from Dave Stadulis at SMSP

you don't want to have the exhaust port on the head exactly matched to
the manifold/header...I've been told to have the header port about 1mm
(.039") larger all around the head port and no larger than 1/16". This
provides an anti-reversion attribute to the header. The same goes for
steps in the individual tubes.
2. Length:

Longer tubes will create more torque at the rpms before peak torque.

How do they do this?

Longer tubes will speed up air flow velocity. The flow velocity of 240 ft/sec and peak torque will occur at an earlier rpm compared to a shorter tube. Changing the length of the header primary tubes does not increase the value of peak torque like diameter does. Instead length changes the behaviour of the torque around peak torque along the rpm band.

If you imagine the torque vs r curve from a dyno to be like a see-saw: then, on a see-saw there is a point where the plank sits to allow it to rock up and down. This is usually in the middle of the see saw and is also called the fulcrum. On our torque vs rpm curve, imagine the peak torque to be the fulcrum, although this fulcrum doesn't necessarily have to be in the middle like the see-saw...it can be moved. Changing length "rocks" the torque curve about the peak torque.

If you have a longer primary header tube, the torque curve will "rock" in such a way that the left side is higher than the right side. There is higher torque at earlier rpms before peak torque. There is less torque at later rpms after peak torque.

If you shorten the length of the primary tube, the torque curve will will have the see-saw with the right side higher than the left. So there is more torque at later rpms after peak torque.

3. Merge Collector Diameter, Length, Angle, and Layout:

In terms of header layout, merge collectors are the portions of the header where the tubes join.

So in a 4-2-1 header, the 4 primary tubes are first joined at a collector into 2 tubes. The 2 tubes are then joined by a second collector into 1 tube.

In a 4-1, the 4 primaries are joined at only 1 collector into 1 tube.

In some cases, the collectors are in a box shape where 2 tubes are stacked directly on top of the other 2 tubes. In other cases, the collectors have the top 2 tubes offset from the bottom 2 tubes. This is called a tri-Y collector. The box collectors give less header ground clearance than tri-Y collectors.

Hytech Tri-Y collector

Hytech Box Collector

The collectors join the tubes and co-ordinate the 4 exhaust pulses leaving the primaries.

Shorter, large diameter collectors have more peak power.

Longer , smaller diameter collectors have more power in the midrange.

The angle of the merge collector tubes should not be steep or sharp, in order to keep the energy or speed of the merging puls coming from the tubes at a high level.

For example, the stock ITR header has a less steep merge collector angle than the stock GSR header (see SurferX's article on the features of the ITR). So, the diameter of the collector affects the flow volume or how much exhaust gas can be removed and how much peak hp can be achieved. The bigger the collector diameter, the higher the peak hp you can achieve. This is why the better headers have larger 2.5 in. collectors instead of the usual 2 in. collectors in some aftermarket headers made to match up to the stock catalytic converter 2 in. flange.

4. How the Header Primaries Are Paired - Sequentially versus Non-sequentially:

the ignition firing order determines which exhaust pulses leave in a particular order. In integras it's cylinder # 1,3,4,2. How we pair the header's 4 primary tubes together at the first header collector determines the horsepower vs rpm curve's characteristics or shape. Sequential pairing allows for a broader powerband and better acceleration properties from an engine.

You can look at your header and see which tubes are paired together: Is it sequential: 1 with 2, and 3 with 4? Or is it non-sequential? 1-4, 2-3?

Originally Posted by SMSP

...the firing order is 1-3-4-2, if we add a few more cycles so we repeat it looks like 1-3-4-2-1-3-4-2-1 etc.

So with a 4-cylinder engine how many tri-y configurations can we have?

If cylinder #1 is paired with #2, then #3 and #4 are paired.

If cylinder #1 is paired with #3, then #2 and #4 are paired.

However, both these set-ups are considered sequential pairing because each secondary gets 2 back to back pulses.

Therefore, these set ups are the same and can be considered as 1 configuration.

[Editor's Note: The HyTech header pairs 1-3, 2-4 sequentially. The SMSP header pairs 1-2, 3-4 sequentially ]

Next we pair #1 with #4, and then #2 and #3 are paired. This is considered non-sequential pairing, since the pulses alternate from one secondary to the other.

