Exhausts - General Knowledge

[HD]

No this isn't another 'wutz da bst eggshorst 4 me byk' thread

I want to know more about exhausts. More specifically about expansion chambers and all of this back pressure malarky.

I know for expansion chambers, the gas going out can expand and is then forced back in but what effect does this have? And what effect do longer, shorter, wider or thinner chambers have?

Back pressure. What is it? On my dad's nitro cars, the exhaust is connected to the fuel tank and forces fuel through (I think) and this is called back pressure. But what is it on bigger bikes? Do different exhausts add or take more back pressure. What are the effects (and maybe advantages and disadvantages) of each.

I hear from reading around that running your bike without a can may damage your valves (4-stroke) or mess around with the back pressure (2-stroke) but how much of this is true?

If I run my bike without a can of course it is loud and annoying but will it harm it? (not planning on doing it btw)

As a side note, how do cans actually reduce sound? The core has holes in it and wadding behind that but how does it reduce noise? Because surely the gas will just get through the holes and get caught up? So does that mean more noise means more air getting out meaning more coming in meaning faster? I mean gp bikes can have stupidly small exhausts. And why over time does the wadding need changing?

Thanks in advance
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Rusty '02 Vito Camper + CBR600F3

[defblade]

[Kickstart]

Hi

See above!

Back pressure is a bit of an odd one. And engine doesn't need or want any back pressure. What can help is the pressure waves arriving at the right time.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[HD]

So a shorter exhaust will give less back pressure? Guessing baffles add back pressure too?

Also forgot to ask, when you hear a bike without a can, when idling why does it sound like a v8 ticking over (then sound incredible when reving )
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Rusty '02 Vito Camper + CBR600F3

[Kickstart]

[HD]

[The View Askew]

This thread is brilliant, keep that information coming!
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"Do what you always did, get what you always got"

[Teflon-Mike]

