Megaphone length

[SPARKY]

stay ' tee    please keep pondering because this is stimulating stuff--- and at most of our ages we love stimulation     especially if we think it might help us go faster

[John Burk]

This is how I understand exhaust tuning . There are 2 main rules . A pressure wave going out returns as a vacuum wave or viceversa . A returning wave bounces off the ex. valve and maintains its phase . A pipe tuned for the 1st cycle needs to be about 6 ft long . The 2nd cycle goes out neg. and comes back as a pressure wave so it's no good . The 3rd cycle comes back as a vacuum and is a good length so that's what headers usually are . What you need is the good 3rd cycle in your power band , the bad 4th cycle below it and the bad 2nd cycle above it . If you know the temperature everywhere along the pipe , the speed of sound at those temps. and factor in the gas speed , tuned pipe length is simple math .

Trombone exhaust with constant 3rd cycle would be perfect .

[rebelce]

You Sir, have a wicked, wicked sense of humor!     

   One pipes not too bad to understand but as I read it, we have those nasty little waves coming and going all over the place. Say we use a merge collector. If originating at an arbitrary 88 degrees and timed to return at TDC (268 degrees later). A wave will return to all cylinders at that time. One will aid in cylinder filling and the rest will be reflected back out, only to return as a pressure wave 268 degrees later, not to mention all those bounces in between that land lord only knows where in the cycle of things. As 268 degrees doesn't fall back on the original timing it just keeps on. Every cylinder that fires contributes to this cacophony.  This may take more than one napkin and a beer to figure out!

[Dakzila]

Practical lessons learned years ago while trying to come up with the perfect megaphone for a DOHC 4 cylinder dragbike engine.

For the example below the header had 1-5/8 primary tubes with a merged collector.

After multiple hours on the dyno with various diameter and length megaphones, a hacksaw and a welder..

Results.....

Short length, large exit diameter (24"X 4.5") yeilded higher torque lower in the RPM range and less HP in the higher RPM range,

Longer length. smaller exit diameter (30"X 3.125) yeilder lower torque throughout the RPM range and higher HP to all the way to redline.

More important.....header primary tube diameter, and design, made a much bigger difference in high RPM HP than any changes made to the megaphone.....

Interesting topic.....

Buzz 1513B

[Interested Observer]

"...lower torque throughout the RPM range and higher HP all the way to redline."

Dakzila:  Please explain how this is possible.

[Dakzila]

I don't have the intellect to explain the physics. Just the observed results of many, many hours on the dyno.

Buzz1513B

[Interested Observer]

The physics is simple:  Hp = Torque x rpm
If the torque is reduced across the rpm range, so is the horsepower.

It is unclear what your observations were, but they cannot be as stated in that sentence.
(Maybe too many hours breathing fumes at the dyno.)

[Caveman]

Ok, someone pick apart this concept:

In the interests of aiding a weak exhaust port to the maximum possible extent, how 'bout putting the cone or 'megaphone' right at the exhaust flange, ala a 2-stroke style header?

Wouldn't the cone then positively affect the sound waves that push out from the port immediately, thus helping scavenging?

Two-strokes have a reverse cone further down the tube, past the expansion chamber that supposedly sends back a weaker, differently timed wave to help with keeping the intake charge from squeaking past the exhaust port, but if you don't build in that feature, wouldn't the concept help a normal 4-stroke port too?

Or would the benefits be outweighed by a normal tube-header design?

[stay`tee]

The physics is simple:  Hp = Torque x rpm
If the torque is reduced across the rpm range, so is the horsepower.

Yes, the physics is simple on paper and in quotations, (most hide behind this) ,, real world application brings conjecture and also facts
 (some step up)

[panic]

putting the cone or 'megaphone' right at the exhaust flange
That dates back 50+ years to the "blooey" pipe. Became a dead issue when Walter Kaaden of MZ figured out most of what has been used since - which begins with a primary before the diffuser.
There's been some improvement, but Jenning's math ("2-stroke Tuner's Handbook") is still very effective as a starting point.

[rebelce]

As we are talking about megaphones and exhaust systems, I need to ask. Are the gasses traveling down the megaphone subject to the Combined Gas Laws?  If so, the wave slows down with the corresponding drop in temperature. Do the positive and negative waves interact with one another?

[panic]

Do the positive and negative waves interact with one another

Always, Blair refers to this as superpositions.

[SPARKY]

 "Do the positive and negative waves interact with one another?"

 "And that Dr. Watson is the elementery Question!" 

[rebelce]

It seems most shoot for the 3rd order harmonic. Anyone know the formula for the 1st order?

[rebelce]

Bell gives the formulas for primary, secondary and collector (including megaphone) lengths in "Four Stroke Performance Tuning". As far as wave interaction, Dr. Jon C. Morrison of The Institute of Mechanical Engineers, concluded wave interaction within collectors was insignificant. When you think about it, in 2-cycles, the reflected positive wave has to pass through itself to push the over scavenged charge back into the port. I ask about full length tuning as I am building a set for Ray and I's next round and a Jimmy has a lot of distance between the first exhaust port and the last. At a 7,500 RPM length, there isn't much room to bring them down and into a collector pointing back under the car. I think there is much to understand concerning the overall resonance within the system and where the returning waves ( positive and negative) fall in the other portions of the exhaust cycle.