Exhaust Modding

[Celadrielas]

As many of you in AZ know, I'm in the process of designing and building my own exhaust. The tragic truth is I've not used my X1 certification in the last 10 years and since I no longer weld or build race cars, my joints are cold and sloppy. Through the hours of research and experience I have picked up in doing this, I realized that many people who jump into or give advice on a project like this really don't quite know what they're up against. It isn't easy to make an exhaust that sounds good, flows well, and isn't too loud. So I'm writing this guide so that hopefully somebody will benefit and make a really nice custom exhaust from it. (Statement not based on this forum in particular, but every forum, guide, and ASE cert book I've been reading. Comparing notes from The forums to my X1 books has been a real eye opener.)

*** THE MYTHS ***

To start with, there are a few myths about custom exhausts that need to be dispelled

1."Custom exhausts are cheaper than bolt-ons"

This isn't necessarily true, in fact, I would say that this is rarely true. If your view of a "custom" exhaust is having a muffler shop replace the stock mufflers, then yes, with the price of a couple cheapo mufflers (~$100) and the welding job (~$100), it will be cheaper than the cheapest ebay exhausts. This is okay if you just want your car to sound nice, but obviously there won't be performance gains. A full custom can still be gotten for cheap, though. You can get mild steel crush-bent by an exhaust shop and then a muffler welded on the end for a few hundred. But it will rust away, crush bends suck, and fitment will suck. Is that really what you want? When we start talking about a quality custom setup, your mandrel-bent stainless steel piping will run you around $300, one or two good mufflers $200-$500, a good resonator $100, a high-flow cat an optional $100-$300, and stainless hangers and flanges, around another $100, and you're now looking at $700-1200 in just parts!

In the end, quality is quality, whether pre-made or custom built. If you cheap out, you'll get crap, but if you save up, research, and make good purchases, your exhaust's quality will rival the best bolt-on's

2. "A little backpressure is important for torque"

This is the default explanation as to why bigger pipes hurt torque. Only problem is, its just not true. Backpressure only ever robs power. Stock exhausts can have up to 18 PSI of backpressure, compared to about 4 from a good aftermarket setup and 2 for a straight pipe. This does nothing for torque- OEM exhausts often have slightly better torque than an aftermarket exhaust simply because they have smaller piping. Without getting too much into the physics, the smaller a pipe is, the faster the exhaust gas flows through it. At low RPM, having a high exhaust velocity helps to clear the exhaust gases out of the cylinders better. Hence, smaller pipe generally equals more torque. However, at high RPM, too small a pipe can mean that the gas can't be moved quickly enough and so you may lose top end power.

On the flip side, restrictive mufflers also reduce exhaust velocity. So restrictive mufflers are just bad for performance, plain and simple. I'll go into piping size and such a little more later.


*** LAYOUT ***

The first thing you want to think about is the layout of your exhaust. If your exhaust is 2.5" or smaller, it would be best to use the stock exhaust's route, as this lets you use the stock hanger locations for best fitment, vibration, and noise levels. If your exhaust is 3", however, it may not fit in the stock channel and you will have to figure something else out. Additionally, it's best to put the cat, resonator, and mufflers in the stock locations as this eases fitment greatly.

Another layout decision is whether you will go single or double. The pros of a single exhaust are that it is lighter and lacks the slight restriction of the y-pipe in a dual exhaust. A dual exhaust, on the other hand, will be more expensive since you will need to buy two mufflers (if you're running them) and more pipe. Welding it up will also be more work. However, you may like the look of a dual exhaust, and it will be a bit quieter since the noise will be split up into two mufflers instead of just one muffler handling sound damping duties.

