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?