We can't pair #1 with anything else and so the fact of the matter becomes there are only 2 ways to configure a 4-cylinder tri-y header.
Here's Larry Widmer's (of Endyn) take on sequentially pairing the header primaries (i.e. 90 crankshaft degrees apart from one another instead of 180 crankshaft degrees):


The reason sequentially pairing of header primaries works is due to the energy imparted to the exhaust charge. If you just do 180 degree timing on the exhaust side, the exhaust pulses are evenly spaced, and they do permit a certain amount of "tuning", as opposed to just dumping everything into one collector.

When you space the tubes so there are more sequential pulses, the energy from one tube will have a much greater impact on the cylinder it's paired with, and the combined energy will have a much greater effect on the other tube it merges with.

Even (non-sequential) spacing (i.e. pairing header primaries from cylinders 1 with cylinder 4 and pairing cylinders 2 with 3) is nice and smooth, but pairing sequential pulses provides more energy to work with.

It's similar to the use of two single cylinder 2-stroke engines. If you want long running and smooth operation, connect the engines where they fire at 180 degrees to each other. If you want ball-busting acceleration, fire them together. It's all energy.

You get the same amount either way, but the combination you pick will allow you to properly select the energy spread.

On the exhaust side, you're dealing with waste heat, so if you can make it help scavenge other cylinder(s), you're simply not wasting as much energy.
5. Header Layout : 4-1 vs 4-2-1:

A 4-1 header layout will have peak torque occurring at later rpms compared to a 4-2-1.

Newer hybrid headers of the 21st century are a fusion of the old 4-1's extra length with the 4-2-1 layout, have stepped diameters, and have large diameter collectors. So you have low end peak torque with enough breathing capacity to support more peak gains (the best of both worlds).



So the old that 4-1 = more peak hp with a loss in midrange torque and 4-2-1 = more midrange torque with less peak hp is an obsolete idea.


Our goal with header design is to control when mean exhaust flow velocity = 240 ft/sec is achieved along the rpm band. Our second goal with header design is to determine the width of the peak torque curve.

Shorter primaries, larger diameters, 4-1 layout, and big collectors will have higher peak hp because 240 ft/sec is obtained later along the higher rpms. The torque is kept higher after peak torque.

Longer primaries, smaller diameters, 4-2-1 layout, and smaller collectors will have peak torque occurring earlier in the rpm band because 240 ft/sec is achieved earlier and the torque before peak torque is kept higher.

We can sort of mix and match though. Typically you can look at the DC Sports headers to see what is the conventional or usual layout of headers: long primaries 4-1 or short primaries 4-2-1.

Are we stuck with only long primaries 4-1 vs. short primaries 4-2-1 only?

Custom header manufacturers like Hy-Tech in California have figured out a way to overcome each design's shortcomings and created a hybrid header. The all motor headers are usually tri-Y in layout but they use big collectors to get more peak but longer primaries & secondaries and stepped diameters to obtain a wider beefier midrange power than the conventional long primaries 4-1. You get the best of both worlds: good midrange power and enough breathing to support more peak power.

The Supercharger headers, on the other hand, can achieve 240 ft/sec quite quickly from the boost and don't need scavenging from the header-cat-exhaust system to generate this exhaust flow speed. They need an exhaust system which will efficiently remove as much of the combusted , inert exhaust gas out of the cylinder as possible. So SC headers tend to be much larger in diameter, shorter, a 4-1 layout, and have huge 3 in. collector.

So the next time you buy a header, think about where you want your peak torque to be along the rpm band first. Do you want peak torque to be at a lower rpm ? Do you prefer big hp in the last 3000 rpms of your rev range? Do you need both? Look at the header design characteristics to determine which header has the right layout for the power curve shape you want.

Now on to exhausts!...


Generically speaking, there are 3 exhaust designs:

1. straight-through

2. 2 pass, twin loop, or sound-cancellation design

3. chambered

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 pipes, or chambers.

The majority of popular 60mm diameter aftermarket exhausts are straight-through designs like Apex N1, Tanabe Racing Medallion or G-Power, Greddy 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 characteristics 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).


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.



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


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.

The trick these days is finding a 2.5 in. diameter exhaust for the longer hybrid 4-2-1 and traditional JDM 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, as yourself this: 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?