Additionally, an important decision is under-axle or over-axle. If you go with a dual exhaust, you will be routing the exhaust through the center. In this case, under-axle is probably better since you will avoid a couple bends in the exhaust and it won't be all that low. It will also be easier to weld and disassemble more easily. However, if you are paranoid about bottoming out, go over-axle. With a single exhaust, things are a little more complicated. If you are running the exhaust over at the passenger side of the car, an under-axle will have to be very low in order to clear the control arms when the suspension is fully extended. This presents a much bigger danger of the exhaust bottoming out. Because of this, I went over-axle with my single. It's not much more restrictive and much more safe.

Lastly, flanged sections are important to plan for, especially if your shop welds the exhaust while its on the car. If you forget this step, once the exhaust is welded together you won't be able to get it off without cutting the pipe.

*** THE COMPONENTS ***

Headers:

There is a whole thread on headers, but basically pairing up a catback with a good set of headers will help open your engine up a lot better. If your car is for the street, though, you will want a cat welded / built / bolted in, which leads to the next component...

Catalytic Converter:

Before going into options, there are a few points to address:

- Driving on the street without a cat isn't okay. Cats reduce noise about as much as a resonator, so without a cat your exhaust will probably be loud and raspy. Extending the secondary o2 sensors to behind a cat will also usually eliminate the header CEL, I believe. It used to work on the 07 Subies, anyway. Additionally, even if your state doesn't have emissions, driving a car without a cat is still illegal. And, of course, its bad for the environment. Please save them kittens.

- Whatever you do, DO NOT gut the stock cats! Gutting the stock cat is actually probably the worst thing you can do. Sure, you will have removed the slight restriction of the honeycomb, but in its place will be an open, empty void. On the horsepower side of things, this will cause flow stagnation, which will cause a lot of turbulence and rob you of top-end power. As for torque, its a large section of open pipe, which will cause the gas to expand to fill it and greatly reduce exhaust velocity, and, well, yeah. You get the idea.


Typical high-flow cat:

Anyways, on to options. Any modern cat, even OEM, has the ceramic honeycomb design. It flows well. The most common HF cat brand I noticed is magnaflow, with a number of equivalent-ish competitors. Magnaflow cats use a few tricks to flow better than OEM cats. A magnaflow cat generally costs very little compared to other brands, though I am not sure ifyou can get one from site sponsor. Just haven't checked.

A step above Magnaflow cats are Random Tech cats. I don't know of any competitors with as high-end of a design; Random Tech uses a different honeycomb design that they claim flows much better than any other cats, and there are some dyno charts out there to prove it out there. Not a huge difference probably, so its only worth it if you really want only the best.

Resonator:

A resonator is basically just an inline muffler that goes after the cat but before the main muffler. You don't really NEED a resonator, but I would recommend one if you don't have headers, and if you do have headers, then I would say a resonator is vital to getting a good, non-annoying exhaust note. All aftermarket Tiburon exhausts as well as the OEM exhaust use a resonator. A typical straight-through resonator will add less than 1 PSI of backpressure, so there is virtually no performance loss. While your neighborhood ricerboy may disagree, the quieter your exhaust, the better.

There are a few considerations when you go to buy a resonator:

- It may or may not have sound-deadening packing in it. If it has packing, it will reduce overall noise level as well as tone out the exhaust, reducing cabin drone and raspy / buzzy sound in the exhaust note. If it doesn't, it will still reduce rasp and cabin drone, it just won't make the exhaust much quieter.

- Be sure to buy a straight-through perforated resonator for best flow. Just about every good resonator is straight-through. In fact, your OEM resonator is straight-through, were it bigger than 2" pipe it would probably be sufficient.

- The longer the resonator, the better the sound absorption. Of course, a shorter resonator with a better design may be better, but length is a major factor.

There are a couple brands of resonator that are very good. The flowmaster hushpower II is for the most part the resonator of choice on the market for generic car guys. Vibrant performance also sells some decent resonators.

(mods: no site sponsor I am aware of sells resonators or high flow cats. If one does, I will be happy to change the thread since I'm all for promoting site vendors.)

