Stuff You Need to Know Before Modifying Your Exhaust

by Deepak on June 25, 2011

The automobile plays an important role in the transport system of India. With increase in population and living standards, the vehicle population also increasing day by day. There is steep increase in the number of two wheelers during the last few years. All this are increasing exhaust pollution and particularly in metros as density of these vehicles is very high. As we know, more the emission, more the amount of fuel we have to burn in engine.

Fuel consumption is directly depends on working conditions of an engine. Particularly for two wheelers, an extensive analysis of energy usage and pollution shows alternative power systems are still a long way behind the conventional ones. Moreover many bikers uses modification in their exhaust to get so called exhaust tune and sporty look.

Here is the live example of it. He has converted his normal TVS bike into dirt bike and exhaust has been modified to get sound like dirt bike. I saw many FZs, FAZERs and other bikes with modified exhaust like R1. Then I decided to observe the effects of this modification and I had performed one experiment.

Engine does a process called scavenging in between completion of exhaust stroke and starting of suction stroke. This process is for removing the burned gases from the cylinder with the help of fresh charge.

The scavenging efficiency of engine is directly affected by speed of the engine, exhaust valve opening, exhaust port dimensions, exhaust pipe location and position, dimensions of catalytic converter, muffler design. All these parameters finally affect the back pressure of exhaust. Back pressure in the exhaust is required to remove the gases from exhaust pipe.

After completion of exhaust stroke, Pressure in the exhaust pipe becomes more than the atmospheric pressure. Because of this, gases flow automatically from pipe to the atmosphere with high velocity and produces tune that we listen from exhaust.

Now to get the smooth (Nearly Laminar) flow of exhaust gases, designer keep the value of back pressure within limit by introducing slant in muffler (You can observe this slant in each bike above 125 cc), muffler design, path travel by gases etc. according to the capacity of engine. Now at the time of modification, we are influencing following parameters:

  1. First of all we are increasing the total length of exhaust because of which the path travel by the gases increases. Now as the gases have to move long way, velocity developed in the pipe decreases by the time. As per the rule, velocity and pressure are inversely proportional. So finally pressure will increase and flow gets chocked. (Chocking means drop in velocity of exhaust gases drastically. It doesn’t mean the zero velocity. If the velocity becomes zero then we can’t run the engine anymore. But as we are introducing more and more gases in the pipe after each stroke, the gases finally reaches to the opening of pipe. But the velocity is too low then the required.)
  2. As we are increasing the exhaust pipe length, we are introducing more bends in the exhaust pipe. Each bend causes flow restriction and friction losses in the gas flow. This leads to chocking of flow again.
  3. As this two major factors result in chocking of flow, the next batch of burned gases coming out from cylinder don’t find any way to move.
  4. Finally the scavenging efficiency of engine reduces drastically and burned gases remain in the cylinder. These burned gases got mix with the fresh charge and burnt again and again. This process leads to generation of more emission.

Experimental Setup

So to check the above said effects of modification, I performed an experiment. The figure indicated here illustrates the HH PASSION just for outlining the set-up. In real experiment, I used other bike. I increased the slant of exhaust pipe up to 90°.

Because this is the maximum angle likely to be exerted at the time of modification. 3000 to 5000 RPM was selected for the test because in usual driving practice this is the most common operating range.

First, I measured the performance with original angle provided by the company to get the standard data. The next step was to carry out the performance test with various changed angles of exhaust opening. Changed exhaust manifold angles would result as higher fuel consumption, low brake thermal efficiency, and high exhaust emission. Below fig indicates my prediction I made before test and also illustrates the changing angle of exhaust.

The above figures indicates the total experimental set-up used for the test. Now I directly switching over to result I got from this test.

Results

  • The value of back pressure for 90° is approximately 3.2 times more than the standard value.
  • Back pressure value increases 0.08 KPa with increase in manifold angle by 1°. This nature of back pressure is applicable up to 60°.
  • After 60°, the increase in back pressure turns out to be 0.44 KPa with 1° rise in manifold angle.
  • Fuel Consumption increases 7.93 gm/hr with increase in manifold angle by 1°.
  • Thermal efficiency of engine decreases 0.17 % with increase in manifold angle by 1°.
  • HC emission increases 2.60 ppm with increase in manifold angle by 1°.
  • CO emission increases 0.054 % with increase in manifold angle by 1°.

