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Thread: Mercs and megaphones--thinking the unthinkable?

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    Default 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?

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    Normally that type of HP gain SHOULD result in a speed increase, BUT, did you take advantage of the HP increase you showed by doing anything else to take advantage of it?

    In most cases a prop with more pitch would help, or even be required, to increase speed so as to take advantage of the HP increase. If you did nothing to take care of the increase and translate it to increased thrust in the water, it would not be surprising that little or no gain would be registered in speed. I have absolutely no experience with open megaphone testing versus a closed exhaust but that increase is considerable, approaching 50%. Are you positive no mistake was made. I am sure there was some increase, but that is really considerable. Not doubting your word, just seems a lot to gain with just that one change.

    You do not mention any changes made other than just the pipes. HP at the prop and the props ability to put the increase into increased "thrust" into the water is one of the keys to increased speed, with any improvement in power. During the time I raced any increase in power from the engine ALWAYS required more pitch or some changes to the prop to take full advantage of the HP increase. I seem to remember from other discussions about HP that there is some loss between the driveshaft and the propshaft, so possibly you are not seeing the entire increase at the prop, although if the increase you show is actual, that should not be the entire cause of the discrepancy. I seem to remember the major outboard mfgrs. changed the way they rate HP a few years ago to "propshaft HP" from powerhead HP to more closely indicate what was actually getting to the water.

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    Hi Bill

    Maybe I can help you understand a little but I have to know more detail questions.
    Lets start at the beginning. you state 40.6 hp and you saw 79 mph.

    What rpm was the 40.6 rpm?

    What rpm was the boat at 79mph?

    Note from your the peak of 40.6hp, what is power 500 rpms higher and 500 rpms lower?

    Need to understand the power band


    With your mod engine, did it make more torque at higher end of the rpm range? This guides you on props needed.

    It takes torgue to get the boat up and going, once the boat is off the water, it only takes rpm to go faster. most race boat speeds are best way past the peak power. If the torque drops to quick after peak power, the engine won't turn more rpms to make you speed.

    Some pipes make more power, but can limit the rpm. at that point you must harness the more power by going up in pitch.
    If you have 50 hp with 12 pitch you will so fast. With 75 hp and the same prop at the same rpm you will be same speed. to go faster you must increase the pitch to carry the load. More rpms alway make more speed, but with high rpm setup motors, they are soft on torque and thats why little props are needed.

    The true measurement of power is torque. Horse power is just a math equation. Where the torque is and how fast it fars off is the key.

    I have been doing dyno development since 1980 on stock, mod and pro engines. I have 3 active computerized dynos.
    The first thing you learn when you start doing dyno work is: Everything you thought you knew, is wrong. You must go back to the basics, and understand each step as you take it.

    Lets discuss.

    Mike Wienandt
    wptracing.com

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    Bill and Mike, you are both using sensible logic that I have also found to be true in pretty much all other motor performance situations. But this case is different, somehow everyone seems to be missing some phenomena with the hp made with megaphones. Let's take for instance the 4 cyl Merc 65, on the dyno it makes around 62 hp with great bottom and mid range power, but falls very rapidly at a certain rpm. When applied to the water on a hydro, everything makes perfect sense there also. It pulls hard up to the mid to high seventies then almost like a governor it falls on its face as expected. The 44 in quiet form has a very good power curve. With megaphones the curve is not quite the same, but it is safe to say that at high speed say 80 mph, the motor will be around 6500-7000 in my case with a 13 pitch prop. On the dyno it will be putting out in the mid to high fifty hp range at that rpm. This is pushing 50% more hp, but will not push the hydro ANY faster.

    I have run a turbocharger on a 44 and saw 90+ hp at a boost level that was safe on 100 av gas, if I were to set the boost at a point where I made the same 57 hp, I would have positively went much faster, as expected with that set up. I'm not trying to be stubborn or argumentative, but something is being missed here with pipes. If I was still obsessed with my 44, I would devise a way to check hp output through the driveshaft while on the water. Possibly a spring loaded coupler that would flex a certain amount and a way for it to 'remember' that point after a run. I feel it is safe to say that at 7000 on the dyno, the hp would not match the hp at speed on the water at the same rpm.

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    Dixon263:

    THEORY is wonderful and can provide many hours of thoughtful questions/answers, BUT nothing beats testing, especially with boats, motors, and most importantly, propellers, when questions like you have are posed.

