[Kern O]

Nice ride, Glen.......

About the variable power: I've going to offer what I know (or think I know) about engines and the EPA. So, I'll do my best to make it understandable. That's not a word often used with the EPA. We need to start with a basic explanation of how a current generation engine is managed. Then, we'll look at what BMW may have done.

The use of a computers to determine fuel, timing, boost and cam position has really improved the efficiency and output of engines. But even with better cylinder heads, improvements in intake and exhaust flow, we still come up against some basic rules in spark ignition engines. The biggest one is the air fuel ratio. Get it too lean and it pings, get it too rich and unburned fuel goes out the exhaust. Since we want the best blend of power and economy, we also need to know where the mixture should be to make the best power and economy. Strangely, power goes up a bit as the mixture goes toward lean and begins to drop slowly as it goes rich.

So, once we get the mixture somewhere near right, we look at timing. Fire the spark too early and the cylinder pressure rises so fast that the engine is being pushed backwards as the piston rises on the compression stroke. Fire it late and you get nice flames out of the exhaust. A lean mixture burns quicker than a rich one. Likewise, as engine speed increases, the spark has to come earlier. Light loads, such as cruise speed, let you lean the mixture and advance the timing. But the moment the load goes up, you better add fuel and take away timing or ping sets in instantly.

The camshaft lets the engine breathe by opening the valves to let fresh air in and the exhaust out. But air moves slowly. If the engine was turning one rpm, we'd open the intake valve with the piston at the very top of the stroke and close it at the very bottom. Then the piston would come back up with the valves closed and compress the mixture. Once it was right back at the top, we'd fire the plug and the resulting explosion would push the piston down. Once it got to the bottom, we'd open the exhaust valve so the spent mixture could be pushed out the exhaust.

That's fine at 1 rpm. But as the engine goes faster, all of those things change because the air and fuel move slowly and the explosion takes time to come up to pressure. At higher engine speeds, the spark can occur at 35 degrees of crank rotation before the piston gets to the top. Any force generated by that explosion is actually pushing the engine the wrong way. but that loss is more than offset by making the most powerful part of the explosion occur when the crank is at an ideal angle to transfer as much force as possible to the prop (or wheels). The same is true with cam timing and fuel mixture.

So, you wind up with a map that is programmed into the computer that shows where the fuel and timing should be for a given rpm point and throttle position. Next, you need a bunch of sensors to tell the computer what is really going on. At every instant, it needs to know things like air temperature, throttle position, coolant temperature, the volume of air actually flowing into the engine so it can make its best estimation based on the pre-programmed map. But it can also cheat. Cleverly enough, it also gets input from two sensors that tell it what is really happening instead of just what is supposed to happen
Those are the O-2 sensor and the knock sensor. The O-2 sensor reads the burnt exhaust gases and tells the computer is the mixture was right or not. The knock sensor listens for ping and tells the computer how far it can advance the timing before ping starts. Put the whole mess together and you get an engine that idles well, cruises economically and will smoke the tires when you nail the throttle.

So, how did BMW come up with the variable power? By using different maps. One may have economy as the goal and the other may have power as the goal. In reality, those are very similar maps, as long as you don't mind the engine bogging as you climb into the throttle. The computer has to be able to watch the throttle position and figure out what you have in mind. If it senses a fast opening, it had better start adding fuel and taking away timing before waiting for the O-2 sensor and knock sensor to say "OK". If it does not add fuel it will ping and stumble. That gets into the issue of driveability, something that has improved a lot - along with starting. Gone are the days of pumping the pedal to start the engine and the engine hesitating as you go to wide open throttle. It is entirely possible to write one map that comes very close to maximum power and economy and many cars have them.

So, why are there different power levels available on command? I'm going to guess here, OK? I'll bet the most economical map is the default map. Why? So the vehicle had no trouble passing the EPA driving cycle for its fuel economy rating. I'll also bet it would face a huge gas guzzler tax if it was certified with the performance map. The EPA driving cycle has led manufacturers to do some "interesting" things. That driving cycle is a very specific program that is run on a dyno, not on the road. It is supposed to simulate both in-town and highway driving. It is also the same program for everyone for every car and etvery manufacturer. So, the manufacturers have analyzed it and have done about everything they can to make their cars get the best results on that specific test. I have a 2006 GTO with a six speed. The transmission is equipped with a device that makes you shift from 1st to 4th if you are driving at light throttle. Why? For better mileage, because it keeps the engine speed down and reduces friction. It is also very, very annoying. Why is it there? Because installing it improved the results of the EPA driving cycle. The automatics have a $1500 gas tax that the stick cars do not, partly as a result of that little solenoid.

Now, onto outboards. If the 200 Verado and the 275 Verado are the same engine with only a different computer (which I don't find surprising), how are they doing it? The Verado is unique in that it is supercharged. Adding boost adds horsepower. So, while we used to see the same basic engines having variations of 25 or so horsepower for different carb jetting or timing, we now see changes of 100 hp on a computer program. The Verado does it by adding boost and fuel. Blowers are wonderful in that regard. They let a little engine act like a big one. You just force more air down their throat..........But even though the blower will make more power, the frictional losses of the engine are a function of its displacement. Even if we use cylinder de-activation, the friction continues. But that friction is only a mionor factor here.

But boats have a very basic problem. What we think of a cruising in a boat, is still going up a very steep hill as far as the engine is concerned. It takes a lot of fuel to push the boat up that never ending hill. Boats also have a lot of drag and are very poorly designed for fuel economy in order to balance other factors such as ride. My 22 Talon would happily run in the high 130's on 600 horsepower. But the 600 horsepower that Glen recently bolted to his transom will only push his boat to half that. Why? The Talon was a tunnel hull and got up on a cushion of air, reducing the drag of the water to almost nothing. That nice big fuel and power eating wake it made at 20 mph was almost gone by 80 mph. At 125, it left nearly flat water behind it.

But put it in anything over a 2 foot chop and you were in for a brutal ride. The tunnel would slam and the sponsons loved to pierce the oncoming crest because the boat did not have enough lift to get the bow up in time to avoid giving you a bath of lake water.

Not so in a deep "V". They are happy to cruise through snotty water and the deeper the "V", the better the ride - at the cost of more fuel used.

The bottom line: I think any major change in fuel economy in boats will come as a result of hull design, not engine improvements.