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Home > Performance Mods > Exhaust > Exhaust Rasp

Exhaust Rasp

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Orin
Axle-back Myths


There seems to be an enigma surrounding exhausts, I'll cover a few topics to help you create the best exhaust system without much confusion.

Contrary to belief, a louder exhaust doesn't mean you're allowing a greater flow of exhaust. There is a lot of hype around getting Brand X because it's special design is going to allow your car to smoke wheels in 2nd gear. You should also buy our our brand of JDM created axle-back exhaust because it's much more powerful than Brand B

Don't believe the marketing!

The first myth is surrounding axle-back exhausts and the amount of back pressure relief. What if I told you to just close your eyes and pick an exhaust and you'll get the same power improvement with Brand A as you did with Brand B

"How is that possible? Brand A website claims 8-10 hp gain from it's axle-back!"

The difference in back pressure relief is so slight by removing the tC's stock system that it really doesn't matter which axle-back you choose. No company is going to create an axle-back system that is non-flowing. If you somehow managed to purchase an exhaust that is more restrictive than stock, you shouldn't be driving.

"...so how do I know which axle-back to choose?"

Your primary concern should be fit, quality & tone. The differences in axle-back exhausts will vary in these aspects. One of the biggest aspects affecting the tone of your exhausts and the volume it produces is insulation. Insulation quiets the hard waves of sound moving vibrating through the metal. Depending on the quality of sound absorption material your exhaust tone and volume will vary. Design will also affect an axle-back's tone. Think of your axle-back brand as a musical instrument. As with all instruments, the sound is affected by numerous variations, the most significant factor being the engine itself.

"How much gain to my wheels can I expect with just an axle-back?"

You can expect anywhere from 1-2 wheel horsepower, on average it turns out to be somewhere in the middle. (1.5 whp)



Exhaust Tone & Rasp


You'll get the best power production with a full exhaust setup and there are many types. You'll hear about, cat-backs, axle-backs, turbo-back and full custom.

Part of completing a full exhaust set-up involves installing a header. The header is a vital part of your cars exhaust and it's performance will vary depending on design aspects.

There are different types of coatings for a header, you can have ceramic coating or stainless steel as well as specialized coatings for heat reduction. Just as with exhausts, the quality of a header will vary, there are important aspects to consider such as quality of welds, coating, design and most importantly fit.

My point of this guide isn't to help you pick a header, I'll be covering the most common problem with bolt on enthusiasts, Exhaust Rasp

"What is rasp?"

Rasp is the awful sound of too much bang and too little hush.

"What?"

Most after market header bolt ons are just bigger chambers of pipes tunneling down a bigger collector or drain pipe. You're car can release it's exhaust at a greater velocity because you've bolted on a part which allows for greater exhaust flow. These pipes are handling extreme temperatures and major explosions coming from the engine itself. These combustions are violent, harsh explosions are bouncing all around and back and forth through the pipes. The frequency of your car's exhaust has been changed dramatically and often the sound is too much for any muffler to handle. When some of those violent sound waves make their way out of your 4 cylinder engine and to your muffler, the sound isn't pretty at all. (although some people like it)

This new explosive tone is called rasp.

Rasp is often the failure of insulating and reflective components within the exhaust pipes and chambers. Most owners are not pleased with how the frequency has been changed and since most aftermarket exhausts are tuned for a factory setup just about all mufflers will experience the loud and violent sound of rasp.


^this video shows you what rasp can sound like, although this setup is a very mild case. Enjoying the sound is a matter of taste.

"How do I quiet down the rasp?"

Since rasp is mainly the result of unfiltered sound waves, anything to deaden or cancel out these violent and harsh frequencies will help greatly. The most popular way is adding resonators.

"What is a resonator?"


^image of resonator before being welded in.

The simplest way I can put it is a chamber that reflects sound back at itself so they cancel each other out. The chamber is designed in such a way that it doesn't greatly affect the exhaust velocity or cause too much back-pressure. I won't go into detail about resonator design because another thread covers it.

The amount or length of resonator needed will vary depending on the tone you'd like to create. Most Scion tC owners prefer a smoother deep tone. It's been discussed that creating this tone would be best achieved by using 30 inches of resonator total. It's not likely that you will find one resonator that is 30 inches long, so welding 2 resonators is often required.

Another step in deadening sound is adding a catalyst converter in addition to the resonators. Because of how a catalyst works, some back-pressure will be created, but not enough to hinder your overall performance. The advantage to having a catalyst welded into your pipes is keeping harmful emissions down as well as canceling out some of the harsh sound-waves.

Since aftermarket bolt ons such a header & s-pipe remove factory catalysts, your vehicle will become emission illegal. You will need to check your County & State Government's clean air laws.

If you live in the state of California, don't even bother reading this post, you're already C.A.R.B. failed for thinking about full exhausts systems.

My best advice for achieving the best exhaust system would be to go custom, this involves choosing the axle-back of your choice, getting a mid-pipe chopped up and welded with resonators (also your pick) and choosing a header & s-pipe that will complement your performance desires.

Most exhaust shops will not do any work that will alter or remove any emissions systems from your vehicle.

Any exhaust shop should be able to provide a steel 2.5 inch diameter pipe without the welding. If they claim they cannot then walk out, they're not a true exhaust shop to begin with.





related thread posts:

Web's Exhaust & Resonator Design Thread
This post will explain the designs of resonators and also explain the need for some back-pressure as well as exhaust diameter.




please do not ask questions about which __________ to buy, you can open a new thread and discuss that

keep posts on topic, off topic posts will be removed by moderators or administrators

last update 08/08/08 ---11:15 EST------minor word changes

[#] Mar 10, 2008 12:45pm
Orin
hopefully this can be used as a reference when it comes to exhaust rasp.

I'll continue to update as needed

[#] Mar 10, 2008 01:29pm
Madpanda331
Orange
California, US
  • Tech Advisor
 THANK!!! this will help alot!

[#] Mar 10, 2008 01:59pm
perrydog85
Sterling
Virginia, US
  • Tech Advisor
 Good information in this post,keep it coming...

[#] Jul 01, 2008 02:26pm
Orin
I forgot I made this.

If anyone wants to expand on this or can think of some extra resources to add to this thread, please feel free.

[#] Jul 01, 2008 03:33pm
krdshrk
New Brunswick
New Jersey, US
  • Godlike Advisor
 ... Fix the video link you have - it's brokededed


Scikotics NJ is sponsored by Lawrence Scion

2006 GReddy Turbocharged Automatic tC
220+ WHP - 14.044 @ 97.02 MPH
[#] Jul 01, 2008 04:12pm
ssm_251
Mobile
Alabama, US
  • Detailer
 ^nice.....thats my video...lol it is a bit raspy, but i plan on gettin a resonator at some point but i have heard much worse.

[#] Jul 01, 2008 07:53pm
DURMIAKI
Pennsylvania, US
  • Tire Changer
 I HATE!!!!!!! that weed wacker sound ...thnx for the solution...even thou its gunna cost me more money

[#] Aug 07, 2008 12:41pm

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