I'll have to disagree with the comment that all of the .46s weigh "about the same."

Actually, Thunder Tiger engines tend to be heavy. OS makes very light 2-strokes. In most planes, and for most fliers, 2 or 3 ounces doesn't mean very much, but on a fun fly plane, you'll get better performance with a lighter engine. The TT .46, from their web site, weighs 15.8 ounces and the OS .46FX, 13.2. That's 2.6 ounces, which can mean a lot in a high performance plane. I own a TT .46 and it is a great engine, it's just not the lightest. I also own an OS .91FX which is lighter than just about any ball bearing .61 and produces tons more power.

When it comes to 4-strokes, the TT .91 is a real heavyweight. It weighs nearly a half pound more than a Saito .91!

Some times, if you are looking for performance, you pay for light weight. If you don't care, you get the cheaper engine, which for many applications, is just as good.

By the way, the reason many people who fly the large acro planes like the ZDZ engines is because they are very light. Great power-to-weight ratio.