[darbikrash]

Thanks to Kern for that very informative description of ignition and valve timing. Nicely done and easy to follow.

The Z06 Corvette has had a "valet key" feature for almost 4 years now, wherein you can place a card into a dash slot and immediatley detune the engine by about 100 hp instantly.

This is great on cars, but I'm not sure about boats, for exactly the reasons that Kern has suggested. In the marine diesel world, data is plentiful and easy to come by. Not so with outboards. Just try and find any specs on BSFC (Brake Specific Fuel Consuption) for any outboard. Not gonna find it.

It is with BSFC data that we can see the effect of engine tune on efficiency.

If you're curious, take a gander over at the Caterpillar Marine diesel site, for example, and look at any engine that they produce. What you'll find is a full range of engine HP ratings for every displacement. For example, a 12 liter engine is available in A, B, C, D, and finally the highest, E ratings. The E rating in all cases is well over the 1hp per cubic inch displacement rule for high performance. Note the BSFC is listed for each HP rating.

Now this is going to sound funny, but the higher the rating, the more economical the engine is, as evidenced by the lower BSFC data. The engines within this rating are all pretty much the same, except for fuel maps and of course, boost. We'll get to that in a minute.

This is because the higher the volumetric efficiency in the cylinder, the more power it makes for given displacement. More power equals more fuel you say? Yes it does, but for two engines operating at the same power output, the one with the higher volumteric efficiecny (and lower BSFC) will get the better fuel economy.

So a 3208 CAT rated at 435 max HP (E rating) will get better fuel economy at cruising power (say 275 hp) that the exact same 3208 rated at 320 HP max (C rating).

I use the diesel example becasuse no such data is available to consumers of outboards, or you could make the same case with Verado's for example, if they'd be kind enough to make this data available.

So I agree with Kern and would conclude that although you may be able to detune an outboard, you would not want to because you would almost certainly get WORSE fuel economy. Similarly, if you were interested in reducing the load on the engine hoping that it would last longer, detuning is probably not the best way to go.

Again taking a page from the marine diesel guys who do this type of thing all the time, it's much better to underprop the boat if the goal is engine longevity, because it is the prop that puts the load on the engine, and nothing else.

But after all of this, if you simply must detune your engine, it's pretty easy to do with forced induction engines such as the Verado. Some type of wastegate can be installed, which diverts excess boost to atmosphere (obviously before fuel is added). It's even easier on turbocharged engines, the Cummins in my Dodge truck went from 12 psi factory boost to 35 psi boost with a $6 spring added to the wastegate....

By using the wastegate approach, much of the O2 sensor algorithims remain intact, as there is just less air (and conseqeuntly less fuel) going through the engine.

But don't expect to get good mileage.