*** Mufflers ***

Whew, where to start. There are tons of different designs of mufflers (perhaps hundreds) out there and its probably the area where you can lose the most flow as well. Its also the area with the most gimmicks and bad deals, so you really have to do your research when you purchase a muffler if you're not going straight pipe. For a long time now, virtually all OEM mufflers have been reverse-flow. Reverse-flow mufflers use a chambered design to reverse the exhaust flow and absorb a lot of the noise. This complex and restrictive piping layout is how a stock exhaust is so quiet, its just not so great for flow.

The old style of performance muffler is the "turbo" muffler. These are just reverse-flow mufflers with optimized airflow. They reduce noise well, but their flow is poor compared to even chambered mufflers. Nowadays, most el cheapo mufflers are turbo mufflers. There are a few name brands that still produce turbo mufflers, so make sure you know what kind of muffler you're getting before you buy!

Next up the food chain are chambered mufflers. These replaced turbo mufflers as the high-end performance mufflers and are a fairly old design. They try to maintain a somewhat-straight exhaust path whilst also having resonating chambers to cut down noise. These are generally louder than turbo mufflers and around as loud as straight-through mufflers and flow a tad better than a turbo muffler. Their quality varies a lot because there are all kinds of wild chambered designs. Here are a few examples: Hooker Aerochamber Muffler.

The most modern and best-flowing muffler design is the straight-through perforated core design. Several companies only roduce these types of mufflers nowadays, such as Magnaflow, Apex'i, and HKS. These mufflers have a straight pipe inside them that has perforations which allow sound to pass through and be absorbed by fiberglass wadding/stainless steel wool packed in the can. Of these, there are several different designs:

The bad:
- Louvered-core mufflers. These are often a cheap alternative to the high-end perforated-core mufflers. They use louvers instead of drilling out perforations. Sounds fine, except that the louvers create turbulence in the exhaust flow, meaning they're very restrictive.

This is what I'm talking about. Don't buy one of these.

- Louvered/baffled glasspacks (aka cherry bombs). These are just bottle enclosures filled with fiberglass and using a baffled or louvered core. They have the same turbulence problem that the above mufflers do, and they are generally pretty loud. Magnaflow actually offers glasspacks with a proper perforated core. These are fine but will probably be pretty god damn loudloud.

The good:

- Offset. These mufflers are straight-through but inside the can the perforated pipe is at an angle. The advantage of these mufflers is that they get straight-through flow but at a lower noise level. Unfortunately they are big and heavy so they aren't suitable for all applications.

These are your best option if you can find a place to put it and don't mind the weight, because they will have the best noise/flow ratio. Magnaflow, flowmaster, dynomax, and a few other companies offer these.

- X/hybrid designs. There are a few interesting straight-through-ish designs that sacrifice a bit of flow for sound deadening. SSR ultraflow X Is decent, and Borla XR-1 is a pretty neat design if you ask me.

- Straight-through perforated core. These are the king of flow. Their difference in flow versus a length of straight pipe is nearly negligible. They are also small and light. Unfortunately, they are also loud, but a big fat and long can goes a long way towards fixing this shortcoming. The best straight-through mufflers have stainless wadding closest to the pipe, as fiberglass will get burned out over time. If you buy one of these, get the longest and largest muffler that will fit. The bigger the can, the more sound that is absorbed, and there is no performance penalty for a longer muffler. All of Magnaflow's mufflers are very good. I would go so far as to say downright best bang for buck, period. Some of GReddy's mufflers are good stuff, too. HKS hi-power; For $300, it better be good, and it sounds like it does deliver. Other companies that offer straight-through mufflers are Apex'i, DC Sports, and Borla.

*** Piping ***

Type of piping is a vital decision in the exhaust design process. The right type of pipe will ensure a long-lasting exhaust with good torque and power.

