Fight droning / resonation!

[Julio Bro!]

In my search for a good exhaust system, with power and low noise inside the car, experience and talk with exhaust technicians have helped me gain some knowledge which I want to share.

Not everyone is willing to sacrifice comfort and hearing health for an aggressive tone or a few horsepower. Many opt for 2.5" or 3" systems for the N/A 240SX, but usually it means an almost unbearable droning, particularly at low rpms. The worst is at the 240SX's 3000rpm mark; beggining at about 2500rpm, the resonation starts and peaks at 3000rpm, then lowers until 3800rpm. On 2.5" or 3" systems this drone persists up to 4500rpm.

Resonation is a complex issue, some of the factors include tube size, length of system, and type of muffler. The OEM formula aims to quiet comfort, with no vibration, and maintaining a performance goal. To obtain this, tubing is small (1.75"-2"), mufflers are tri-reverse-flow, and resonators are used.

The OEM type muffler relies on reducing tube diameters and noise cancellation by means of resonating chambers within the muffler, all this to quiet the sound and provide a "pleasant tone". Some cars, like the 240SX, have additional resonators in the system. These are not mufflers, they work more like "frequency filters", taking care of molding the tone more than the loud factor; although some decibels might be substracted. The 240SX's resonator can take about 7 decibels off the sound, but reduces the diameter to almost 1" inside the unit, which decreases high end power.

Just as important is the tube size. This has to provide a fast moving path for the exhaust gasses; you see, these must flow fast enough for the scavenging effect needed for proper torque in the low range. Slow flow, low torque; and more noise.

Why more noise? Combustion is essentially an explosion, explosions are mostly low frequency; big sound waves with lots of energy. The tubing maintains the size of the wave in check until it exists the tube, it also helps increase it's frequency so it isn't tooo low. Large tubes provoke vibration and drone at low rpm, because of the big low frequency wave traveling; also torque is reduced because of the low speed of gasses, which makes the engine work harder.

Try this, take air in your lungs and blow with your mouth wide open and feel the airspeed with your hand. Now take in air again and blow, but slowly close your mouth; now you can feel the airspeed increasing as you reduce the size of your mouth. The same applies to the exhaust system, it needs some restriction to produce good torque at low rpm. So, unless you are planning to race, large tubing is not good for everyday use.

The size issue also applies to the mufflers, the bigger the tubes inside the muffler, the larger the wave inside, and so, more noise. Mufflers are key in all this; a 2.5" system with just an OEM muffler will sound great with no droning and good torque, just not much of a power increase. The same system without a muffler will have a little resonation, not good torque, very good high end power, but will be veeery loud outside. Decreasing the tube to 2" will have less resonation, good torque, good high end power, and will be loud, but less than the 2.5".

Most performance mufflers rely in absorbing sound, which alone don't eliminate the quantity of decibels needed for near OEM sound. To get more sound control, absorbtion needs to be increased and so longer mufflers and pre-mufflers maybe needed. Some brands, like Flowmaster, use noise cancellation technology, but their aim is at a "power tone". So even though droning is reduced, is not that much.

You get 2 things here:

1) The closest you get to a straight tube, the less droning you'll get; but louder outside.

2) For N/A engines, the larger the tube the less torque and more noise you'll get; but more high end power. The exception would be highly modified and nitrous engines, which would have high compression pushing the exhaust harder.

All in all, if you would like to maintain a level of comfort with little droning and good overall increase in power, stay with your OEM tubing, exchange the resonator for a matching size glasspack type muffler, and exchange the end muffler with a matching size straight type or turbo type (use the longest one you can fit). Turbo mufflers tend to give a bit more droning because the wave get's exposed inside the case, but are quieter outside, and maintain torque.

The glasspack type pre-muffler is not a resonator, but being a small diameter straight tube muffler, helps to control droning and takes decibels out of the end sound. Just fit the longest you can so more sound gets absorbed.

To increase power you may have new 2" or 2.25" tubing made at the muffler shop or buy complete systems. The BRM systems sold in the forums are excellent choices, but stay within the conservative 2.25" for good torque and low resonance. To still have good torque, but louder sound, you can eliminate the pre-muffler (resonator) and/or increase the size of the end muffler.

Other quality brands are Magnaflow and Dynomax.

If you have a current drone problem, begin by decreasing the size of your tubing; you should notice an immediate increase in low end torque and a smaller range in the drone rpms. If you don't have a pre-muffler, install one matching the new smaller diameter, or switch your current one with a smaller diameter unit. If still not satisfied, change the muffler size and or type.

Remember, there are no zero drone performance systems, yet. I just hope this helps many of you to make a better choice and not waste as much money as me; at least you can make your decision based on knowledge and spend it well.

[Vkoslak]

The following is from "http://www.specialtyz.com/"

--------------------------------------------------Having had tons of hands on experience and "sit down" chats with engineers. 2.5" piping is recommended on a car like the Z32. One 3" pipe can flow 900 HP, imagine that? We have dual piping and you want 3" piping? We hope you can make 1800 HP! 3" piping is way too big and slows the exit speed of the exhaust. Not to mention it can create unwanted turbulence; loss of low end response & power too. If you insist on 3" we do still offer it. --------------------------------------

It would seem that a 3" exhaust would be overkill on damn near any engine. If one 3" pipe can flow enough for 900hp, what can a 2.5" or 2.25" exhaust flow?

When I get around to ordering my BRM exhaust, I will most likely get the 2.5" since I plan on shoving an rb of some type into the 98 240sx

[Julio Bro!]

He, heee! I don't want 3" piping, I'm mentioning that some people have installed those in the 240SX.

I don't have those numbers, but I'm sure they're in the forums somewhere. What I know from experience is that for 2.5", high end power is explosive and acceleration is fast and strong. With 2.25" is not that explosive, but still strong and a lot better than OEM.

What is "rb"? Roots blower?