Exhaust pipe diameters

I think there is a possability that at full power the throttle is fully open . Or on the stops , as was the jargon of the time .

Rohan said:

No mention of the exhaust gas speed out the pipe ?
Will obviously depend on the bike, AND throttle opening.

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Triton Thrasher said:

And even the exhaust pipe diameter.

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Oddly enough, I sort of assumed that 'bike' would sort of come with exhaust pipe(s, each of some spec,
so yes the pipe diameter obviously has some bearing on this.
Hence the title and subject of this thread.

We still haven't come up with an actual gas speed out the exhaust....

A request was made earlier in this thread for the speed of sound at various air temps, and typical exhaust gas speed in a pipe. Regarding the speed of sound in different temperature air, this calculator from NOAA should fill the bill, and at the bottom of the conversion table is a link to the actual formula.

http://www.srh.noaa.gov/epz/?n=wxcalc_speedofsound

For reference the speed of sound in air at RT, 500, 1000 and 1500 F are 1130, 1518, 1872 and 2170 ft/sec respectively

Below is a link to an exhaust article by Burns Stainless LLC where it is suggested that the average exhaust speed is over 350 ft/sec and that the speed of sound in an EX system is on the order of 1700 ft/sec. Using the NOAA SOS calculator, this speed would equate to an average EX temp of ~750F.

http://www.burnsstainless.com/Theory.aspx

To speculate further on the subject of exhaust gas speed, we could perform a simple back of the envelope calculation for an 850 CDO and assume for the moment that the exhaust speed is due solely to piston motion (it isn’t, but this at least allows us to ballpark estimate what contribution piston speed alone might make).

At 6500 rpm the instantaneous maximum piston velocity is 104 ft/sec occurring 74 deg BTDC
The bore area is ~ 7.21 sq in
The 1-3/8” ID EX pipe area is ~ 1.48 sq in

The maximum instantaneous pipe speed would then have to be higher than the piston speed by the ratio of piston area to pipe area, or 507 ft/sec (104 * 7.21/1.48 = 507).

Again, we are talking here about piston motion only to get a glimpse of what contribution the piston alone might make. The plot below shows the relationship of piston speed to EX gas speed for the system as described above, from BDC to TDC.


All in all this information seems to compliment the suggested average EX gas speed of “over 350 ft/sec" provided by Burns Stainless.

Now we need a plumber to figure out what speed air would travel in a 1-3/8” ID pipe with 70-90 psi pushing it.

WZ507 said:

Now we need a plumber to figure out what speed air would travel in a 1-3/8” ID pipe with 70-90 psi pushing it.

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I will guarantee it is well over 507 FPS.....Jim

comnoz said:

507 FPS

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How/why did you come up with 507 fps specifically, and not say 505 or just ~500 fps. ?

Obviously too this will depend on throttle opening, so is an average, or maximum ?

At idle the low pressures will mean a lazy exit for the lukewarm gases,
and at full throttle the full cylinder pressure will be somewhat warmer and more violent....

Rohan said:

comnoz said:

507 FPS

Click to expand...

How/why did you come up with 507 fps specifically, and not say 505 or just ~500 fps. ?

Obviously too this will depend on throttle opening, so is an average, or maximum ?

At idle the low pressures will mean a lazy exit for the lukewarm gases,
and at full throttle the full cylinder pressure will be somewhat warmer and more violent....

Click to expand...

From the Phantoms "wz507" post above.

comnoz said:

Rohan said:

comnoz said:

507 FPS

Click to expand...

How/why did you come up with 507 fps specifically, and not say 505 or just ~500 fps. ?

Obviously too this will depend on throttle opening, so is an average, or maximum ?

At idle the low pressures will mean a lazy exit for the lukewarm gases,
and at full throttle the full cylinder pressure will be somewhat warmer and more violent....

Click to expand...

From the Phantoms "wz507" post above.

Click to expand...

LMAO!!