Material- If you want to go cheap, don't just get mild steel piping. It will get ugly and rusty almost immediately, and will rust out eventually. Aluminized steel is a better alternative and will keep decently. But really, just get stainless. It isn't THAT much more expensive and will last basically forever. Now, there are two different types of stainless steel. 409 stainless is the cheaper option. Over time it will fade to a dull brown color and does corrode very slightly over time. It also cannot be polished. There's no structural danger, it just isn't as pretty. 302 stainless doesn't fade and can be polished to a mirror finish but is more expensive.

Bends- OEM exhausts use crush-bent tubing. Crush-bent tubing is cheap and easy to shape, and as such, cheap neighborhood exhaust shops are also going to use crush-bent steel in a custom exhaust. However, crush-bent piping is very restrictive. I have been told that a 90 degree crush bent pipe's flow is reduced by 50% when compared to a similar mandrel-bent pipe. That's a big difference. Don't cheap out here. A smaller mandrel-bent pipe is better than a bigger crush-bent pipe. Remember, exhaust velocity is just as important as absolute flow.

Size- compromise in size is very important. Too big of a pipe is going to destroy torque, but too small will choke the car off. On smaller engines, 2" piping is just fine. i4 people would probably be just fine on 2" pipe so long as they stay N/A. 2.25" pipe is also a good N/A sweet spot for both i4 and V6 people. However, if you have plans for F/I in the future, a 2.5" exhaust is a good compromise. Plenty of people here run big power with a 2.5" exhaust with no trouble. Any more and you'll lose a lot of torque while you're still N/A. If you already have a ripping turbo setup, then 3" is the size for you. Of course, if you are running a turbo, you probably already know that and know more about exhausts and tuning than me. heh.

www.summitracing.com & www.jcwhitney.com sells mandrel-bent piping of all sizes and material at good prices. I got 6 302 stainless mandrel bends and 4ft of 302 straight pipe for around $200 when I built my Subie exhaust in 2007.

Hangers/Flanges:

Honestly there's not too much to be said about this aside from get the right stuff. Getting mild steel hangers and flanges will defeat the purpose of expensive stainless piping. Hangers come in a couple varieties. You can get hangars with plastic bushings or pre-bent stainless hangers from magnaflow. You can then weld these to either the piping or quick-remove exhaust clamps. You can also get stainless steel rod (I think I used 3/8" rod) and bend it yourself. If you're getting a stainless exhaust done by a shop, I would buy these myself. Most shops don't have stainless hangers or flanges. Don't forget gaskets either! Exhaust shops ought to have various sizes on hand.

WELDING:

No, I'm not going to write a guide on welding. Truthfully, I've forgotten how, run cold joints, and wouldn't trust myself on my own exhaust much less to even consider helping others do their exhausts. I don't know that much about welding. However, there's a few things you have to take into account when you take your stuff to the exhaust shop:

- Type of welder. Make sure to ask your shop what type of welder they use. The cheapest welders are flux-core arc welders. These do not work for stainless steel, and even for mild steel they're not any good. If the shop tells you that they use an arc welder or won't tell you what kind of welder they use, I would recommend finding another shop Next is MIG welders. So long as you know what you are doing, MIG welders work fine for stainless. The welds won't be super-pretty and they will get slag on them that you would have to grind off, but MIG welding stainless is functionally fine. However, best is TIG welding. In the hands of a skilled welder, TIG welds on stainless will be strong, pretty, and should be slag-free.

- Type of wire. If you use stainless steel, its crucial the welder use stainless steel wire. Otherwise the welds will be weak and rust away, which defeats the purpose of using stainless steel pipe. If you use aluminized steel pipe, you will also have to paint the welds or something, because they will rust much sooner than the pipe.



That's more or less all I've got. Hopefully if you've read through this novel you'll be well on your way to building a good custom exhaust. Happy modding! *** ALSO -- if my information is outdated, please please please let me know. I will be happy to update it and I hope this is helpful since I am working on getting what I want on my FR-S.