So guys, here you have the results of modification done in exhaust. I did the modification only with opening angle of exhaust; many other factors are yet remaining to be examined. Many other factors also contribute to these effects.

You can judge the results what if all the factors contribute together. This is what generally happens when we modify our exhaust with local dealers or garages. We blindly are increasing pollution and fuel consumption of our bikes. Many calculators are available on net for calculating correct required back pressure for your bike engine. You have to insert some technical specifications and it will provide you the correct back pressure.

I personally suggest you not to modify your exhaust ever. Because it is optimum design provided by the manufacturer and designed by their best designers. Even then if you are so passionate about noisy bikes (there is vast difference in tune and noise), first you get the calculated back pressure and then get the correct dealer for correct exhaust.

- Dhruv Panchal

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{ 53 comments… read them below or add one }

fas June 25, 2011 at 11:42 am

I think exhaust is best left untouched. Does not make sense to risk it.

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Ysak June 26, 2011 at 7:35 am

gr8 topic..
bt wat abt chngng my p150′s silencer with a p220 silencer.????
ny probs in tat???
plz reply..

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Dhruv June 27, 2011 at 2:15 pm

Ysak, here your problem is about capacity of your engine. 220 is burning 70 cc more charge then 150. so I think p220 silencer will provide less back pressure than the required. may be after incorporating this, you will not find any sudden change in your bike performance, but i am sure, this will definitely affect after some time. I mean, fuel consumption will increase and efficiency will decrease.

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Abhimanyu June 30, 2012 at 6:59 pm

No problem, its nice n u ll get nice pick up n speed…. And if u thinking abt decreasin in mileage den its obvious …. Cause ater fitting that silencer u ll make sound by raising throttle and cause of this mileage ll decreses… if u ll ride normally den it ll be good

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Prabakar June 26, 2011 at 11:50 am

Great article. Very usefull for youngsters who blindly change their bike’s exhaust.

Not only tat, the important factor is the noise pollution made by open silencers, infact it is illegal too…

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Gurdarshan Singh June 27, 2011 at 12:58 pm

Sir,
I am getting two type of silencers in classic 350, one is straight and other is bendy.Which silencer you will recommend? Both are company fitted.

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Dhruv June 27, 2011 at 1:36 pm

Gurdarshan Singh, Don’t worry about it….you can choose any one..be cause as i said in my article, companies are spending lots of money to get their vehicles approved from ARAI. They do many research and tests on each silencer. so for you, its just the matter of looks…choose which ever you like.

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Pankaj June 27, 2011 at 6:58 pm

Straight one will be ideal
Because bent one if it is bent down then no problem in going for that
But if it is bent upwards,then it is not recommended to go for that
I am telling this on basis of the experiment given in this article

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som June 27, 2011 at 1:48 pm

Extremely appreciable and to-the-point experimental review….really of great importance for any over-enthusiast biker,who wanna modify his dream-machine.Thanks a lot for your study report on this.

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Ramiz June 28, 2011 at 5:35 am

How can i make my R-15′s voice like racing bike….?????

Bruummmmmmmm….

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Akash August 26, 2012 at 6:36 pm

Switch ur engine with R1.

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rahul July 25, 2011 at 12:45 pm

i wanna modify my honda cb twister 110 cc exaust

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saurabh September 6, 2011 at 10:05 am

Hi,
Dhruv, Very nice study by you. Since you have digged out motorcycle exhaust i have few doubts on design of Motorcycle & i feel this is not the correct portal to discuss hardcore techy things. Drop me a test mail from your email ID.

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saurabh September 6, 2011 at 10:09 am

sorry frogot to add my email ID.. saurabh.bulandani@gmail.com

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aachal garg October 26, 2011 at 2:13 pm

hi Dhruv

All you said there sounds perfectly logical…but can you please share some info about using “TailGunner Exhaust” for bikes…I want to know if it may harm my Avenger 220 in any possible way if at all I decide to use one…..