The header-cat-exhaust system works as a single unit not as 3 separate distinct parts. You don't add a header and it makes 6-7whp and then add an exhaust and it makes another 3-4 whp. That's the old way of thinking.

These days we see header-cat-exhaust systems, if they have the right complementary characteristics to each other, capable of making 10-18 whp up top and 10-12 whp in midrange. This was unheard of for street headers even 2 years ago. The characteristics you see in these big systems are matching diameter from header to exhaust and added length in both the secondaries and primaries on a sequentially-paired, stepped tri-Y layout.

The single most neglected part of this unit is the catalytic converter. Everyone upgrades their header and cat but keeps the stock cat. The aftermarket scene now has affordable higher flowing cats. The most recommended brand is the 94000 Series Carsound (not Random Tech) by the major custom header-exhaust builders for proven-to-be-fast Hondas like HyTech and SMSP. The Magnaflow cat is a Carsound cat.


Originally Posted by Dave Stadulis of SMSP

The larger cat is the 2-1/2" 94006 built to fit a JDM header and a stock B pipe flange location. The smaller cat is the 2-1/2" 54006 built to fit the early GSRs with a JDM header and stock B pipe location. In the link you'll also see the difference between the 2-1/2" ID cat outlet and the stock ITR B pipe inlet of 1-3/4" ID.

As for differences, I use a slightly different style floating flange than some of the others but this doesn't make a difference in fit or performance. I do tack weld the heat shield in multiple locations down each side now. Some of the Carsound spot welds had broken in the past causing a rattle. I don't know of any rattles since I started doing these extra welds back in the early summer.

The 94005 is the 2.25 in. version and flows more than a stock NSX cat made for a 3.2L engine. In fact, the 94000 series cats are capable of flowing enough for a 5L engine!

You will need to weld on flanges and in OBD2 cars a second oxygen bung. Some shops ( www.ipsracing.com , SMSP email smsproducts@novacoxmail.com ) sell this cat with the flanges and bungs already welded on.

For people with big cams and headwork, it's a must that you upgrade to a 2.5 in. collector header, cat, and ex system. The Carsound 94006 2.5 in. cat is available.

You will need a local muffler shop to help you install this cat, since it is longer than the stock ones. You'll notice that the newer longer headers do not use the stock mounting points as well. It takes a little hassle and working with a local muffler shop to install these but in the end you will separate yourself from the crowd who has settled for convenience over performance.

You may need O2 sensor wire extensions for some Carsound Cats, if the aftermarket header is longer than stock headers. There are newer Carsound models which have the O2 sensor bung hole for the back sensor (OBD 2) at the stock location even though your header is longer (contact SMSP for details about these newer models).


A properly sized exhaust SYSTEM (from the header collector to the cat to the exhaust), "releases" or extracts the entire power potential of an engine. The engine had made this power but due to ineffiencies in the exhaust system (backpressure and reversion), the power was stunted and never showed up at the wheels. So in reality, a good exhaust system does NOT create power. Instead it RELEASES power already made. Most people use the term "uncorking the engine". If you have the wrong size anywhere along THE SYSTEM, you have effectively choked or corked the engine from releasing the power it is capable of. Exhaust gases sitting in the cylinder and slower exhaust gas speeds exiting the cylinder do nothing for you.

Here is an example of how an exhaust with the wrong size can hinder or hold back performance even though it does not "make" hp.

This is the Skunk2 B20B built by Steve "Omni Man" Rothenbuehler.

-Machined Stock CRV Pistons yielding approximately 11.2:1 CR
-Custom Crower rods
-GSR Head Heavily Ported And Polished (By Omni)
-Crower Titanium Retainers
-Skunk 2 Dual Valve Springs
-GSR Valves
-Skunk 2 Cams
-Skunk 2 Intake Manifold (Extrude Honed)
-Type R Throttle Body
-RC 310 Injectors
-MSD 6AL Ignition
-B&M Fuel Pressure Regulator (Set At 58 Psi)
-Skunk 2 ECU
-Apex-i VAFC
-Comptech Cold Air Ice Box Intake
-DC Sports 4-1 Header (2 1/2" Collector)
-Skunk 2 Test Pipe
-Greddy Exhaust

They removed the "too small diameter" Greddy exhaust(uncorked the backpressure) and ran an open header with the Skunk2 testpipe only just to see what they got:

Now do you still want a little backpressure? I hope not.
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