[FR-S Matt]

Nice writeup, although changing anything in the catback area is pretty negligible unless you're turbo. You're pretty much only changing sound at that point. You should mention the overpipe\front pipe section if anything when talking about restrictions. Your header description was also pretty vague considering your options of equal length, unequal length, and long tube designs. These are probably the most important points to consider in an exhaust when going for performance.

[Celadrielas]

Originally Posted by FR-S Matt

Nice writeup, although changing anything in the catback area is pretty negligible unless you're turbo. You're pretty much only changing sound at that point. You should mention the overpipe\front pipe section if anything when talking about restrictions. Your header description was also pretty vague considering your options of equal length, unequal length, and long tube designs. These are probably the most important points to consider in an exhaust when going for performance.

The exhaust research I did has been primarily resonator-back though I need to eventually get into headers. There are SO many options and I understand there has to be a lot more research into that.

Regarding the front/over pipe, is it really that restrictive? I've not even touched on that in research. I thought it was basically an empty piece of tubing. Not much to change shy of it's size.

I will do research on headers and front pipes. get to adding that in.

[Cycotech]

Originally Posted by Celadrielas

Regarding the front/over pipe, is it really that restrictive? I've not even touched on that in research. I thought it was basically an empty piece of tubing. Not much to change shy of it's size.

@Cross was talking about the over pipe at BWW one night. He said it was restrictive as well.

[Cross]

I have to remember to come read this I have a feeling I will enjoy having a conversation with you. I also might ask you to show me WTF I am doing since I just bought a welder after all these years lol.

[Cycotech]

Originally Posted by Cross

I have to remember to come read this I have a feeling I will enjoy having a conversation with you. I also might ask you to show me WTF I am doing since I just bought a welder after all these years lol.

[FR-S Matt]

Originally Posted by Celadrielas

The exhaust research I did has been primarily resonator-back though I need to eventually get into headers. There are SO many options and I understand there has to be a lot more research into that.

Regarding the front/over pipe, is it really that restrictive? I've not even touched on that in research. I thought it was basically an empty piece of tubing. Not much to change shy of it's size.

I will do research on headers and front pipes. get to adding that in.

Yeah, you definitely are spot on with the mufflers. Sacrifices to be made by going straighter pipes, chambered mufflers, etc. Sound vs Power. So many decisions. Ultimately though, when you swap catbacks, you'll find the power does not change much versus straight through mufflers.

I'd love to get you a good comparison of the N1 versus Q300. Not sure what the Q300 muffler looks like inside but I'm assuming it's chambered based on the noise reduction where as the N1 is flow through dual canister. N1 you may see a few extra ponies simply due to that, just like you explained.

Front pipe is restrictive, but not as much as the header. Having a regular cat in the front pipe is slowing the flow as well as an extra resonator.

#1 restriction is the stock header though, hands down. The overpipe is not necessarily the best bent piping which I would consider a restriction as most aftermarket ones appear to flow better. Header > Overpipe > Front Pipe are my top 3, then catback for sound adjustment. You would literally have to dyno test each individual part for gains which would be A LOT of time.

[kuhlka]

When I was pricing out how much it would cost in parts to get all of the components I'd need to build a header back exhaust it would probably be close to a grand with all the 3" stainless, resonator, cat, muffler, etc. I've got the equipment and know how to weld, but getting mandrel bends done by a shop would put me close to the price of a full Nameless. Not worth the time, effort, and potential wasted money if I screw up a section and have to redo it. A custom DIY exhaust would be great if you've already got parts lying around or can get a BIG discount. Summit and Amazon didn't have great prices.

