Megaphone Theory 101

[Raymond]

I started a discussion about megaphone theory in another members thread (Budget d-mod build up) and I don’t want to hijack his topic any further but I do want to pursue the megaphone theory discussion and resultant understanding; so I have started this thread. Besides I thought it would be good content for the rooky lounge.

Please follow along and chime in.

[Raymond]

Ok, this is how it all stated out…there were some pic’s of the final build with megaphones installed and I posted … “I am real interested in learning more about megaphone theory. I understand how headers helps scavenging in my GTO and how an expansion chamber can ‘stuff’ flow-through fuel charge back into the cylinder but…I am still working on how a megaphone optimizes exhaust flow and power. I suspect that it could have something to do with the increasing volume of the pipe and its affect on ‘pressure’ but…I have read some conflicting literature and am a bit confused. Anyone got a good explanation? “

[Raymond]

ricochet112 responded pointing me to http://en.wikipedia.org/wiki/Expansion_chamber

[Raymond]

I responded saying

“Yea, I got expansion chamber theory but megaphones don't have anything to reflect the pressure wave off of like an expansion chamber does so...how does it...do what it....does to improve performance? “

[Raymond]

Sam responded back and said

“When a wave leaves the end of a pipe, a weaker reflected, negative wave returns back into the pipe. If the pipe is the correct length, the negative wave will strike at the exhaust port just before the exhaust closes and slightly increase the pressure in the cylinder, but reflect again at the end of the pipe, returning as the exhaust port opens, drawing the next exhaust out behind it.

The first function of the tapered megaphone is the promote flow out, the second function is to increase the strength of the returning wave by concentrating the wave.

The working function is a double bounce, but each reflection is of reduced energy; so its not as powerful as what happens with an expansion chamber, nor does it have an expansion chamber's stinger to hold the pressure up so much.

You can hear the multiple reflecting waves by patting your hand on the end of a long thin length of pipe (open on both ends) ... listen carefully and you will hear a descending "perrf" or "perrrungg" echoing in the pipe, not just a single "pop". The echo comes from the negative wave formed when the original, main wave bursts out into free air. “

And now we are current on this discussion.

[Raymond]

Sam,

Everything else is clear to me except that I am not seeing what the reverse wave is reflecting off of. How does this reverse pressure wave get generated?

In fact what I see happing is a huge pressure drop as the wave propagates down the cone, which in my mind just helps to pull/draw all the exhaust gasses out of the chamber; much like the effect of header theory.

I want to understand

[fbref5269]

quincy looper web site

raymond,

back in the 60's/70's the quincy looper had the megs on their engines. they went thru 3 stages of developerment with the final ones liiking kike a trumpet. they were developed from research from one of the christner's sons in law, i beleive it was frank volker(sp?). you may want to go to the quincy loooper web site, it has a like on the main page here and also go to the boatracingfacts.com web site. on that site look for the quincy welding thread, there is a lot of info there from back in the day. in my looper collection i have the first 2 types of megs. i do not have the generation3 style.

frank

[mdaspit]

I said it in my original post ... when a wave goes out the end of an open pipe, a reversed wave echos back into the pipe.

The proof is in patting your hand on the end of an open pipe and listening closely ... there is not just a single "pop" but a sort of purring sound ... "purngh" or something like that.

Yeah, but...
Why doesn't it just flow out, what does it hit to bounce the echo back?

[hydroid]

Exhaust vs ambient air temp/density?

Hmmm, thinking as Bill Nye the science guy, I will ask:
Could the reflecting wave/echo be created when the hot expanded wave front moves out of pipe, it pushes the colder denser ambient air out ahead of it, as far as the physics allow. Once the wave front has passed(out the end of pipe), there is a lower density partial space behind the front which will be filled by colder/denser ambient air to fill in...???...does this work at 7000rpm?
Theoretically I understand how when the high temp(200-300-400+F) low density expanded air hits the ambient air(60-90F), it will bounce back,..the echo.

Remember those A/H-bomb videos, the wave front moves out from the center, pushes the high density air out ahead of it to the edge of its energy enveloppe?...then the ambient dense air rushes back in to fill the low density/vacuum space...that's your pressure echo coming back.

There, I have equated your 302 chamber explosion to an H-bomb,...don't tell the EPA!!!

Thoughts?...or is this acrock of s**t? LOL...I need another beer...where is that NASA racer guy when you need him?

