Mercs and megaphones--thinking the unthinkable?

[Fastjeff57]

Power BAND, not power alone, is what makes a racer fast.


Jeff

[GHMiller]

Power BAND, not power alone, is what makes a racer fast.


Jeff

Jeff, your are correct. That is what I'm trying to figure out. I think that is where the "black magic" comes into tuning with megaphones. Once you hit on the "right" porting, flow, carburetion of your set up, now how does your megaphones affect all of this? By changing length vs. cone degree you need to get the power band tuned into where you desire the rpm range to be. With our snowmobiles this was easier because all we had to do was change the clutch settings to obtain the desired power band to rpm ratio. But with outboards you are somewhat limited to what changes you can do. I believe that is the obstacle with tuning an outboard whether it is an expansion chamber or megaphones.

So if I understand this right, by having a short wider cone you raise or have a narrower power band with higher rpm's? Thus increasing horse power but not torque? By having a narrow longer cone then you would have a lower or flatter power band with lower rpm's? This would increase your torque but not horse power? Where would your compromise be? Maybe a longer but wider cone?

[Fastjeff57]

Back (way back!) in my motocross days, I built hundreds of expansion chambers to pay for my racing expenses. One thing I learned is that narrow angles equals more torque and a wider power band, where sharper angles does the opposite. Not sure how this relates to megaphones, but my best guess is that wider angles MIGHT make more peak horse power though with a narrower power band--great for top speed, but lousy out of the corners. The length of the megaphones, generally speaking, should effect the rpm where the horse power peaks: longer equals lower.

Hope that helps.

Jeff

PS With just a pipe (and bolt on flywheel) I turned by ferocious TM 400 motorcrosser into a mild mannered endure bike. It's amazing what a chamber can do.

[champ20B]

simple solution to megaphones:

The fact is, there is only so much one can do in modified class as far as porting ect is allowed. Everybody doing this that has done it, even for a little while, are doing these things. All the Mercs out there are the same engines that have been ran since way way back. Everything that could be done has gone all the way for these older motors. Unless it is a looper, its no good for anything like pro and even loopers are more-over outdated. A good Rossi or Konny would destroy them in any race. Mercs are still pretty good for mod classes though. I believe driving technique and instinct are more attributed to champion drivers than "magic motors". I don't believe in Magic motors, just good set up and proper specs, and good driving. The cones that have won on these engines can simply be copied and they will do the job the same. Nothing new or any improvements can be made beyond what already exist.

[Art Kampen]

Wrong,Wrong, Wrong

[Seagull 170]

The problem with copying the megaphone lengh from another similar motor is that the powerheads are similar, not identical.

The length of the pipe is dependant on the exhaust port duration, & the exhaust gas temperature which is affected by the mixture, compression ratio, ignition timing, megaphone taper angle & scavenging efficiency.

The answer to all these variables is testing, testing & more testing!

[champ20B]

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.

Just as I already explained as quoted above, with feasible theories to support before...... I know about the areas of tuning in design.

The only thing left to explore though, is to take advantage of the loopholes in the tech rules to build faster hulls and (gearcase customizing, if possible to allow). That is something it seems a lot of racers aren't really focusing on enough. Those things seem to stay the same as "tried and true". All there is, is engines and trying to magically squeeze more power from them all the time. I say, try to make more speed of the horsepower you've got, and win the race. I just don't see how (with the limitations on technical rules), much more could be done to engines that are the same to start with as what has been on the field for 30-50 years and has been raced and built, improved, and again raced and built by the best of the best time and time again. Advancements can only go so far until you have to come up with a whole new kind of engine altogether. I just don't see how one can keep reinventing what has already been invented for too long, until everything has been thought of that would make any possible positive difference.......... This is more interesting to me than thermodynamics associated with soundwave technology!

[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.

[champ20B]

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.

[GHMiller]

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.

Neat idea....