[Cross]

Ok I did some reading yeah we seem to share the same views just a slightly different layout as I say you covered much of what I was thinking should be present later on. I will say pricing really varies and knowing where to buy makes a huge difference. If you are spending the say 100 suggested on hangers you might want to think twice about trying to do it yourself.
Also this concern about rusting from an AZ member is interesting. From a Classic Car’s stand point the rest of the world comes to places like AZ because rusting is not an issue here, you know that pun on “It’s a dry heat”.
Also Stainless is not only more expensive but it is also heavier. So while the normal overall goal is a performance gain when you remove that huge factory muffler and piping and go with heavier replacements how much of a negative are you causing? Was it worth it when your climate did not warrant it? Could a Coating have been all you needed?
These are just points to add to this informative article, it’s apparent this is meant to help so I am hoping this will just be taken as additions to help it.

You covered most of this when you got to mufflers but I thought of this during the myth section, #1. When you look at the make-up of a stock muffler sometimes it does make a good amount of difference based off the fact the design of many aftermarket mufflers is unbaffled, a flow through with material on the side. Think of a bigger pipe say 6” with a 3” pipe inside it with almost a thick mesh screen acting as that inner pipe. Inside it is a material used for damping sound giving the muffler its tone or basically what you hear.
However many stock mufflers have piping size changes that are drastic inside them, baffles and resonance chambers. Getting rid of all of that for a flow through can be a noticeable change not to mention a weight loss.

When it came to gutting cats and the rear O2 sensor I thought you might want to add this information as it may help. Another common trick used for the secondary O2 CEL is to take a Spark Plug Antifouler (The long one, if you look on many header designs for our cars you will see the secondary O2 port now has this really long pipe essentially on it, it’s the same idea) drill out the tip and it threads right into the O2 Hole as well as the O2 Sensor threading into it moving it away from the exhaust gas in an attempt to fool the sensor. For some car’s it works great, others it just will not fool the sensor.

As for gutting cat’s the above can be true but there is a very simple solution exhaust shops have been using for years. When you gut a cat you do leave a large open space that can come some odd resonance noises and flow changes. But most openings inside the cat are bigger than the flange connections. Also most exhaust shops can expand the ends to allow another in.
The point of this is to take another pipe (Say you have a 2.5 Exhaust, you slightly expand the hole after gutting the cat and install another straight 2.5 pipe inside it) and put it through the inside of the cat. This eliminates the gasses expanding because as far as they are concerned they are just going through a pipe there is nowhere for them to expand. This does take a shop with the tools to do it but most shops have these tools. This means you can punch out your cat and stop the concern of the gases expanding in the free space. The nice thing is you also should not be adding weight considering everything you just removed.

[Cross]

Originally Posted by Cycotech

Seriously I bought one in my twenties that was a non-gas model, used it for a few months and was ok with it then sold it when I moved. Last weekend I found a new Weldpack 150HD on sale and decided what the hell lol.
I have never used gas though so its going to be a learning experience.

I also just took stuff to Matt when it needed to be welded and said I want this. A few beer's later I had it or a better version we thought up while drinking... which sometimes didn't quite work out as we planned.....

[Celadrielas]

@Cross - Coming from CA (with the seabreeze air in CA) and family from the midwest, whenI was learning to work on cars, my father mandated I consider things like rust. I guess it still runs in my logic.

You have great points though, and your information is great and things I overlooked. The silliness to all of this is I am still running on bone stock exhaust due to funding and when considering the mod, I am trying to look at all angles.

@FR-S Matt -- Exhausts like the N1 and Q300 are both fantastic for these cars, especially when paired with a tune. The differences in them are minor and the companies have spent a lot of time and energy on the R/D process making it amazing changes for both the sound and the feel of the exhausts.
@kuhlka -- Building your own exhaust can be EXPENSIVE. especially if you're trying to R/D your own for the best performance and power for you. You're absolutely correct. I checked the new pricing on JC whit and Amazon as well as summit and a couple other places. Piping is just rediculous.



Thank you to all users who read this. To be honest, I sort of felt like this thread would be just for me and left to the way side, but I am so thankful to have the input from people who've gone past my mod level and have some amazing set ups. Not to mention the PMs I've gotten from people. Thank you so much for the added information.