Sexy bell mouths, place your bets...

Bell mouths do look sexy but be warned and I go by experance here, in 1980 when I first built my 850 Featherbed I had open stacks on my carbies, the Norton ran so well with them but one day I was leaving a mates place, gave the bike one big kick it back fired throught the carbie shot out a flame, the bike started but unknown to me it also started a small fire, which in turn started a bigger fire, I was sitting on it at the time, in a few seconds my whole bike was on fire, what a mad panic this caused, the flames just got bigger as I jumped off the bike and it layed down, lucky my tank wasn't bolted down and once the fuel lines melted we were able to remove the tank, ended up suffercating the fire with a large tarp, most of the damage was done to the top of the bike, new paint job completely distroted, seat melted, speedo gauge smashed and wiring completly melted, what caused this to happen, just before it happen I found one of my brand new fuel taps to be leaking, back fire through carbie, leaking fuet tap, bang, moral of this story, run air filters and also think about all the find dust and dirt that will get sucked into your motor from road riding.

Ashley

In complete agreement. I have more that a couple of stories about engine fires on old airplanes on startup.
On motorcycles the air filter should be called a flame suppressor.
No way would I ride a street bike with open intake.

So which is the best proper or logical path, long term security or maxed out breathing for a risky intervals?

hobot said:

So which is the best proper or logical path, long term security or maxed out breathing for a risky intervals?

Click to expand...

Hi hobot.
I have a piece of string. How long is it?
Ta.

When will the dyno results appear?

hobot said:

So which is the best proper or logical path, long term security or maxed out breathing for a risky intervals?

Click to expand...

Since when does anything that you do have to do with "best", "proper" or "logical"?

In your case it is absolutely "maxed out breathing for risky".......and a foam sock.

In my high performance section in my workshop manual, the stage 2 race engine they recomand using stacks with foam sock filters for best performace.

Ashley

We use K&N filters on mikuni carbs on methanol fuelled H1 and H2 two stoke speedway sidecars. If any motor is going to suffer from an air filter it would be a two stroke. With the Kawasakis, it is not a problem - they fly !

Forum has discussed and comnoz tests that anything in front of funnels, screen, foam etc hinders their full potential, unless inside a big ole air box that's difficult or taboo to even picture for Commando. Could have foam balls on a tether or foam over screen support caps, to reach down and open up when encountering a hot shot bunch of bikers to tease with then put back for normal extended use. I found a 1" thick white air filter foam still in wrapper at rural recycleler to trim for front of Drouin over or instead of its metal mesh and cloth layer filer. I am not in denial about being nutz to motorcycle so wallow in it. Oh yeah much as a velocity tube may help likely extending the intake length behind carb would do even better. What might help and keep stones and birds out would be a air flow straightening flat plate grill.

My understanding of tubes / bell mouths is that their entrance must be in clear air i.e. actually or extend INTO an air cleaner rather than flush mount or cover the entrance so the air flow presents unfettered. This is why the rubber bellows are as inefficient as end mounted filter pods. Using hobot's earlier photo, imagine how a grille over the mouth or a filter / foam attached to the lip would restrict flow. Say, place a filter 10+ metre away extending above and below the surface and it would have minimal restrictive effect for a cleansed water flow.
Ta.

Gravity or low pressure difference will draw a fluid through any opening, after that what matters most before throttle is smoothing the flow from turbulence. Some stacks & filters - usually cone k/n type- that are wider than the bell mouth and bigger volume than bellmouth can tactfully help flow power especially for top end. Hm stacks are double sexy being phallic tubes as well as sleek intakes.

I lived to see something similar to the turbine water intake, large slow turning whirl pool 25-30 yd wide with bushes and trash circling so dove w fins/mask into crystal clear spring water down about 30 ft to see a yard wide hole I kept my distance to mild distinct current pull as I watched surface stuff get pulled down then fast accelerated into cold black void. A buddy Bud Man with the righteous Hog chopper I rode and who ran my P!! open pipes thru college campus after midnight dropping it for me to race out next day, would enter caves in upstream directions and said caverns were so huge could swim out till serious dive lights did not hit anything in the clearer than air water.

Heres why some bell mouths roll the lip all the way around and filter should be wider at base
[video]https://www.youtube.com/watch?v=VUDu9n2bsjQ[/video]

Hello All,
Just a question, isn't there a Formula for the length of intake & exhaust tubes, for the harmonics of flow?
Peter

Hi hobot.
This is my 1980's 'tube' showing distance projecting into the air cleaner box - K&N cleaner fitted / box is spring retained at 5 points.
Ta.

Hi Chili.
Scribbled notes (veracity not established) I made in my 1973 Chiltons Norton manual says:

Induction length (inches) to rear of valve head = 228 x duration / (max. power revs - 500)
Primary pipe length (inches) from rear of valve head = (5100 x exhaust valve duration (opening to TDC in degrees)) / ((max. power revs. - 500) x 6)
Primary pipe diameter (inches) = ((cc's x 2) / 16.4) / Primary pipe length (inches)
Ta.

Alrighty Mr. Plumber Galore that is impressive tubage craft to view - makes me want to hear/feel it howl inside best filter set up going. Btw what was the answer to your stack length equation?

hobot said:

Alrighty Mr. Plumber Galore that is impressive tubage craft to view - makes me want to hear/feel it howl inside best filter set up going. Btw what was the answer to your stack length equation?

Click to expand...

I can't recall the equation result back then but will measure the actual and work back for the optimum revs according to my jottings then compare them to an induction length internet calculator. Keep in mind it is a pair of slow revving siamesed singles.
Oh, I recently refitted the perforated hamcan as the open filter 'looked' as bit shabby. Induction roar is still very evident.
Ta.

Chili said:

Hello All,
Just a question, isn't there a Formula for the length of intake & exhaust tubes, for the harmonics of flow?
Peter

Click to expand...

I found this interestinghttp://www.emeraldm3d.com/articles/emr-adj-length-intake/
If you want best torque at 6000 rpm , you'd make the intake length 15" from valve to bell-mouth, and only have 1/4" radius on the bell-mouth. The inside diameter of the ram tube would be only slightly bigger than the carb diameter. For 3000 rpm , you'd make the length 30".

There is a shorter alternative, about 3/4 of that, that uses a different harmonic. I don't have the exact formula for that handy. The intensity and effect will be less.

Exhaust length can be calculated from L = 850 (exhaust lead in degrees + 180)/ rpm , for where you want best torque improvement. Exhaust lead is the number of degrees BBDC where the exhaust valve opens. That won't necessarily be the at timing and lift given in the manual, and could be 5 degrees more. It's not much use if you put a muffler on the end of the pipe.

Hi hobot.
That jotted down formula does not account for throat ID so I used one at http://www.velocity-of-sound.com/veloci ... lator3.htm.

With a measured overall induction length of 232mm (9.1"), 32mm throat, 5 waves and 328° opening (Combat cam in 1973 book) the tuned rpm is 5620. Using 304° opening (as now fitted - Commando cam from 1973 book) the rpm are 5925. No consideration of the step created at the carby inlet is allowed for so Ludwig's design should be better / more accurate.
Ta.

Chili said:

Hello All,
Just a question, isn't there a Formula for the length of intake & exhaust tubes, for the harmonics of flow?
Peter

Click to expand...

It has to do with the speed of sound. At each intake event (valve opening) a negative pressure wave moves from the intake valve seat towards the open end of the intake tract (bellmouth, in this case) As that wave, moving at the speed of sound, reaches the end of the intake, a positive pressure wave returns to the origin of the negative wave and the timing of that wave is critical to using the positive pressure to stuff the intake charge at more than atmospheric pressure and thusly extra volume, into the combustion chamber. Valve timing, total lift and overlap all come into play. During overlap, the intake begins to open but the exhaust is not fully closed, creating negative pressure that helps suck in the intake charge, again at more than atmospheric pressure, a sort of 'free' supercharging. Exhaust tuning works the same way, with a negative pressure wave moving back towards the exhaust valve seat at the speed of sound in response to the positive pressure of each exhaust pulse. Simple, no?

The length of the standing wave in a commando exhaust pipe is the speed of sound divided by half the engine revs (don't mix the system of units you use). That gives you the wave length node to node. In many exhausts you have multiples of that wavelength to give you the optimum pipe length. In a two into one system , the header pipes should probably each be one wavelength long. In the end it comes down to experimentation rather than theory. You need the optimums to coincide with what the cam dictates. I usually set cam timings to makers specs, and adjust the exhaust and intake lengths and configurations to suit

On the subject of induction system length and shape, Prof Blair had some advice, offering a simple formula for calculating IN runner length as follows.

L = (1100 x S) / N

Where L = runner length in inches, S = number of crank deg ABDC that IN valve closes (if later than 75 degrees, use 75), and N = mid-point of normal power range(rpm).

Plugging in values for a stock Commando one might come up with the following.

(1100 * 75) / 5750 rpm = 14.3" IN runner length

This dovetails nicely with X-file's suggestion of 15" (maybe the same formula?).

The following link provides Blair's insight on bellmouth shape.

http://profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

Here's something I found on another site "www.velocetteowners.com" , that takes into account ambient air temperature for intake tract length.
Any formula you find for a car engine normally assumes a high under-bonnet temperature, which you don't have on a bike.

"Gordon Blair from Queens University Belfast, who has more expertise than most in this area gives the following empirical formulas for induction lengths (see: “Design and Simulation of Four-stroke Engines”). He maintains that it is accurate to about 3%.
The induction wave is reflected up to five times, and the equation for the length is:
L = (aC /N)
Where L = Induction length from valve head to end of bellmouth in mm.
a = reference speed of sound at 340 at 15 degrees C, 346 at 25 degrees C
C = Dimensionless intake ramming factor
* for first ramming peak C = 8900, second C= 6600, third C= 5150, fourth C= 4150.
N = rpm

If your playing around with lengths I suggest you tune the length so you get the best peak at 6000 RPM, which automatically determines the next ramming peak. This I work out to be secondary ramming peak with a length of 377mm, at 20 degrees C. This gives the third peak at 4685 RPM The theory is that then you get the
exhaust to resonate between these two peaks."