Sorry, the message I was trying to give is that many of the labels are just that, labels. They often vary by manufacturer. You can usually find information about every engine from usually major manufacturer on the web. The Engine product guide here in RCU is also a good source of information. I looked around some, but could not find an engine with an LS designation, but I found that many car manufacturers use LS. Who's the manufacturer? Perhaps you meant the Magnum XLS engine?

Power and weight are independent. Some engines are light and powerful, some are heavy and powerful, some are light and not so powerful, and others are heavy and not so powerful. It's all up to how the engine was designed, and is often related to cost. Lighter weight and higher power engines typically cost more.

Since weight has an effect on flight quality, one would think the lighter the better. For the most part, this is true. However, if the designer messed up and made a part too light, and not strong enough, then the part may fail. This is why I suggest looking at the reputation of the manufacturer and also of the particular engine.

Personally, I don't think light weight and a smaller block are reasons to select a particular engine for a trainer. I would think the selection criteria would be ease of use, reliability, and decent power. As a beginner you want an engine that is easy to start, has good idle and midrange speeds, needle settings that are not overly sensitive, and that is durable enough to handle the inevitable mishaps. You need to spend your time at the field flying, not tinkering with the engine settings. An engine that's easy to adjust will allow you to fly while you develop the skills to tune the engine properly.

Another important thing to keep in mind is whether the engine has the right power range to be used in subsequent planes. For example, many of the trainers say they will take .40 to .45 engines. Some people might go out and choose a decent .40 engine for this trainer. That's an ok choice, for the trainer, but when they move to their next .40 size plane they will discover that this engine may be marginal. If instead they had chosen a .45 or .46, they would have an engine with more power, but only a little heavier, and which could handle a much wider range of planes and flying styles.

FSR is a label for Schnuerle porting. If I'm remembering correctly, on OS engines this label meant Front Schnuerle Ringed. I've read the details on exactly what this does long ago, but unfortunately those details have moved to the part of my brain where all other information that's not really needed goes.

There are lots of good sources of info on the web. MECOA, a company that bought ownership of many different brands of engines, has a pretty good FAQ. It's here: http://www.mecoa.com/faq/index.htm. Among other things are descriptions of Schnuerle porting, ABC versus ringed. Someone in the extreme speed prop plane forum recently referenced a prop speed chart in the following online book: http://www.bolly.com.au/book/. I've looked at it a little and it seems to have lots of useful information.

So, back to the LS engine. I don't know the person who recommended it to you. I assume it's the guy you mentioned in another thread who offered to teach you. He's either a straight up kind of guy who is steering you in the right direction, or someone trying to unload some inventory. In this hobby you'll find both kinds. I'd like to think he's looking out for you, but a bit of verification, that you're obviously doing is never a bad thing. Look up the specs on the web. How do the weight and horsepower compare to other engines with similar displacements and prices? Look at the RCU product guide to see if anyone has rated it. Do some searches to see if people have posted their experiences, good or bad. If it looks like a good choice, go for it. If not, choose something else. The nice thing about this hobby, is there are many choices, and often many of them are the "right" choice.