Mercs and megaphones--thinking the unthinkable?
There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.
Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?
possible theory of megaphones
Quote:
Originally Posted by
GHMiller
Thanks for the reply's. But....I'm looking for a bit more on the technical side. Like the how's and why's of the megaphone. What I mean is, I've always just run whatever length and degree of cone that came from whoever made them. And accepted it as the best possible combination. Is there a better combination? I don't know and thus my questions. I've never tried to tune them and want to know how and why they were made or came to the final dimensions of the finished product. And how those final dimensions affect the performance of the motor. Please keep this discussion going and I think it will tie into the original questions that started this thread. Mr. Dixon263, I hope you don't mind, if you do I will move this to a thread of it's own.
Thanks,
One thing I used to think about what really happens inside a megaphone is......
As the sound waves move outward as the cone diameter increases, the waves divide thinner and thinner, more and more. It is like if you drop a pebble in water, you will notice an initial wave made that spreads out around and divides into multiple ripples as it goes outward from the disturbance. This effect basically creates a barrier of waves that act like an expansion chamber's belly, deflector and stinger pipe. Now, revisiting the "pebble dropped in water theory", the next point is that as the pebble enters the water, air follows in with it naturally. But as the pebble is encased by water, the water then collapses around the trail of submerged air and resonates it back into the atmosphere. Basically, imagining all of this, the wave ripple theory explains how the sound waves can create a situation of a density like the water is to air in the fore-mentioned figurative scenario. The pebble represents the sudden shot of powerful exhaust gasses piercing this accumulation of waves and hot gas. The excess fuel that is wasted out behind the exhaust is like the air above the dropped pebble that enters behind it and the air being pushed back is basically like the collapse of the waves /gases resonating the fuel back into the cylinder. This can probably take place as the initial exhaust gases are going to be strong exiting whereas the fuel/air following will be weaker and easy to drive back. Maybe this could be a bit off the wall though.
A second theory of megaphones
Quote:
Originally Posted by
GHMiller
Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.
Somewhat relative to my first theory, I have a second possible idea of the true nature of these tuned pipes....
Now, in this possible scenario, as the sound waves multiply into higher-frequency/thinner waves they move slower exiting, thus the exhaust barrier is created. As the shot of exhaust collides into these waves from behind, a springing action occurs. Basically, the inner waves are pushed against waves toward the open end of the funnel therefor being compressed together. As this happens, heat between the waves causes a return resistance/expansion that drives the excess air/fuel that was scavenged behind the exhaust to be resonated back into the cylinder. Think of it like a spring....
the mechanics of the shape
Quote:
Originally Posted by
GHMiller
Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.
I have been thinking of a clear way to finally address the reason of the cone's general design, as to how it's various characteristics affect the physics involved.....so here it goes.
generally, the whole idea of a "cone" design is what I wrote of earlier. The cone affects the division of the wave into more waves. For example, If you have a straight pipe the exhaust will blast through in a wave like a shunt. However, if the pipe increases in diameter from beginning to end, the wave will break and divide (one into two ect) as it tracks along the wall of the pipe naturally. That is the purpose of the angle of the cone. As you increase the relative angle, the wave multiplies more actively in shorter distance. The only drawback is that the angle can become less affective in excess. Basically, if the angle increases too much the sound waves will not track along the inner wall and will affectively break loose. At this point, it will be no better than a vintage open exhaust. The diameter ( at the start) of the pipe is relative to the area of the exhaust port opening. This also affects length particularly taking "duration of relative time open", into account. The cone angle ultimately affects your power band. If you create a barrier of mega-waves closer to the port, your resonation of scavenged fuel will take place quickly, good for high RPM / lower port duration engines like the modified class. This can be accomplished with a higher angle cone. The needed length of the cone is due to duration of the port to ensure that the full desired function takes place before the waves exit or desipate in the pipe thus loosing the barrier, particularly due to the limit of RPMs the engine can handle or what the carburator can efficiently supply due to CFM for effective power affecting the exhaust strength.
possible adjustable-angle cone for testing
Quote:
Originally Posted by
GHMiller
champ20B,
I'm not trying to "reinvent anything" as you say, I'm only trying to learn how to make what I've got more efficient. As far as hulls go I've got a pretty good one that has won championships. Now my goal is to put together a complete package. Thank you for your input as it does add to the information that I've compiled.
I have an idea for a quick adjustable angle cone that might or might not work just for experimentation.....
One way you could test is to take thin sheet steel and roll it like a paper snow-cone cup in a sort of way. Cut some rings of plywood (one for small and one for large end of cone) to hold it in a rolled position. Next, spot weld some little tabs around the big end, the center and then the small end. With these tabs, you can put adjustable hose clamps around these three points and they wont ride or slip back. To seal the seam, just use metal tape. With this make-shift set up, you can adjust the angle of your cone in a minute with the turn of a socket driver and find the perfect angle for tuning.........I hope this would help if it hasn't already been tried.