India being the largest two wheeler market has an immense potential for customisation and modifications. As far as two wheeler's are concerned the most common modification is the switch to an aftermarket free flow exhaust/slip on. The most important point to be made here is that 99% of all the exhaust mods available are not legal. As there are norms to regulate the noise and emission level of any motorcycle/scooter, the noise should not exceed 85 dB and the vehicle should conform to BS IV standards. But until recently the RTO and Traffic Police have been showing a blind eye to the same but there has been a recent crackdown in Bangalore, Karnataka on the most popular bike manufacturer raking the highest number of exhaust mods, it is none other than our very own Royal Enfield. Click on the link to read more about this.
Most of the exhaust mods are meant only for off-road use and are not road worthy, but many users do not realise the same. There is no doubt that Royal Enfield tops the list on exhaust mods, especially because the newer bikes do not come with the same thump which the older Enfield's have but apart from the mods being illegal is it really worth to change the exhaust?
It is not surprising that most after market exhaust manufacturers have extra ordinary claims like a 20% increase in performance. To be very fair most of the exhaust systems rarely increase the engine output by such a huge margin. Like everything the engine is a system and barely modifying one component only puts more strain on the other components rather than being productive. So in our opinion for you to really benefit from the exhaust change you would need to change the intake system, valve timing, valve openings, ecu remap etc. But this would surely burn a hole in your pocket, instead one can opt for a better performing stock motorcycle.
On top of all this there is a lot to be considered while designing an exhaust system, one important factor is backpressure from the exhaust. Although a lot is done to reduce the exhaust back pressure as much as possible the pipe diameter, exhaust length is linked to the valve timing/overlap, valve opening etc. Once an exhaust mod is done this tuning is disrupted resulting in greater losses which allows the fresh charge (air-fuel mixture) to be expelled from engine without getting burnt. This results in a spectacular looking after burn from exhaust in some cases as shown below.
 |
| Exhaust flame from Royal Enfield Electra with Short Bottle Exhaust |
Although this looks spectacular it is burning off the unused fuel resulting in a loss in economy. Another downside to this is it increases the exhaust temperature which can result in engine damage by warping the exhaust valves which would reduce compression. This can also cause mis-fire because the exhaust valve might ignite the charge before the ignition time. It also reduces fuel economy. To avoid this the mixture needs to be made richer, which is easier on a carbed motorcycle but for EFI bikes it would require a remap or piggyback ECU.
 |
| Overheated Bend Pipe of a Motorcycle. |
The effects of an exhaust mod is different on various bikes. For example enfield's have low compression and lower rpm's which ensures the engine is not damaged to the extent a high revving short stroke engine such as KTM Duke 390. However engine damage is inevitable with exhaust mods. One needs to understand that motorcycle manufacturers spend a great deal of time and money testing the engine and combinations of exhaust systems, hence if someone claims a miraculous increase its generally not the case.
What if i have improved power as well as FE? An exhaust mod generally changes the power band of an vehicle. This could improve driveability based on the exhaust design like a short bottle silencer would result in a loss of a low end torque which is better in case of a long bottle silencer. So if you are driving in a city a long bottle silencer would be better. Fuel economy may be marginally better but that is because of your driving style and probably because your engine is still pristine but mostly it results in an FE drop unless its perfectly tuned and the newer layout suits your driving style.
To conclude the discussion an exhaust mod is definitely not worth it unless your need is justified (racing/off-roading). There are ample choices but most of the cheaper exhausts are poorly designed and generally affect the engine. More expensive options may cost around 30-35k but however they don't justify their price and as said earlier it would make more sense to invest the money in a better performing bike. Also because most of the exhaust mods remove the catalytic converter it results in more emissions, add to that its illegal and it really doesn't make sense.