We didn't actually get an explanation from Jim as to WHY he said/disputed that though....

Too funny!

Rohan said:

We didn't actually get an explanation from Jim as to WHY he said/disputed that though....

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Actually I didn't dispute the figure. I just said that a Norton exhaust pipe is capable of flowing at more than 507FPS with 90 psi of pressure behind it.

Danno said:

Was it :?:
I am to understand that the 500 twin went better with this set up as vouched by dyno results by Paul Dunstall in his tuning book. :?

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All you have to ask yourself is which one of these "tricks" has survived in modern designs and which ones have fell by the wayside as another feeble attempt to extract more power out of an outmoded design?[/quote]

OK, fair comment – then what new way has happened on exhaust pipes since the 1980s :?:
On the two strokes there o are or were some engines with an adjustable powervale to control the exhaust valve height to broaden the spread of power throughout the rev range.
I seem to remember in the 1960s before the noise tests came along one or two four stroke riders using megaphone at the rear with a cable operated butterfly valve to close off the outlet when riding around town, so not to attract the wrong attention, similar units are available on flea bay for the tin box brethren. There also appears to be no dyno results on big twin cylinder engines showing what, if any, power advantage there is with a butterfly valve used at different rev ranges.

Bernhard said:

Danno said:

Was it :?:
I am to understand that the 500 twin went better with this set up as vouched by dyno results by Paul Dunstall in his tuning book. :?

Click to expand...

All you have to ask yourself is which one of these "tricks" has survived in modern designs and which ones have fell by the wayside as another feeble attempt to extract more power out of an outmoded design?

Click to expand...

OK, fair comment – then what new way has happened on exhaust pipes since the 1980s :?:
On the two strokes there o are or were some engines with an adjustable powervale to control the exhaust valve height to broaden the spread of power throughout the rev range.
I seem to remember in the 1960s before the noise tests came along one or two four stroke riders using megaphone at the rear with a cable operated butterfly valve to close off the outlet when riding around town, so not to attract the wrong attention, similar units are available on flea bay for the tin box brethren. There also appears to be no dyno results on big twin cylinder engines showing what, if any, power advantage there is with a butterfly valve used at different rev ranges.[/quote]

And then there are the 4-stroke exhaust tuning systems, like the Yamaha exup, that basically uses a butterfly valve to restrict the exhaust path at lower rpm to provide a stronger reflected wave for better low and mid range performance without affecting top end. The Triumph 675 had a similar system, but I've forgotten its name. Wonder if something like that could be adapted to a Commando?

Ken

It would probably be possible to use the two stroke tuning formulae and develop an expansion chamber for a four stroke, however it would probably not fit on a bike. The other thing is that a big four-stroke fitted with a megaphone is more difficult to ride well. An expansion chamber on a four-stroke might be horrible as the power band starts and finishes. Two strokes don't develop anything like the torque which comes from a commando engine. You would probably stand the bike out sideways as it came on song ?
I've ridden a two stroke with incorrectly designed chambers with greatly increased it's torque - it was not good overall.
I have no doubt that an exhaust could be designed to give much more power over a much reduced rev range - but who could ride the bike ?
I disagree that this is about FLOW - it is about standing waves and mass transfer of gases. The fact that the exhaust has a note which varies with the revs, should tell you that. Think of it in terms of an organ pipe. If you put more gas through it, the note becomes louder.

The theoretical optimum exhaust pipe length is a multiple of the wavelength at the frequency which is determined by the revs. The optimum only occurs at one frequency so the exhaust is always a compromise.

https://www.youtube.com/watch?v=jSLw01C9Pv8

Its pulses of gas pressure (-ve and +ve) that are being tuned with megaphones and expansion chambers,
not merely the associated sounds.
You could do the same with high pressure (bottled) gas if you wanted to model it,
without the associated sound effects of the internal combustions...