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Pulin December 22, 2011 at 9:29 pm

Hey Dhruv, thanks for spreading the knowledge! Never knew that the exhaust manifold angle was responsible for the performance of the bike in so many ways. The test that you’ve performed above is very much appreciated.

I’d like to know the pros and cons of replacing the default exhaust of a bike, say an Avenger 200, with a free-flow exhaust.

Thank you!

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jeeva December 31, 2011 at 5:28 pm

i have Suzuki Max100R two stroke engine it sounds a lot while driving, i dont have anyidea to sell this bike in my lifetime bcoz its my dad’s First bike, but want to change the silencer which should very very silent….. , is there any possibility?? tell me plzzzzzzzzzz

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prateek October 16, 2012 at 11:42 am

get an exhaust from a bigger capacity (150cc) four stroke bike,cut out its muffler and weld it with the existing muffler.as the bigger muffler will contain more baffles the sound pulses generated will be cancelled and you will have a quieter exhaust.but the problem is that you will find a separate muffler only on the fz16 so ask for its exhaust system in the biggest trash yard of the city or get a new exhaust system.a new one costs rs 8000.

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Ashwanth March 21, 2012 at 11:43 am

Can we keep another scilencer exhaust for Hero Honda HUnk is it advisable Mr.Deepak

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Stipen March 27, 2012 at 3:02 pm

what modifying your exhaust pipe with the same angle but shorter one. How will this effect the performance of my bike.

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shaik March 27, 2012 at 6:05 pm

I want a noisy exhaust from my modified tvs central what I want to do for that

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yassar nagi May 31, 2012 at 2:27 pm

The author is absolutely right and the article is very informative. Instead of trying to mimic under seat exhausts with poorly built tin canisters, people should buy tuned exhausts by reputed tuners like leo vince etc. these aftermarket exhausts are built according to the the dimensions, cubic capacity etc of a particular bike.

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Balasubramanian June 28, 2012 at 2:58 pm

Guys,

I am facing problem on my two wheeler(TVS Centra), when the engine is ideal on running I am getting a burst sound in the cylencer. Fuel consumption is very high compare to early i.e befor this problem occured. Requesting your suggestions for this.

Regards, Bala

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Abhimanyu June 30, 2012 at 7:03 pm

I ve R15 version 2…. I m not happpy with the pick up so need to put racing silencer to increase the pickup and nice sound…!! with normal speed i get 45km/ltr. I’m ok if i get 35km/ltr after modification.

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Preetesh July 20, 2012 at 7:11 pm

hello Dhruv,,,
Very good article…

I have karizma R, & i also want to change exhaust,,,, not only exhaust, but
1) NGK Spark plug IX iridium,
2) K& N air filter r 1100,
3) CBR250R 11-12: Yoshimura R-77J CF tip-SS Slip-on, ( que is wt will be effect if i install exhaust shorter in length or should i install in default position )

so plz let me know, wt will be the positive and negative effect on my bike…

Thank u
my email id is preetesh1@gmail.com

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Raaj August 6, 2012 at 3:55 pm

I have to change my karizma zmr exhaust sound like r1 what should i do please reply

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dev September 9, 2012 at 10:22 pm

hi ……….this is Dev and i have removed ma bike’s exhaust , it sounds good but am worried about the average of ma bike , can u tell me it does effect ma bike in any way.

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ROHIT September 30, 2012 at 6:38 pm

i have an bajaj avenger 180cc which i had modified to some extent n its average is awesome
but i want a noisy silencer for it like bullet 350
how much effect will be there on my bike

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$reeniva$ October 5, 2012 at 6:23 pm

Hi i wanna change silencer for my YAMAHA R15 V.2
does it impacts on mileage ?
how much mileage will be decreased incase i change the silencer ?

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Webddict October 7, 2012 at 9:18 am

Nice to read this. I was thinking of modification but i think i should give up that after reading your article.

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prateek October 16, 2012 at 11:31 am

in bikes provided with resonator (bajaj’s exhaust tec and yamaha’s midship exhaust system)all the necessary back pressure required for scavenging is developed as it reaches the catalytic converter and reduces after it flows back from resonating chamber to its way out to the muffler,so we can modify the angle,position of the muffler but not the header pipe and resonator.

