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Author Topic: Subaru Legacy N/A , Achieving Da Boxer Burble.  (Read 14958 times)
(((buRRRble-B4)))
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« on: April 23, 2009, 12:51:11 pm »

GUIDE: Achieving the Boxer Burble on an NA Subaru --------------------------------------------------------------------------------

In light of the recent flood of people asking this same question, I have decided to write a guide to hopefully answer all the questions and remove the need for a new thread every day. If one of the mods could make this a sticky, it would be great.

It should be noted that the information and conclusions in this guide is derived from my understanding from my own personal research. I am not an exhaust mechanic and am far from an expert. You should seek professional advice from multiple sources before spending your money.

If you see anything in this guide that you disagree with, let me know and I will annotate this post. Once you can see the annotation has been made, please remove your post to keep this thread purely "information Only". No chatting in here please guys.

HERE GOES...

STEP 1: INDUCTION.

The stock induction system on NA Impreza/Legacy is very restrictive. Essentially, there are two different types of induction systems.
Pre-1998 EJ20 engines had what was called a torque chamber which sits behind the intake manifold. The piping then runs down to the airbox which contains the air filter. Many different configurations are available for this design. I have experimented with all of them and found THIS achieves the best results when used in conjunction with a pod filter.

The second option with this design is to replace the paper air filter with a high flow panel filter, remove the silencer as explained in the above link, and simply duct the air box to the front guard.

The third option is to remove the torque box and replace all the plumbing with a Ganzflow style induction system. This is not recommended as you loose a lot of low end torque.

On Post 1998 Imprezas, the torque box has been replaced by the air box. The air box is mounded behind the intake manifold. The best system for these model cars is to replace the paper air filter with a high flow panel filter and duct the air box to the front guard. you can do your own ducting or purchase a Ganzflow induction pipe.

Option 2 is to remove the air box completely and replace the entire system with a injen style induction system.

STEP 2: EXHAUST

The standard exhaust system is quite restrictive. I would recommend installing a new exhaust system from the Catalytic Converter back. For Imprezas, you should not use a greater diameter than 2.25inch because you will loose too much back pressure and performance will be adversely affected. However, speak to your exhaust mechanic for professional advice.

It should be noted that if you wish to fit MRT extractors at a later date, you should have the whole lot done now because MRT extractors do not bolt up to the original exhaust system and you will end up replacing part of the mid section of exhaust again if you use them.

STEP 3: MUFFLER.

Obviously this step should take place at the same time as step 2 to save money on installation.

I can not stress how important it is to choose a high quality muffler. Cheap mufflers are likely to give you quite a nasty raspy sound when the rest of the exhaust system has been modified, especially if extractors have been fitted. As the exhaust note desired is a matter of personal choice, I highly recommend coming along to one of our cruises and having a listen to our different exhaust setups, or shopping around for sound clips on the internet before making a decision regarding a muffler. This should give you a good idea of how to achieve the result you want.

STEP 4: HEADERS

Boxer engines are capable of producing a wonderful "burble" sound when the exhaust is configured correctly. The headers, along with the muffler will have the largest overall effect on how your exhaust sounds. Before learning about what options are available, it is important to understand how this "burble" is born in the first place.

On horizontally opposed engines such as the EJ20 and EJ25 on the Impreza, the exhaust is forced out from each side of the engine. The exhaust then directed by the headers into the center pipe which then carries the exhaust to the muffler. It is the path which the exhaust must take to reach the center pipe which ultimately defines whether the engine will sound burbly or more like a straight 4 or 6 engine. If the exhaust from both sides of the engine must travel through the exact same length of pipe, or the same velocity before reaching the center pipe, the exhaust note will be constant and in phase and the burble will be for the most part lost, headers which use this method are known as "Tuned Length".

The burble is created when the exhaust from one side of the engine has to travel a different distance, or is forced to travel at a different velocity to the other side of the engine. This causes the exhaust note from one side of the engine to be out of phase with the other and created the burble sound we refer to. Headers which use this method are known as "Unequal Length". The rest of the exhaust system will essentially act as a filter for the sound, so the more restrictive the system is from this point on, the more the burble will be filtered out.

But the fun doesn't stop here.

Tuned length headers are designed for maximum high end power. Fitting these to a vehicle with a high flow exhaust system and modified induction system give on average a 5-10KW increase in power. The idea is that the exhaust velocity and restriction between both sides of the engine is matched to give maximum flow.

There are currently two companies in NSW which fit tuned length extractors for NA imprezas, these are MRT and X-Force. Outside of NSW I am not sure what is available.

Unequal length headers will not give the same performance gains as tuned length headers. I am not sure of the actual difference, but I know that they do not perform as well. But if you want maximum "burble", they are the way to go.

So where do the standard headers fit in to all of this? Well in terms of sound, they fall somewhere in the middle. If you leave them intact and replace the rest of your exhaust system, you will have more of a burble than if you fit tuned length headers, but less than if you fit unequal length headers. In terms of performance, both untuned and tuned length headers will yield small yet noticeable improvements in performance over the standard headers.

Apparently, MRT stock an unequal length header system, although I have no information on it. Best to contact someone at MRT for details. The most popular manufacturer of un-tuned length headers is Borla in the USA. I am not sure if any Australian companies import them.

However, no matter what you do, it is never going to sound exactly like a WRX unless you fit a turbo. A turbo adds a separate dynamic to the entire flow of an exhaust system and changes the note.

CONCLUSION:

You will never make a Naturally Aspirated Impreza sound exactly like a WRX. However, you can still achieve a nice boxer note. While fitting unequal length headers will give you the closest end result, they are very expensive to import and the improvement both in performance and note over a CAT-Back system with standard headers is simply not justified.

I recommend starting with a CAT-Back system and modifying your induction system and seeing if this suits your needs. This configuration will give you a noticeable increase in performance over a completely stock car and will whilst not sounding like a WRX, will achieve a nice boxer note.

If this is not enough for you, look at importing some unequal length headers.

I hope this helps to answer questions.
« Last Edit: April 23, 2009, 12:53:48 pm by (((BURRRBLE-B4))) » Logged
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« Reply #1 on: January 20, 2010, 05:54:33 pm »

 
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JASUBARU
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« Reply #2 on: January 20, 2010, 10:27:24 pm »

Good post Burbs 2 thumbs up
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