For expansion chambers and resonance tuning in the two stroke engine, you need to research a chap called Walter Kaaden, and Kaaden 'effects'. He's the chap that basically 'discovered' how the expansion chamber exhaust worked, and exploited it to get two strokes to work more effectively.
https://s178.photobucket.com/albums/w269/teflons-torque/050_B_001_Motor/Arbeitsweise_Zweitakt.gif
Principle is pretty simple, and can explain how 'silencers' work too.
Inside the engine, on the power stroke, you have high pressure gasses pushing the piston down.
The exhaust, open to atmosphere contains gasses at atmospheric pressure.
when the exhaust valve opens, or the piston uncovers the exhaust port in a two stroke, you have a HUGE pressure difference between whats in the cylinder and whats in the exhaust, and the high pressure exhaust gasses rush out through the opening into the low pressure exhaust.
Engines work on a cyclic basis, so you get 'pulses' of echaust gas dumped into the exhaust, 'sound' is a cyclic 'frequency' or pressure wave, and the exhaust gasses rushing out of the exhaust so many times a minute, give you 'one' sound frequency.
Like a musical wind instrument, though, a flute or whistle, or blowing over the top of a milk bottle, the actual movement of gasses sets up 'harmonic' sound waves on top.
Now, the 'silencer'.... two main means of silencing an engine, but the basic principle is to even out the high and low pressure peaks of the sound wave so they cancel out.
Best way of doing this is using baffles... and I describe the 'muffler box'
TaKe your 2" diameter exhaust pipe, and vent it into a 5" diameter tube. As the high speed gasses hit the change in section, they slow down to fill the larger volume of the new pipe.
You then stick a 'wall' infront of them, and put a 3" hole in it, but off set from centre, so the gasses dont just rush straight through, they sort of bounce back and forth between the walls of the chamber, or at least the pressure wave does, before going through the hole, where it hits ANOTHER wall, with another offset hole.... BUT teh distance between the walls this time is slightly more or slightly less than the first 'chamber'....
Principle is, as the pressure wave bounces backwards and forwards, like a flute, it creates new harmonics, and the different distances and different volumes and 'stuff' basically break up 'harmonics' so that it 'jumbles' the frequencies, and each time a low pressure peak hits a high pressure peak, they cancel each other out, 'dissipating' the pressure differences, like scrambling an egg, until you have no big pressure peaks, and the gasses are traveling fairly slowly, so when they come out of the end.... the sound wave has hammered itself to bits..... its a bit like rocks on a beach. as teh water level changes it makes the waves get smaller and faster, then they hit rocks that break them up, and the reflected wave from hitting a breakwater wall, when it hits an incoming wave, flattens it out a bit.... so out at sea a few hundred yards you can have huge breakers maybe a foot high, but at the harbour wall, they are barely an inch or so.....
OK, two stroke......
As animation..... as the exhaust gasses vent into the spannie, you get a shock wave. Change of section of the expansion chamber allows that shock waver to dissipate, and slows it down, so that it takes longer to hit the reverse taper, in order to 'hold it back' the length of time the exhaust port is open, so that it hits that reverse taper and is reflected back through the chamber to teh exhaust port JUST as the exhaust port is closing, in order to 'reverse scavenge' the exhaust.
This was Kaaden's genius.
Scavenging is the process of clearing out all teh last lingering exhaust gasses in the cylinder before the next combustion cycle. any lingering exhaust gasses reduce the volume of 'charge' that can be sucked into the cylinder, so reducing performance, but also, they contaminate the charge, and hinder combustion, so you dont get as complete combustion.
So its 'good' to get them out.
Four Stroke engine's, having a positive displacement exhaust stroke, the piston rising forcing the exhaust gasses out of teh cylinder are very good at scavenging themselves.
Two strokes, simply letting the exhaust gasses escape by opening a window in the wall of the cylinder and hoping they bugger off on their own, and not so wonderful!
In the 1930's there was a lot of 'interest' in the two stroke engine for various reasons, but significantly under the German Nazi industrial reconstruction, a very simple piston ported two stroke engine which was incredibly cheap and easy to manufacture, was designed for and produced in the DKW RT125 motorcycle.
Nothing particularly inventive about it, apart from being amazingly simple and cheap to manufacture...... and unfortunately woefully ineficient.
Licence to manufacture the DKW RT125 was apropriated by the allied forces after WWII under war reporations, and it was built in england as the BSA Bantam, and in the USA briefly by Harley Davidson, before the Harley tooling was shipped to Japan, and given to Yamaha as part of their economic recovery work, having nuked the poor little blighters! (Hey, I'm sorry, I just dropped an H-Bom on you, matey, Here, have a little motobike I nicked of the Germans last year!)
I mention this becouse after WWII the DKW factory fell in the Russian sector of Germany and became the MZ factory, where they carried on making DKW RT125's after the war, and Walter Kaaden, who had been a development technician before hostilities became their chief development engineer.....
Other boffins had been playing with much more sophisticated two stroke engines, most notably NSU, who had an advanced 'split single' which had two cylinders, and two pistons, one behind the other over the crank shaft, connected by a common forked con rod, and sharing a siamesed combustion chamber, the inlet ports in the rear cylinder, the exhaust ports in the front, the forked con rod meaning that the rear cylinder reached TDC ahead of the forward cylinder, the forward cylinder lagging begind the rear when it came to the ports at the bottom of the stroke, SO, you got 'asametric' port timing.....
Conventional single cylinder engine, the ports open as early before bottom dead centre on the power stroke, as they close on the up stroke of compression.
Slit single, the timings controlled by leading and lagging cylinders, the timings could be made more advantageouse.
Means that the inlets could be opened early and flood the rear cylinder, spilling into the forward cylinder as the piston started to ride in the bore, while teh front cylinder 'lagging' still had the exhaust open, so washing exhaust gasses out the cylinders.
Kaaden, was rather hampered by not being allowed materials to do anything so 'clever' or sophisticated, ALL he had to work with was the simple piston ported RT125 engine.
So he got interested in 'porting', and the first thing he discovered was that by using a flat topped, or very shallow domed piston, a hemispherical combustion chamber and very flat and directly 'opposed' transfer prots, he could create a fountain like flow pattern inside the cylinder, mimicking what a split single did.
Charge rusing in the transfers, accross teh top of teh piston, met an oposing flow coming from the opposite piston, and hit head on.
Flat piston, couldn't go down, so had to go up, straight to the round combustion chamber, where it would flood and push back down, forcing exhaust gasses out the exhaust port, for better scavenging.
Unfortunately, more successful he was at 'porting' the engine, better scavenging he achieved, more fresh charge he'd 'loose' straight down the exhaust port, into the exhaust......
BUT, if he used this expansion chamber thingie....... he could get a shock wave, JUST as the exhaust was about to close, that would push a good proportion of his 'lost' charge back into the cylinder.....
Hence 'reverse' scavenging', and it made the engine work a lot better over a wider range of engine speeds, though normally tuned more precicely to get FAR better power over a smaller range.
Clear as mud?
Picture explains it better than me, but Kaadens the chap you need to read up on!
As far as 'Back Pressure' is concerned, this is it in operation, on a two stroke, and that 'back pressure' can actually be essential to get them to run, or all teh charge would be lost into the exhaust pipe.
On a four stroke, as Keith says, basically the engine just wants a clear shot to pump all the exhaust gasses out the chamber.
But you have a similar situation during the vale 'overlap' phase at teh top of the exhaust stroke, start of the induction, when the exhaust valves aren't yet closed, and the inlets are starting to open.
the piston is slowing almost to a stand still, turning around from going up, to gong down, but the exhaust gasses, having been given a heck of a shove out the door are traveling quite quick, and like a truck, leaving a void behind them, or vaccum.
this can be useful, and tuners can exploit it to get a mini supercharging effect, using the 'suck' from the exhaust pipe through the exhaust valve during overlap, to create a vaccum in the combustion chamber to help get the stationary charge in the inlet tract moving into the cylinder......
Only trouble is, if you have too MUCH 'suck' it can actually suck that charge straight out the exhaust valve, usually, though not always, making the mixture in the chamber go 'weak', resulting in detonation, and burned valves and 'stuff', which is why when fitting free'r flowing 'race' exhausts to four strokes they usually screw the carburation jetting, and normally make it go weak, or create flat spots, and people talk about 'needing' a certain amount of 'back pressure', or at least a certain lack of exhuast 'suck' to hold the charge in teh combustion chamber.
silencer 'packing'?
alternative to baffles, rather than having walls and holes or other 'obstructions' in teh exhaust pipe to chop up and disturb teh flow, create and distroy resonance and mix up teh soun waves to kill noise levels, simple 'packing' normally a loose wool of glass fibre or similar, as said can give a 'fuzzy' zone where the energy in the high pressure waves is absorbed causing pysical movement, which reflected back into the gases, creates turbulance evening out the pressure waves.
Generally not as effective as baffles, but at the same time, often free er flowing for a given section.....
which brings us to 'section'....
Old rule of thumb with exhaust pipes is tha the 'header' or the section of pipe imedietly after the exhaust valve, ought to be roughly the same volume as the cylnder.
So, big fat pipe or long thin one.
Big fat pipe favours peak flow, when the engine is making most gas most often, and getting it out the engine as fast as possible, hence better for high rev peak power.
Thinner longer headers, can be restrictive to higher flow rates, but, the smaller section promotes a higher gas speed through the pipe, so it can provide better teh kind of tuning harmonics that create scavenging vacuum, al lower engine speeds or throttle openings, so tend to be better for low and mid range 'torque'.
Baffles are very efficient at removing noise, but to get good silencing requires very large chambers, slowing gas speeds right down, and a LOT of well spaced baffles, so that the turbulance they create and the reflected harmonic pressure waves don't impede flow. That means that they tent to be big and heavy.
Making them more compact, you start to create impediment to flow,
so its a compromise between making a muffler that flows well or one that silences better.
Hence why 'race' pipes tend to be louder than road legal production 'cans'.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[keggyhander]

