Exhaust pipe diameters

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Biscuit said:
And just a further observation. V12 Rollls-Royce Merlin aircraft engines of WW2 had virtually no exhaust system at all, just a flattened stack at each port, but they were tuned to run at a pretty constant RPM of around 2800.
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Messing about with pressure waves in exhaust and inlet to persuade a little more mixture into the cylinder may be less worthwhile in a supercharged engine.
 
Biscuit said:
And just a further observation. V12 Rollls-Royce Merlin aircraft engines of WW2 had virtually no exhaust system at all, just a flattened stack at each port, but they were tuned to run at a pretty constant RPM of around 2800.
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Merlin engines as fitted to a variety of aircraft are forced induction normally by two stage supercharger. So tuning of inlet or exhaust to aid power output is not as critical. Some versions of the engine were used to power tanks but without forced induction and considerable less power output.
 
Norton Commandos don't exactly have critically resonance tuned inlet OR exhaust systems, or a big power output stock, so changing/modifying the airbox or exhausts doesn't make much difference. UNTIL you start chasing more power, as a number of the race guys here have detailed. Merlins don't have a huge output either (bhp/litre), as do most aero engines, so open exhaust stacks for aero use is more convenient and workable.

By contrast, if you look at say Manx Nortons, where the cams almost never shut (a slight exaggeration) and their inlet and zorst systems strictly set at 14" and 28", you had better know what you are doing or it will be slower.

I'd comment also that for more than 50 years, it was considered that getting the exhaust out was considered the impediment to better performance. It was the 1930s !!! before it dawned on folks that a bigger INLET VALVE and induction system was the key to more performance. I have an earlier engine with quite large exhaust valve DESIGN - it has been commented, many times, with 20/20 hindsight obviously that if they'd turned all those earlier engine head designs around, they'd have found true performance much much earlier.

Back to the subject of exhaust pipe diameters.
It would take some serious dyno time to sort all that out. ??
Nortons did go smaller in the pipes, and went faster, sometime in the late 1950s or early 1960s..
And my old Model 7 dommie 1950ish has a quite small carb - 1" - dommies didn't get a hotter cam for another 10 years or so, and had fairly big pipes each for a pair of 250cc cylinders....
 
Danno said:
Bernhard said:
What about the double diameter exhaust pipes that are 1 5/8 at the port then 1 3/8 at the muffer :?:
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Probably just another attempt at having your cake and eating it too. Flattened-section "Cobra head" pipes were the rage for a short time. Didn't last.
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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. :?
 
Rohan said:
acotrel said:
For Rowan about standing waves. When sound passes through an open ended pipe it reflect backwards from the open end and forms a standing wave.
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For Ellen, we are all familiar with this concept.

The problem in this comment is that exhaust gas flow in exhaust pipes is a PULSED FLOW.
For 3 quarters of the time, there is not much flowing in the pipes - so the 'standing wave' dissipates almost as soon as it forms - stand behind a Commando with peashooters sometime, you can feel the puffs of gas emerging.

And, its not sound waves that we are interested in to harness for better exhaust scavenging etc, its the pressure pulses of exhaust gas and the associated reversion waves, as Tex mentions above. There is a difference.
The sound is merely the musical accompaniment...
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You are not quite right there. Exhaust scavenging has a couple of mechanisms, as has been described here there is an acoustic mechanism that can be used to create a low pressure at the exhaust valve for a relatively small rpm band, generally the variable here is pipe length. The second mechanism is inertial tuning and this is what you are trying to describe, however I am not convinced that this will have a huge effect on a commando power
 
Its not 'acoustic', its a LOW PRESSURE PULSE, AS A REVERSION WAVE arriving at the exhaust port at the right time. !!!!!!!!!!!!!!!!!!!!!!!!!!!!

Pressure waves travel through the ocean without any sound,
earthquake waves travel through the earth without any sound, etc etc.
Pressure waves in exhaust pipes have sound associated with them,
but its not the sound thats being pulse tuned, its the pressure pulses..

Study the ripples in swimming pools, if you want it simple.....
 
Cheesy said:
The second mechanism is inertial tuning and this is what you are trying to describe, however I am not convinced that this will have a huge effect on a commando power
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I didn't even mention inertia, anywhere.

Thats more totally applicable to inlet systems.
And is terribly important in race bikes and say manx nortons, where max power is being chased.

But not very important in Commandos, like you say - having an air box and an air filter rather filters out the affect. The lengths of those manifolds and rubber connector tubes to the airbox will have been investigated though, you can bet your rubber booties on that...
 
Rohan said:
Its not 'acoustic', its a LOW PRESSURE PULSE,.
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The question you have to ask, and answer, is could you do the same tuning with pulses of hot gas down a pipe,
WITHOUT the internal combustion fire and noise.

The answer is of course .......

P.S. The entire point I was making here initially was that how can you have a 'standing wave' with a PULSED gas flow. ??
 
