The makings of exhausts

Hello big-brains!
I was wondering, what makes road OEM exhausts quiet? Why do the fumes go back and forth a few times before coming out? Does this hinder power? How are race exhausts different, and why do they give off more power? What makes and exhaust louder?

[dizzyblonde]

race cans are like this



a standard can has a perforated tube down the middle, and packing around it.....or something to that effect. As you can see, a race can has none of this. The fumes as you put it, go in and out and around the perforated tube, in a race can,,,,,its through that great big hole down the middle, and straight out. standard is not as free flowing.

but what do I know....not seen a standard can long enough!

[yorkie_chris]

Race exhausts are ones with perforated tubes down middle. Idea is the sound waves can be damped out by the packing around the outside, but the straight-through design does not present an obstacle to flow.

Standard ones tend to have solid baffles and convoluted paths for the gas to slow and expand the gasses without robbing too much power. The standard systems are actually pretty cool on modern stuff how they manage to get so much power and still make them quiet. On old bikes standard pipes were like a cork up the back of it!

Thank you so much! That's what I guessed but I thought I'd ask anyway.

So for racing, why do people use race cans? Why don't they just use... no cans?

Also, what exactly happens to the fumes in a stock exhaust? I am told they go towards the outlet, then back, then back again. Why, and how?

[yorkie_chris]

Who knows what they do in there, cut one open and have a look
The down and back thing is for that expansion and slowing of gas, in places where it may be going out and back in same space it is to make the pressure waves (the noise) hit each other and hopefully cancel out.
The "how" is just arrangement of metal baffle plates inside.

Tracks have noise limits too. Stock cans give power without noise, race cans give more power with acceptable noise. Also exhaust length is important, if you have no can you can often lose power. There are a lot of factors at play which I'm not even familiar with. The reflection and speed of waves being important, as is gas scavenging by which the exhaust uses the pressure waves to suck some more exhaust gas out of the cylinder.

If you fit a stubby can you'll often lose midrange, especially on a race system. Makes it feel peaky. Whether this adds anything at the top is anyones guess.

I've heard that midrange thing about stubby cans. You know your stuff. Squirrel better watch it or he'll lose his place as Sensai.

Thanks so much brother, much love <3

Originally Posted by yorkie_chris

If you fit a stubby can you'll often lose midrange

sounds good though

and to add if you want to make a loud can very loud take the packing out

Just my 2 cents (I did my work experience with MHP exhausts, so know a little bit, having built both a race and road system and repaired a fair few more)

In terms of performance, an end can on its own will actually only give you minimal power increases - some of them claim up to 5bhp, it's closer to 2 or 3. What you want to increase power is a full system, as these will have pipework which has significantly fewer bends and kinks, and those bends and kinks that are present will be smoother. This allows the gasses to flow better, which in turn increases performance.

On the 'why not use any end-can' question, the end can itself produces back-pressure which is important in the cycle of an engine. The exact details elude me but they are nessecary to maintain or improve performance, especially midrange power. Stubbying an exhaust basically removes the resonant chamber of the exhaust and reduces that back pressure, hence the loss power which is most keenly felt in the midrange.

[STRAMASHER]

I have a full race system on my big retro and as well as the points made above it has a significantly bigger bore of pipe from header to where the generic race can attaches.

Apparently the small bore headers give better midrange at the expense of top end performance..... they are also double skinned so the chrome finish does not tarnish from the heat.
The race system is 4-2-1 as oppossed to 4-2 and only has a slight loss at the bottom end, negated by the weight saving anyway. Midrange more, top end much more.

A claimed 17bhp and a weight saving of 14kg certainly turns it into a different machine.

Sport bikes OE systems though were getting pretty trick until they started adding cats all over the place. Some big gains to be had on some of the new stuff, sports or otherwise. That perf killing carbuncle on Z1000's would have to go for instance.

Too much is never enuff.

[dizzyblonde]

Originally Posted by allantheboss

So for racing, why do people use race cans? Why don't they just use... no cans?

Would you really like to hear a V8 engine under you, at full chat, for long periods?
Cause thats what you'd get if you were sat on my VRap.....and it ain't feckin quiet at idle with no cans!