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Conan Cardoz November 5, 2012 at 7:47 pm

When it comes to Royal Enfield Unit Constructed Engines. I have a Electra Twin Spark. I wanted to modify my silencer to the short bottled from stock for more noise in that how do you calculate the back pressure. How is the short bottled silencer for my royal enfield bullet.

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Arunav November 23, 2012 at 11:56 pm

Druv: its great to see someone who took real effort to write something on their experience n experiment, that has been a great help for all the bike lovers i guess. But i wanna know one thing as i ride yamaha 2 stroke modified for rally basically as dirt. I have also done modification with the exhaust pipe naturally. My bent pipe from head is 75• slant and 16 inch down n draged upto 3 1/2 foot long with keeping 3 1/2 inch diameter thickness of the resonater n then attached with a twin net muffler. I somehow managed to increase the innitial acceleration bt not satisfied with my top acceleration. It dropped down to 131 km/hr. How can i increase my innitial n top both at the same time..? Where as my rear wheel is 18 inch. Please suggest..!

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Giridhara January 16, 2013 at 5:04 pm

Very useful article pertains to Bike silencers…

Kindly do one more experiment for us:
For Royal Enfield Classic 350cc Bullet, if we change the silencer as Short Bottle or Rocket model Silencer – What will happen in engine & Thump?
Expecting your reply..

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Dilip January 25, 2013 at 6:05 pm

A very good analysis by Dhruv Panchal. The exhaust pipe/muffler should be replaced with a proper one keeping in mind of bike performance and emission control, if one is doing so, otherwise I think it’s simply a waste of maney… and all that.

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Vishnu January 28, 2013 at 9:21 pm

Hai..
I have tvs star city..i getting 65 to 70mileage per liter…i had to change my bike exhast for sound…
So plz tel me sme idea abt exhaust change without afecting of mileage…

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Jithin March 29, 2013 at 3:00 pm

Hi dhruv;
what about upgrading a yoshimura pipe with stock on honda Cbr 250 R, does it affects the efficiency of the engine?? Plz reply

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chetan March 31, 2013 at 1:49 pm

i hv….bajaj discover 150 ….bt this byk acceleration..n exhaust sound ….iz vry poor….so…plzz giv me….suggestions ….can i upgrade…..k & n air fillter….to my 150 discover….????

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rashima June 2, 2013 at 12:51 pm

Very technical information.
But presented so well so that everyone can understand.
Easily one experiment as detailed by modifying the exhaust.

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jaydip June 8, 2013 at 1:36 am

i want modify my cb twister to r 15 old virson. . .with only body. . .how much expense? It’s good or bad??

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PH July 21, 2013 at 10:28 am

Hello Dhruv,

I am planning to install RE Electra or RE Thunderbird’s exhaust on Bajaj Avenger 200cc. It is possible with some welding. What is the impact on engine?

Avenger’s exhaust is straight and the RE electra’s exhaust is also straight. However the length of exhaust of RE electra is lengthier than Avenger’s.

Will this have an impact? What modification need to be done brother?
Response and suggestions are appreciated!

Thanks!

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Shazad Killedar August 29, 2013 at 4:32 pm

Can you help me I need to change the exhaust & air filter for ktm duke exhaust Akraprovic & air filter k&n or Bmc where will I find these & who is the best person to go for this job in mumbai

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deepen September 17, 2013 at 4:42 pm

Hey I have apache rtr 160 hyper edge… can I make modifications in ma apache…wid sporty muffler…!!??? Pls suggest ur views..

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Anupam October 7, 2013 at 3:05 pm

I have a Bajaj Avenger 220cc….wanted to change the exhaust…long n a big one…..pros & cons plz…..

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Anupam October 7, 2013 at 3:08 pm

I already hav a K&N airfilter installed…..its beeen 1yr roughly I have purchased the bike……& also…wanted to go on a long ride frm blr to goa……keeping the ablove in mind…how does my trip look like…?

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alfiyan November 8, 2013 at 3:37 pm

I have a Bajaj Avenger 220cc….wanted to change the exhaust…double silencer and tyre…..pros & cons plz…..

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Dhawal November 22, 2013 at 2:41 pm

Hi Everybody,

The above article is a great knowledge spreader. Ideally, changing exhausts on Indian motorcycles is a no! for enumerable reasons. Most of them being loud and do more harm than good to the engine and performance. Aftermarket exhausts are for motorcycles with larger capacity producing more power and higher compression ratio’s.

OEM exhausts as Dhruv said are made with great research and are optimum in terms of performance and emmisions and are approved by ARAI.

Below mentioned are two types of exhaust which are available for a lot of motorcycles. I have tried to summarize the pro’s and con’s of doing the same.

SLIP-ON Exhausts:
These exhausts are ‘end-cans’ which are replaced from stock ‘end-cans’. An ‘end-can’ is the larger part of the motorcycle exhaust towards the end. The reason the exhaust is fatter is becuase it encases the Catalytic converter, which is helpful to reduce sound, control emmisions and increase engine braking pressure ( if you do engine braking ). Some motorcycle’s like the KTM D200 have an external Catalytic converter to salvage the enderbelly space. Please remember, the Slip-On exhaust systems are compatible only to motorcycle exhausts which have expansion springs which make the stock ‘end-can’ easily removable.

Advantages of Slip-On exhaust:
The back flow of the exhaust is not changed drastically.
The exhaust header pipes remain stock.
Does not drastically affect fuel economy and gas movement.
Reduces overall weight.
Emmision is under control most of the time.

Disadvantages of a Slip-On exhaust:
Only looks good with a minimal increase in power.
Motorcycles which need welding the Slip-on DO NOT provide any increase in performance of the motorcycle.

In my opinion, any motorcycle enthusiast who owns singles, or twins should go for a Slip-On.

FULL SYSTEM EXHAUSTS:
These exhausts are full systems which start from the exhaust manifold to the ‘end-can’ Usually made of light weight materials like Aluminum and Titanium. Some of them are also made of steel. Basically different shaped pieces of metal are welded carefully to maintain a free flow of exhaust gases and increase performance. These exhausts are either homologated for road use or are strictly for track use because of emmisions and noise. Most of them come with a baffle/ dB killer for road use which is an insert in the end-can to compress the sound.

Advantages of Full System Exhausts:
Performance figures change.
Engine braking is usually taken care electronically. For motorcycles with no electronic engine braking & Slipper clutch, hard braking could be an issue.

Disadvantages of a Full System Exhaust:
Loud.
Emmision Control goes for a complete toss if the Exhaust is not homologated for road use.

A kind request to all the fellow motorcycle enthusiasts to use exhausts made and recommended by the manufacturer and not cheap local brands.

About Air filters: K&N, Pipecross, BMC and other brands increase air flow into the carburottor/ injector which in-turn increases fuel flow ( post tuning ) and definitely decrease fuel efficiency. Increase in power is minute and mostly pyschological as the exhaust note is better than stock. Air filter is recommended with a Full system exhaust to experience overall increase in performance.

Any questions, please shoot me at dmak4956@yahoo.com

Regards,
Dhawal M
dmak4956@yahoo.com

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Sachith January 13, 2014 at 2:30 am

i have yamaha r15. Can i make it sound like yamaha r1??

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Saad January 13, 2014 at 12:55 pm

For that you need to have an inline 4 motor… ;)
In short NO!

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sadruz February 2, 2014 at 2:10 am

I have a bajaj pulsar 150 dtsi I need a silencer who give me sports bike sound in my bike so plz suggest me what i do…

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Shriharsh February 8, 2014 at 3:44 pm

AM thinkin of modyifying my pulsar 135 exhaust to that of P220 ……… can i do it?, will this make a diff?? …….. or else suggest me any other exhausts under 5000 for it ,…. for gud performance .,…….. i app. all ur thoughts

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javeed February 14, 2014 at 1:10 pm

I have a karizma zmr ….i hate that sound…,so i wish to change….its possible mean….tell me the solution…

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alan March 30, 2014 at 2:02 pm

really informative article,Thanks a lot
Is there any way to have a louder voice from silencer without drastic change in mileage>’>

Reply

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