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post #1 of Old 09-10-2012, 12:46 AM Thread Starter
Spartacus
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Want to build a custom exhaust? Read this first!

As many of you know, I'm in the process of designing and building my own exhaust. 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.

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 18PSI 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 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 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. If your Tib is an 05-06, extending the secondary o2 sensors to behind a cat will also usually eliminate the header CEL (see the CEL sticky for info on this). 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 the 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 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 around $100, you can get one from site sponsor NOPI.

However, 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. Not a huge difference probably, so its only worth it if you really want only the best.

Here's a random tech cat's honeycomb:

Random tech cats generally cost around $180-$200

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 1PSI 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. Vibrant performance also sells some decent resonators.

You can find both of these resonators at www.summitracing.com
(mods: no site sponsor sells resonators, I checked. If one does, I will be happy to change the link)

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.

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.


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Last edited by Spartacus; 09-15-2013 at 01:40 AM.
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