Rick, "smoothness" is a direct function of port design and duration, and ignition timing, as well as how much compression an engine has. If you look at a DA, 3W, or BME (top 3 100cc "class" motors) you notice a few design differences.

For the DA, the intake ports use ports in the piston skirt with matching ports in the cylinder wall on one pair of ports, and the others have obviously high-velocity designs, all of which promotes very smooth bottom and mid-range running, at the expense of a little top-end power lost.

The 3W has about the same exhaust timing as the DA, but has wider and less "refined" (for low rpm usage) intake ports, so the mixture has to get moving faster in order to promote cylinder efficiency. This requires that the engine not be idling, but be in a slightly higher rpm range than the DA for smoother running characteristics. The 3W basically "comes alive" when the ports start flowing properly, so the midrange and top end are better on the 3W. That's how the 3W guys like to make their engines run.

BME has taken a slightly different approach, using more intake ports, with higher velocity designs but shorter duration. This is effectively called a "short-duration, high efficiency" port that works well at lower rpm, but still has enough area to allow good high-rpm power. BME also uses 6 ports instead of the 4-5 that DA and 3W use.

DA and 3W use about 152-155 degrees of exhaust duration, whereas BME uses 170 degrees of duration. This shows the designers intent on how the exhaust system should be designed for each brand of engine. DA and 3W will get moderate gains of 200-300 rpm from an optomized "canister" muffler setup that is properly tuned to the correct length, whereas the BME is actually designed for these types of exhausts and will get a much stronger increase in power when a properly tuned system is in place. Expect 500-600 rpm from a "tweaked" exhaust from the BME.

What this all boils down to is that each engine has it's own design parameters and running characteristics. The DA WILL have the smoothest idle (which impresses a lot of people and is great for sales), simply because it is designed for that, as well as a "smooth" transition and mid-range. But, do not expect the DA to make as much top-end power as a 3W. The design trade-off basically prevents this. It's not a LOT of a power difference (whether DA owners want to admit it or not) but it's there, on a consistent basis, and about 150-300 rpm in most cases compared to a comparable 3W using the exact same prop

The 3W, conversely, WILL have a slightly rougher idle, and a greater tendency to "burble" in the lower mid-range while in the air and unloaded, but the engine is substantially stronger on the top end, which is what the 3W guys intended.

The BME, on the other hand, is an entirely different animal, and exhibits idle/midrange characteristics close to the DA, but is just as powerful (if not more so) than the 3W 106, with the added potential for even greater power boosts if a properly tuned exhaust is added.

The differences in engine design are the main reason you see people going gah-gah over one engine brand or another (then there's the marketing hype as well). Some features scream out "better design" to people who expect an engine to run a certain way. Guys who like the butter-smooth idle and mid-range will prefer a DA, usually. Guys who like a decent mid-range and very strong top end power will probably prefer a 3W. And after people learn the new running characteristics of the BME (110) there will be a hard-core following of THAT engine.

Hope this explains it.