? For exhaust guru's

[mr hunt]

I've always understood fossil fuel driven engines to be nothing more than air pumps - compress as much air as possible, light it up with as little fuel as possible, then get it out as quick as possible. Based on this understanding I can't comprehend why a single pipe system is 'better' than a pipe per cylinder for torque. Can anyone explain this to me?

 

[Blackthou]

Back pressure?

 

[EasTexRIII]

then get it out as quick as possible.

Did you answer your own question mate?
Gases flowing out of less overall area?
Just guessing

 

[alpental]

I'm sure others will chime in, but you can start by looking up exhaust scavenging and reading here:
Header Theory (Part 1): Looking at the Science Behind Exhaust Header Tuning

 

[Paul Bryant]

Back pressure?

Blackthou, you really have to loose that term "Back pressure" it is the most over used and inaccurate term possible.
Back pressure is what you get when you put your finger over the end of a running hose, not at all good for getting a lot of water out of the hose.

A hint about exhaust systems, when we are talking about performance, we really are not dealing with exhaust gases but sound, or more to the point shock waves.
The shock wave travels the length of the pipe where it hits atmosphere, (end of pipe) reverses, heads back up the tube, hits the closed exhaust valve, reverses again, if all is correct this happens at just the right time when the exhaust valve is opening so as to "draw" that next spent charge out.
The diameter and length of the pipe, along with the cams used and compression ratio all conspire to make this work at any given RPM
All formulae for exhaust design ask what RPM you want the power at.

Mr Hunt, when you have a "collected" system each pulse draws the next one out as opposed to a single pipe per cylinder where (basically) the gas/shock wave just falls out the end.

There is of coarse a lot more to it than just this, but I hope this gives you some understanding.

 

[mr hunt]

Thanks @Paul Bryant ... I get the sound/ resonance stuff (I think)...but wouldn't there be an area of low pressure directly at the exhaust exit that was proportional to increasing speed of the bike? Particularly with big tapered pipes like the stock roadster. Meaning very little atmospheric resistance for the charge in the pipes? Like scavenging but in reverse?

 

[mr hunt]

I'm sure others will chime in, but you can start by looking up exhaust scavenging and reading here:
Header Theory (Part 1): Looking at the Science Behind Exhaust Header Tuning

Good link...better than any I've found so far...but now I'm even more confused

 

[R-III-R Turbo]

I can't comprehend why a single pipe system is 'better' than a pipe per cylinder

My understanding is that Cylinders don't fire at same time
(At least not when on same set of headers)
each one creating a venturi effect for the others = "scavenging"

 

[Blackthou]

Hi Paul,
Thanks. Perhaps I used the term too loosely as you say. My reasoning was that in flow measurement of liquids a more stable flow at lower volumes is obtained if discharging against a nominal back pressure. It helps the liquid amass momentarily then exit more consistently and as you say draws the liquid out which I guess is what your returning shock waves do. The one think that always baffles me (no pun intended) is why do some tuners vent through short, open cut pipes?

 

[Paul Bryant]

Thanks @Paul Bryant ... I get the sound/ resonance stuff (I think)...but wouldn't there be an area of low pressure directly at the exhaust exit that was proportional to increasing speed of the bike? Particularly with big tapered pipes like the stock roadster. Meaning very little atmospheric resistance for the charge in the pipes? Like scavenging but in reverse?

I think that the low pressure area you are talking about at the exhaust exit compared to the exhaust charge would be like comparing a fart to a Hurricane