Why would you want an expansion chamber on a 4 stroke ???
2 strokes use them so that the convergent cone puts a pulse of (positive) gas pressure back at the exhaust port - to keep the incoming charge in the cylinder, and not flow out into the exhaust pipe.
4 strokes have separate inlet and exhaust valves to prevent this happening, more or less.

Torque - given that the last of the 250cc GP bikes were putting out 110 to 120 bhp,
that equates to 440 to 480 bhp/litre.
That is some serious mumbo.
When you work out the torque values for those engines, that is WAAAAAAY more torque
than equivalent 4 strokes can ever put out. In a fairly narrow band though, like you say.
Torque is a fairly broad term (applicable across the entire rpm range), you use it very loosely....

You keep using that "standing wave" phrase, when the flow in an exhaust pipe is PULSED.
Every pulse is a new adventure for the gases.
Constant gas flow is required to produce standing waves...

acotrel said:

The theoretical optimum exhaust pipe length is a multiple of the wavelength at the frequency which is determined by the revs. The optimum only occurs at one frequency so the exhaust is always a compromise.

Click to expand...

Obviously since the exhaust pipe length is fixed, any reversion waves arriving back at the exhaust port will be more effective only within certain rpm ranges.

When you work out the speed of the reversion waves and the length of the exhaust pipes,
there are not too many "multiples" involved at all...

Y'all will have seen those electrically adjustable-length inlet trumpets that a number of race teams are using on their engines. These expand the rpm range that the tuned length of the inlet trumpets operate at,
quite a useful extension I believe.
Anyone seen any dyno figures, such stuff is usually labelled top secret.

Bit trickier on exhaust pipe lengths ( and less influence too).

acotrel said:

The theoretical optimum exhaust pipe length is a multiple of the wavelength at the frequency which is determined by the revs. The optimum only occurs at one frequency so the exhaust is always a compromise.

Click to expand...

Alan, I think you missed the point of a system like the Exup. It allows you to tune the exhaust for best top end performance, but also have improved low end and mid range performance. At high rpm, the butterfly valve in the exhaust is wide open, and has no effect on the performance. At lower rpm it is progressively closed, until it almost completely closed at idle. According to Yamaha, the smaller opening at the end of the headers provides a stronger reflected signal, and improves the performance at low and mid range rpm. It gives you a broader powerband with no high rpm penalty. Not as efficient as using a servo system to change the exhaust pipe lengths, but a lot more practical to implement.

Ken

For anyone interested in the details of the exhaust power valves for four-strokes, these two sites provide some more info on the Yamaha Exup concept.

http://www.snowmobile.com/products/insi ... -1178.html

http://www.yamaha-motor.eu/designcafe/e ... &id=441168

I'm sure there are more detailed explanations available. These are just two that I found with a quick search.

Probably also some more info on these sorts of systems by searching for the Honda H-VIX, Suzuki SET, or the system on the Triumph 675 triple. They are all variations of the exhaust power valve concept.

Requiring and ECU and sensors to operate, I don't see them really fitting with the basic simplicity of the Commando personality. Just another interesting technological for enquiring minds.

Ken

A lot of recent 2 stroke engines have a power valve, or similar (patented) names that operates in the exhaust, to give stronger torque at lower rpms.


My old 1910 Triumph (project)(4 stroke sidevalve single) has a 'Town & Country Exhaust'
- something not dissimilar to this concept,
although much simpler (and less closer to the exhaust port.), and manually operated.
Its a little lever you press with your foot, and it opens or closes exhaust exit holes just down in front of the engine.
Very useful for climbing hills, I believe.
You can see some of the pepperpot holes low down under the front of the engine in this view, selected at random off the net.
http://www.go-faster.com/1908%20Triumph/IMG_0199.jpg
Banned in the UK more than a century ago, I understand.

Two points; 2-stroke racebikes (Yamahas were first) had powervalves to raise and lower the exhaust port (not the valve, they didn't have any) to alter tuning and the phrase "recent two-strokes" is a bit of an oxymoron similar to "military intelligence" or "business ethics". :lol: :lol: