Topic: Understanding duct physics Part one

[Old Scrambler]

TT.........this is one reference that may be helpful for futher discussion............I LIKE this type of info..........Rather than saying pods are bad..........the discusison will conclude that any mods from a stock system will have an effect on the performance.  Event the shape and capacity of the pods will cause different effects........anyhow........Keep it going..........

 http://www.landracing.com/forum/index.php/topic,7200.0.html

[Old Scrambler]

This was copied from the Engineer and Land Speed competitor known as Wablywalrus on the LSR.com site under Build Diaries..........

The intake valve opens on the Triumph.  This creates a low pressure area at the intake valve and it sends a vacuum wave out toward the inlet end of the intake tract.  The wave goes through the carb venturi and it reaches the end of the velocity stack.  The stack opens into an empty area and the wave is reflected back to the valve as a pressure wave.  An open pipe end reverses the wave direction and polarity.  The pressure wave arrives just before the valve closes and it pushes additional fresh fuel air mixture into the combustion chamber before the intake valve slams shut.

This simplistic explanation describes a phenomenon that would happen if I had a very long intake tract.  In reality, I do not.  I have a short tract and the wave must bounce back and forth several times before it hits the intake valve at the right moment.  The pressure wave hits the closed intake valve a few times.  This is a closed pipe end and it reverses the wave direction but not its polarity.  A pressure wave is reflected as a pressure wave.

The waves travel at the speed of sound and the engine rpm varies.  This means that these waves will help me in a narrow rpm range.  I need to tune the inlet tract so everything is in harmony at the desired rpm range.  I do this by adjusting the inlet tract length.  This controls the length of the bounce and the instant it hits the intake valve.  A short tract causes more frequent bounces and a long tract makes fewer.

There are some good theoretical equations for determining the length.  I need to know the speed of sound in the inlet tract, the inlet tract length, the desired rpm, the intake cam timing, and the inlet tract diameter.  This input data is problematic.  It is hard to determine the reflective length of a curved inlet tract with complex shape around multiple inlet valves.  Any errors of estimate are compounded because the waves go back and forth several times.  The speed of sound is dependent on gas temperature and this is hard to estimate.

No equations, no matter how good, can give accurate predictions if the input is garbage.  I have never got the equations to work well.  My inlet tract tuning method is trial and error on a dyno.  The next few posts will explain how I do it. 

[Old Scrambler]

More from Wabely on his modern Triumph Twin.........

The inlet calculations I did in 2010 are redone so they are legible.  Tuning for the third bounce can be done within the space constraints of the Triumph.  The first try uses 1100 feet per second for the speed of sound.  Many reputable tuners use this value.  It is the speed of sound in 45 degree air.  Some tuner use 1300 feet per second.  This is the speed of sound in 245 degree air.  Note how the sound speed makes a big difference on the inlet tuned length calculations.

Both pages will be posted separately so they will be large enough to read.   

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wobblywalrus
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Age: 58
Location: backwoods Oregon
Posts: 1216



    Re: Team Go Dog, Go! Modified Partial Streamliners
« Reply #648 on: January 11, 2012, 11:54:06 PM » Quote 

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This is the second page.

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I gear to run 7,500 rpm through the mile and I am trying to move the peak horsepower up to that rpm.  It is a 500 rpm shift and I can do it with intake and exhaust tuning and a different spark advance curve.  I hope.

All of this intake theory study is paying off.  I am hesitant to rely on calculations alone to tell me the correct runner length.  This is what the theory and math are telling me.

1)  Tuned length is not significantly affected by cylinder size.  I do not need to change the runner length when I go from 865 cc to 996 cc.  This is a good thing.

2)  Cam timing does change runner length.  I will need to adjust the length when I go from the 813 cam to the more radical one.  The length change is expected to be reliably calculated by the equations.  No need for an additional session of tuned length dyno testing when I fit the bumpier cam.

3)  Intake air temp significantly changes the optimal tuned length.  I need to get my cold air intakes done before the dyno work.  Also, I need to develop some length correction factors for running on exceptionally hot and cold days.  Also, I need to figure a way to adjust easily adjust the runner length on the salt.

I thought the bike would be as fast as it would go and built in five years.  It is seven years and I am just getting started on figuring out a bonehead simple NA motor.  A crazy hobby.

[TwoTired]

keeping within the humour we like to have around here mjstone,,i can photo copy part two if you want,and email it as a pdf,if thats ok with Lloyd?

I'll be waiting for that "photo copied pdf" , Dave.

MJ

I think this goes toward proving my previous point, MJ.  Educational discussions or dialogs are not tolerated on this forum if not supported by the populous faction that already has all the education and information they care to accept.

A PM distribution list sounds good, even efficient.  However, it is a text only medium.  Photos/drawings must come from another site I must either pay for or allow the owner of that site to use those photos/drawings with advertising at their discretion (Sometimes BOTH factors are within their sign up agreements).  I made some videos, that no one will host without remuneration of some kind.  There's very little altruism in this world.  And, it is actively suppressed by many who would make profit from competitive or alternative offerings.

I've been thinking along the lines of in my own web site or a book.   Of course, those are projects in themselves.  And, I'm an excellent procrastinator. 

Cheers,

[TwoTired]

TT.........this is one reference that may be helpful for futher discussion............I LIKE this type of info..........Rather than saying pods are bad..........the discusison will conclude that any mods from a stock system will have an effect on the performance.  Event the shape and capacity of the pods will cause different effects........anyhow........Keep it going..........

Thanks for your posts.  You've got the idea about further discussion/education.
I will comment that the pressure wave topic is a 2nd order effect, and there are still more basic physics that should be understood before adding that complication, such as Membranes, pressure vs/mass/volume, obstruction turbulence, venturi's, etc.  imo
Further, pressure wave tuning, whether for inlet or exhaust, really narrows the effective operating range of the engine.  This is not normally suitable/practical for street driving unless you are carrying moonshine, perhaps.     The factors that add natural supercharging effect using that technique, generally reduce the efficiency at any RPM outside of the "sweet spot".  IE RPMS yielding peak effectiveness at high speed, make lower cruise operation more costly, inefficient and produce less power than would otherwise be possible.

Cheers,

[Old Scrambler]

TT............You are correct..........I am playing around with a single-cylinder Triumph Cub to 'study' the effects of tuning mods...........When I am just tooling around on that bike, it sounds terrible, but good in another way. It has very good pulling power for its size but limited to just over 7m rpms.

My goal is to move up to multi-cylinder bikes. I have thoughts of not going for records but merely tuning a CB750 sohc to be the fastest of its kind.  Some of the builds on the HYPO page are amazing but are focussed on road racing or drag-racing.  Holding the throttle wide open for more than a mile is required to be competitive on the SALT.

I have used this page from GOOGLE for basic info.

http://www.bgsoflex.com/intakeln.html