RascalAngel

hello guys,

there are lots of guys i see on road who have swapped their stock exhausts for FFE's.Though it improves performance to some extent, it also emits lots of noise.I was wondering if the noise part could be dealt with a silencer ala the ones fitted to guns.Since its the CAT which restricts air flow, fitting a silencer ala guns should not effect the perf. gains AFAIT.

What you guys say?

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sunilg

General biking discussion approved.

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satyenpoojary

Theoritically yes,

and PS: the catcon isnt the only thing that acts as a restrcition... the moment you introduce a baffle to deflect the air flow inside you will be loosing out on power...

Will wait for the expert explaination on this one...

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ROCKRZ

yup dude. the sound can be reduced but for that, you need a very big exhaust.

also, most of the times, the ffe does not do anything other than making a racket of noise. a exhaust has to be tuned with the intake capabilities of the engine else the perfornmance actually drops off. you just feel that you are going faster but actually you are not...

believe me i have a k&n, with a race carb and a ffe but later i found out that there was degradation in the performance of the bike and so i removed the ffe and now its only k&N and the tuned carb. and the bike is now better than stock

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RascalAngel

Thanks Sunil.


But CATCON is the major ExGas flow restricter and for baffle one cannot have cake and eat it too

thats there (like Cars) but the way i think is,if a silencer on gun can mute a bullet shot to just a whine then i wonder a slightly bigger silencer(as an ADD-ON to FFE) of such type should cut down on noise to fair levels while still giving substantial perf. gains over stock exhaust.

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jd666

its called muffler for a reason. the baffles reduce the energy of the exhaust gases by making them take a longer route through the exhaust, before exiting at the end. so that reduces the sound emitted, but also creates a certain amount of back pressure which the piston while going up for the exhaust stroke has to overcome to push the gases out properly during the exhaust stroke. This reduces the rev-ability of the engine, but does improve low end torque as well.

The pulsar tuned expansion box for which they have a patent as well ( forgetting the name ), kind of is a balance between the exhaust back pressure and the freedom with which the piston can move, improving performance and efficiency.

So like pointed out above, either use a massive end can, or increase the length of plumbing through which the gas has to travel. but the loss will still count.

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aargee

Exhaustec; its a patent technology from BAL

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jd666

^^ yes sir!

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Old Fox

To understand whether 'noise' is an unavoidable evil when free-flow exhausts are used, we need to know what causes this noise in the first place.

Exhaust gases have two characteristics as they exit from the engine. They exit at a high pressure (having been ejected by the piston at high velocity) and are hot. Which implies that they are at a higher energy level than atmospheric air. As this high-pressure and high temperature gas exits the exhaust pipe and escapes into the atmosphere, the contained energy gets dissipated. Lets get in basic physics here in the form of the 'Law of Conservation of Energy' that states that energy can neither be created nor destroyed but can only change form. So the heat and pressure gets dissipated respectively as 'heat' (goes into heating the surrounding air) and a pressure wave that travels as a concentric ripple around the exhaust exit till its pressure equalizes with that of the surrounding air and the wave dies out. Now this pressure wave is audible to us as our ear drums are sensitive to pressure changes. The 'heat' adds some bit of additional energy to this 'pressure wave' (remember Charle's Law of expanding gases where gases expand when heated) but its the exit pressure and velocity mainly that determines the quantum of noise. The higher the exit pressure and velocity, the greater the noise.

A 'silencer' is a combination of expansion chamber and baffles. Both of these reduce the effective pressure and exit velocity of the gases, also cooling them off a little in the process. In short, the overall energy content of the gases is reduced which in turn reduces the noise. But these 'restrictions' have a price, in that they restrict the flow of gases. And as the gases continue to be ejected by the engine while those in front are 'delayed' by the silencer, there forms a resistance to flow or back-pressure for the latterly ejected gases. FFE's work at reducing this back-pressure and there are two ways of doing this: 1) by reducing the restrictions and allowing 'free' flow of gases (hence the name FFE) and 2) by tuning the exhaust pipe length in such a manner that just as one exhaust pulse is exiting the pipe (and so leaves a partial vaccum behind), the next exhaust pulse gets ejected by the engine. This following exhaust pulse finds the pipe 'empty' and so flows out unrestricted. Clearly, this can only be done for a certain narrow RPM range for the engine and hence we call them Tuned Exhausts.

Moving now to your 'gun silencer' analogy where the exhaust from the barrel can be silenced almost to nil. The situation here is different. The bullet gets pushed through the barrel of the gun by high-pressure and high temperature gases that are produced by the rapidly burning gunpowder. This high-energy gas can expand in only one direction i.e. towards the open end of the barrel and as the bullet head is in its way, it gets pushed to a high speed. Now these gases would make a lot of noise if allowed to exit behind the bullet as is the case with an unsilencered gun. But since the energy of the gases is useful only till the length of the barrel lasts as it is only inside the barrel that the bullet accelerates, the energy of the gases serve no useful purpose once the bullet has attained its peak muzzle velocity. That's where the silencer, which is attached or screwed on ahead of the barrel, can come in. The gases are expanded through a series of perforations (and each perforation or 'orifice' causes a drastic drop in gas pressure) so that almost the entire pressure energy of the gases is dissipated before they exit into the atmosphere. But there's a price here too vis-a-vis silencered guns have a shorter range than unsilencered ones though in case of small arms, this does not make so huge a difference from a utility point of view.

Such a silencer in an engine would create a strong and long lasting back-pressure wave that would almost choke the exhaust for the following exhaust pulses. Moreover, an engine breathes enormous amounts of gases in and out compared to what ejects after a bullet is fired.

Nutshell: FFE's will be noisier than true silencers. Forget the physics and see the purpose: FFe's are for performance while 'silencers' are to meet with noise restrictions. The design briefs are entirely different.

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psr

Old Fox.... a very good technical write up on FFE and conventional Exhaust.You are not only correct on the thermo dynamics but also on the acoustic part.The depth of your analysis and the easy language you used are truly commendable . Do keep writing.
Acoustic resonators(Helmholtz resonators) are now being used more ,both in the inlet and exhaust to reduce noise and also to improve performance,in specific RPM and power bands.Though the invention was primarily by a German , The Japanese were the first to understand and exploit it in the two and four wheelers.