    One of the most thoughtful, talented motor builders I have ever known who was responsible for many engines that set records and won championships, including several built for me, used to have a saying. It went this way. "If everything was known about the two "P's", everyone would be a World Champion". The "two P's" are pipes and props!! Theory tells little about these "P's". Testing tells a lot more than just formulas and numbers, but not everything, and even then there are still mysteries to be unraveled. If you were to run a poll amongst boat racers, or even average boaters, they would probably without fail tell you what has them scratching their head most is props. That is why you have to either spend the money with a good prop guy, beat on them to learn yourself, or be forever lacking in probably THE most single important thing regarding knowledge about boats and making them go fast. That is of course if you first have the HP to achieve what you are striving for. If you tell us anywhere in your original question you have gone thru the process I am describing, I missed it. Also I did not see where you mentioned the type boat involved. That as I am sure you realize is also very important in your problem.

    I am sure Mike Wiendandt, who has built a bunch of those championship motors would agree. Unless you have actually TESTED, with larger props, or different configurations of same, you are spinning your wheels. (no PUN intended) If you like to do that, fine. If not, it would be helpful to try to answer Mike's questions. Even the family Dr. needs a little help to determine whether you just have an ingrown toenail, or the problem is terminal. Few on this forum are as knowledgeable about MOD engines as he, both as a builder and as a competitor himself.

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    I've spent several years drag racing snowmobiles where a dyno was almost a must for finding the very small range that the horsepower peaked. This range on our very narrow power band of a hundred or so rpm's was so critical that we took into consideration the pipes heating over a 5-6 second run and used a varying helix clutch/converter that would follow the horsepower peak throughout the run, only changing slightly from starting line to finish line. As the pipe heated over that 5-6 seconds the peak hp would rise around 2-3 hundred rpm's. I am very aware of propping to achieve this effect and I have a wide selection from 11 to 15 pitch in several styles and diameters and it is pretty easy to match dyno peak rpm on the water. I believe the answer to my question can only be answered by thinking out of the box on this one, as I have exhausted the simple basics covered here that seem to work on other applications.

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    Default megaphone temp

    Quote Originally Posted by Dixon263 View Post
    I've spent several years drag racing snowmobiles where a dyno was almost a must for finding the very small range that the horsepower peaked. This range on our very narrow power band of a hundred or so rpm's was so critical that we took into consideration the pipes heating over a 5-6 second run and used a varying helix clutch/converter that would follow the horsepower peak throughout the run, only changing slightly from starting line to finish line. As the pipe heated over that 5-6 seconds the peak hp would rise around 2-3 hundred rpm's. I am very aware of propping to achieve this effect and I have a wide selection from 11 to 15 pitch in several styles and diameters and it is pretty easy to match dyno peak rpm on the water. I believe the answer to my question can only be answered by thinking out of the box on this one, as I have exhausted the simple basics covered here that seem to work on other applications.
    Have you tried wrapping the pipes with insulation? Lower temps when running in the wind can cool the pipes and slow down the sound wave effect. I'd try keeping those pipes hot. Lucky for me, my Yamato megaphone is encased in a mid tower where it doesn't get too cooled. But for any outside pipes on any other kind of engine, I would wrap them up.

    Also, a slight less prop pitch will free up the power curve with megaphones. When the Mark20H popper conversion was offered, 16:21 gears was included to replace the 1:1 so it would help the tuned exhaust do its job and survive the competition of hotrods. Anyway, Lesser pitch props somewhat accomplish the same effect. Maybe these ideas might work.

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    Champ, I've never tried insulating pipes on an outboard only on the drag snowmobile. The pipe temp would be a variable that would be different from dyno to real world though. First off there is no arguing that pipes help acceleration in the mid range. There are only a few things that I can come up with that could somehow cancel the large gains you see with pipes on the dyno at top end speeds. The pipe temp will be cooler, high speed air flowing across the pipe that could change pressure waves and the slight amount of weight that the pipes change the motors center of gravity rearward.

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    Love to know what RPM each setup was made at the boat's top speed. If each set up used the same prop, then the top speed should be the same, right?

    Jeff

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    This thread is pretty interesting. I would like to ask a question about megaphones. What would be the change in performance, if any, if you had a narrow cone vs. a wider cone? For example, an 8 degree cone vs. a 12 degree cone? Or a longer cone vs. a shorter cone? Again for example, an 8" cone vs. a 14" cone? And then any combination of said examples what would be the performance advantage? Thanks for any input, just trying to learn.
    Gardner Miller
    Lone Star Outboard Racing Association

    "Water is for racing. Asphalt is for the parking lot."
    Remember....Freedom isn't...."Free".......

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