Bernhard said:
Danno said:
Bernhard said:
What about the double diameter exhaust pipes that are 1 5/8 at the port then 1 3/8 at the muffer :?:
Click to expand...

Probably just another attempt at having your cake and eating it too. Flattened-section "Cobra head" pipes were the rage for a short time. Didn't last.
Click to expand...

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?
 
Bernhard said:
Danno said:
Bernhard said:
What about the double diameter exhaust pipes that are 1 5/8 at the port then 1 3/8 at the muffer :?:
Click to expand...

Probably just another attempt at having your cake and eating it too. Flattened-section "Cobra head" pipes were the rage for a short time. Didn't last.
Click to expand...

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?
 
I would like to know what pipes Norman White had on the bike when he ran the quarter in 12.2 at Orange County Speedway. He had the advantage of small stature, but even so 12.2 is an amazing number for the bike.
It seems to take about 90 horses to do that now.

Dunstall experimented with pipes quite a bit, starting with larger than stock diameters . The big pipes gave him poor race results. From my reading, he eventually fitted 1 3/8" pipes to his Dominator 600. With the small pipes and other changes, his model 99 based racebike pulled away from short stroke Manxes on the straightaway.
I've also read that it was Dunstall who advised Norton to use 1 3/8" pipes on their new Commando model.

Glen
 
Rohan said:
Its not 'acoustic', its a LOW PRESSURE PULSE, AS A REVERSION WAVE arriving at the exhaust port at the right time. !!!!!!!!!!!!!!!!!!!!!!!!!!!!

Pressure waves travel through the ocean without any sound,
earthquake waves travel through the earth without any sound, etc etc.
Pressure waves in exhaust pipes have sound associated with them,
but its not the sound thats being pulse tuned, its the pressure pulses..

Study the ripples in swimming pools, if you want it simple.....
Click to expand...

You're right, it's not the sound, but the negative pressure waves move back to the exhaust valve seat at the speed of sound. Seven hundred and blah blah miles per hour, which can be translated into feet per second or any other unit of measurement that's handy for calculating pipe length, megaphone cones etc. to make the negative pressure waves arrive at the proper time to aid in extraction.

Megaphonitis is the term that was used to characterize a powerband that was enhanced at certain rpm to the detriment of that powerband at other rpm.
 
And as we all know, the speed of sound increases, considerably, as the temperature and pressures rise.
So in hot exhaust pipes flowing (released) high pressure gas, the reversion waves will be CONSIDERABLY more than that ~750 mph.

Also, anyone have any numbers for the actual speeds the exhaust gases flow out of the aforesaid exhaust pipes. ?

In inlet systems, when the flow exceeds the speed of sound (?), it gets somewhat noisy. (??)
Can the same happen in the exhaust pipes ?
Or is that the exclusive domain of supercharged beasties...
 
worntorn said:
I would like to know what pipes Norman White had on the bike when he ran the quarter in 12.2 at Orange County Speedway.
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Well, according to Norman it was a standard 750 Commando, and the year was 1972, so it would have had the standard 1 3/8" pipes with peashooters.

Ken
 
Rohan said:
And as we all know, the speed of sound increases, considerably, as the temperature and pressures rise.
So in hot exhaust pipes flowing (released) high pressure gas, the reversion waves will be CONSIDERABLY more than that ~750 mph.
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And that's why you can slide a megaphone back and forth on an exhaust pipe., to get optimal extraction in the fat part of the rpm range at normal operating tempertures. Over the 30" + or - length (estimated) of an exhaust system, the difference between 736 mph and 750 mph is fractions of an inch.
 
Rohan said:
And as we all know, the speed of sound increases, considerably, as the temperature and pressures rise.
So in hot exhaust pipes flowing (released) high pressure gas, the reversion waves will be CONSIDERABLY more than that ~750 mph.
Click to expand...

Danno said:
the difference between 736 mph and 750 mph is fractions of an inch.
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I was quoting the speed of sound in general terms at ~750 mph.
(its quoted online as 768 mph at 20 °C in dry air at sea level, but who flies at sea level ?)
(no atmo/barometric pressure quoted, we notice)

It varies, CONSIDERABLY, with temperature and pressure.
In an exhaust pipe, at some hundreds of degrees, and quite some pressure,
I think I once calculated it out at approx 900 mph. (averaged ?).
(Its quoted online at ~1250 mph if say the gas temp in the pipe was 500 °C, not impossible at full throttle)
(and even more if there is any pressure associated with it).
Not insignificant numbers ?

It will vary with throttle too, obviously, (temps and pressures go up and down) so is quite a moving target.

Anyone know the formulas ?
Anyone know the exhaust gas speed in the pipes, or formulas.
Beyond my ken what I knew back then....
 
Phil Irving, in the link above, recommends using 1500 ft per second as the speed of an acoustic wave in exhaust gas, 1100 ft per second being the speed of sound in air.
 
So he has settled on an average of ~1000 mph for the pressure inversion

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