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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72 COMMENTS

    • 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.

    • 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

  1. 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…

  2. 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.

    • 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.

    • 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

  3. 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.

  4. 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.

  5. 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…..

  6. 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!

  7. 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

    • 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.

  8. 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.

  9. 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

  10. 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.

  11. 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

  12. 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.

  13. 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

  14. 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 ?

  15. 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.

  16. 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.

  17. 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..!

  18. 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..

  19. 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.

  20. 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…

  21. 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….????

  22. 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!

  23. 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

  24. 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…?

  25. 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

  26. 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

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

  28. hi m kamlesh i want to know if i can put 1 more silencer to my avg220 n wud dat effect my milege or else help me to reduce my back pressure n provide more milege

  29. Hi Friends if you have built classic 500 cc You put silencer mini punjab and band pipe jali remove from band silncer . now all silncer a holo pipe then sound very good or like Gun 09813108946

  30. Hi Friends if you have built classic 500 cc You put silencer mini punjab and band pipe jali remove from band silncer . now all silncer a holo pipe then sound very good or like Gun 09813108946 sound is very good cost only 650

  31. You have talked only about the angle of the exhaust setup…there are other very important factors…Length of the exhaust system and the muffler (with its baffles). These 2 are also equally important for the performance of exhaust system. Do you have any data to show the effects of modifying these 2 factors?

  32. Anybody here! can u please tell me which exhaust i should use to get a sound like racing bikes, that cool sound, not that louder like bullet type, please suggest?
    !!!

  33. i have my 2yrs old bajaj 220cc avenger. i want my avenger to sound loud like bullet or louder than that. can anyone please help me un suggesting the best outcome for it ??

  34. Hi, I’m Santu…
    I have Avenger150…
    I want to change with punjabi long bottle silencer…
    So plz advise me, if I exchange the silencer, have a any problems ???

    Thank You…

  35. A-1 Answer. As an aeronautical engineer I like your explanation. I think, whatever (type of exhaust) is fitted with your bike this is the best as they are designed, modified and fitted by the originator / manufacturer by calculating highest performance of your engine in all range.
    Thanks.

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