This thread is brilliant.

[Teflon-Mike]

[Frost]

[Wozza]

I'm doing a dissertation on this sort of thing actually.

The term 'back pressure' refers to a positive pressure wave being intentionally reflected back towards the exhaust port of a two-stroke engine, as Mike explains above. Any such positive pressure wave arriving at the exhaust valve of a four-stroke engine is never a good thing and so is never intended.

There are two main ways in which a four-stroke exhaust is 'tuned' - inertially and acoustically. Since gas has mass it must have inertia (or 'momentum' if you want). So when an exhaust valve closes, the gas wants to keep flowing. This causes a drop in pressure on top of the valve, which helps scavenging when the valve next opens. In a narrow exhaust port/header pipe the gas has to flow faster and so it has more inertia, creating a greater drop in pressure and better scavenging. Obviously going too narrow just makes the system restrictive at higher revs.

Now then, acoustical tuning. When an exhaust valve opens, a positive pressure wave is created which travels at the speed of sound (hence 'acoustic') through the exhaust gas and along the exhaust pipe. When it reaches the end of the exhaust pipe it dissipates, creating a negative pressure wave which travels back down the exhaust pipe. If the timing is right, this negative pressure wave arrives at the exhaust valve while the valve is open, helping with scavenging. This is the 'tuning' part - since the speed of sound in a certain gas of certain temperature and pressure is fixed, the timing of these acoustic waves depends on the length of the exhaust. So if you want to improve performance at certain revs, you do some calculations to work out how long your headers need to be. It is common to tune for the third pressure wave since utilising the first pressure wave requires impractically long headers (if the wave hits a closed valve it simply carries on up and down the exhaust, but gradually losing magnitude).

That's the basics of it. The same principles exist for the intake system, but with opposite pressures. This is why fitting shorter trumpets helps with power at higher revs, and why bigger carbs are not always a good idea.

[Teflon-Mike]

Reading Wozza's post, made me think.
A note on 'Tuning'
In the case of engines and how they run, it can have three distinct, but confusing defanitions!
As Wazza explains there is 'harmonic' tuning, which is as per tuning a mucical instrument, setting up the acoustic properties of waves to get best effect.
However, from the same oragin, we get 'tuning' as in an Orchestra 'Tuning Up', and all setting thier instruments so that they play in harmony together. From THAT use of the word, we get 'Service Tuning', as in setting it all up, and adjusting stuff that has NO 'acoustic' properties, like the ignition timing, balencing the carburettors, etc, so that everything, like an orchestra is working together to best effect 'in harmony'.
Last use of the word, is an extension of the second, and is an abreviation of 'Performance Tuning', basically going further than 'service tuning' setting the engine up to best effect, but looking for ways to get more power out of the engine...... which may have nothing to do with 'service tuning' practices, but completely redesigning the engine, and sticking a turbo or super charger on it, or changing the compression ratio, altering the combustion chamber shape, two stroke port timing or four stroke cam profiles etc etc etc.... and then BACK full circle, having made those 'big' changes, to 'acoustic tuning' to make them work, changing exhaust pipe diameters, induction tract lengths, etc..... And then making them all work to best effect with 'Service Tuning'.....
Its all 'Tuning', but only one of them is using the term in the strict scientific or musical sense of sound waves.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'