[krazy karl]

Pipes

Though I might post a couple of pictures from the book "We Were the Ramchargers". On their first drag car, The High and Mighty they tuned the exhuast as well as the intake. First time for tuned exhaust in drag racing. The car was virtually unbeatable and set numerous records. NHRA legislated the thing off the track under GM pressure. Sorry Bowtie guys. Took pictures out of a book with a camera so quality isn't too good.
kk

[roperz111]

1970's megaphone pipe experiment

Ok, this discussion got me thinking about an experiment my Dad (Randy Johnson) tried with his D Alky Hydro 44 Quincy Looper back in the mid-70's. His motor had the "bell" shaped pipes, first straight megaphones, later he had the "trumpet" type.

He connected a water hose to each one of the exhaust headers and running back to a cockpit mounted water valve. The water valve allowed water through the hoses into the exhaust manifolds as a spray. A switch on the steering wheel activated the valve which I believe he told me he pressed it coming out of the turns (just like pulling up the pipes).

Supposedly the water spray would decrease the pressure, but I cannot remember why this would increase power. Maybe someone could explain it again? I'm pretty sure I remember this experiment as promising and at least partially successful.

Chris Johnson

[dwhitford]

Injecting water into the header pipe cools the exhaust stream, which then slows down the wave propagation through the exhaust system. Doing this makes the exhaust system ''look'' longer to the engine. Longer exhaust systems tune at a lower RPM. So injecting the water tunes the system for a lower-RPM torque (power) peak for more punch coming out of the corner.

[nbass]

More Thoughts

Throw the following into the discussion.

First point, if the gasses traveling from the exhaust port are traveling supersonic, the expansion through the megaphone causes the gasses to accelerate and because the area is ever increasing the gasses will draw hard significantly reducing the pressure at the port. However, if the gases are not traveling fast enough to exceed the speed of sound, the megaphone will cause the gases to slow and build some backpressure. Problem is that three hole exhaust ports are significantly more difficult to develop good flow greater than the speed of sound because often the pipe diameter at the port must be too large to encompass the ports and still develop gas flow at supersonic speed.

Second point, It is desirable to have a length of leadout from the ports with a very slight taper (maybe 1-degree). There is a negative (sucking) wave that is created when the gasses transfer from the 1-degree to maybe a 3-degree tapered megaphone. This negative wave will ideally return to the port before the transfer port opens. Problem: finding the right length is a bit of a guess. Most megaphone pipes are too short for a decent leadout. I have tried some different lengths on a 44 block and found longer to be better if any of the rest of the pipe is working.

Third point, as others have stated, the gas flow sends back a negative wave when it reaches the end of the megaphone, or when the gas drops below the speed of sound. In the case of pipes, the speed of sound varies with temperature and is in the range of 1800 feet per second.

Keep in mind, the gas is loosing temperature all the way out the pipe. Also, something to watch for is any sudden changes in the internal pipe shapes. Changes in shape will reduce gas temperature (bad) and will create restrictions.

Last point, add water only if you want to waste the exhaust gas energy and de-tune the pipe. Because the water will cool the gasses, the speed of sound will decrease, thus loosing useful energy. Water injection is added to many pro engines so that the pipe that is tuned for 12,000 RPM will not load the engine at 5,000 RPM by injecting water when coming off the beach or to slow the engine.

I love the topic and have broke more than one engine playing with pipes. However, breakage usually results in learning.

Have Fun!

[ZUL8TR]

2 cycle exhaust and tuned pipe

In here is a very detailed section of tuned exhaust theory that should answer most questions for why and how among other things for 2 cycle engines:

http://www.datafan.com/TunersHandboo...efiltered.html

[mdaspit]

In here is a very detailed section of tuned exhaust theory that should answer most questions for why and how among other things for 2 cycle engines:

http://www.datafan.com/TunersHandboo...efiltered.html

Awesome! Thanks for posting that! That should keep me busy for..... say the next couple of years or so.

One of the hardest things about asking questions is that "I don't know enough to ask what I don't know"

Thanks.

[nbass]

Reference Book

This is an excellent reference book that I have read front to back several times. However, note the copyright date, 1973. There has been lots of development on tuned exhaust since 1973. This book does not bring to light all the issues of trying to tune the exhaust on something like a Merk 44 engine. I have not seen anything more recent that clearly presents all the issues. Dies anyone else have a better or newer reference?