I have a 4*40. It is running with a .40 Super Tiger and is running somewhat sluggishly. I had an idea. I have two Super Tiger engines. Both are the same physical size, so they should be interchangeable. One is the .40 size which is currently on the 4*40. The other is a .51. They are the same except that the .51 has been bored out. The .51 has never been run and is a spare. I was wondering about replacing the .40 on the 4*40 with the .51. What do you think?

You should be okay with that switch, however you might need to practice throttle control. Good Luck, Dave

Also, if there is a weight difference, you may have to move the battery or add weight to keep the proper CG. No big deal though.

The .51 will be overkill but not insane. What's wrong with the .40? Why not try and get it running well?

Like Dave says, and, you will probably kick yourself in the shorts for not doing it sooner.. You have probably been tweaking that 40 for some time trying to get it to run the way you want. The 51 should cure that.

I was told at ,y club to put a 55 in it (although one guy said to put a .40) and when I asked here the board was 50/50 on that being a great/terrible idea.

I plan on going to a ST.51 or Aviastar.53 (from SIG) as the OS55 is just too expensive.

I had an OS 50 SF on mine and it flew just fine. This plane will probably take up to something like a 55 AX without any problems. Just prop it correctly and it should be fine. It sounds like the engines are the same weight, so the CG should be ok.. just make a quick check before flying it.

CGr

I had the ST51 in mine for quite awhile. APC 11x5 gave it good speed and thrust. On cooler days it would go up untill out of sight. The 4*40 is not supposed to be a fast plane, I did get a lot of comments on the speed though.

APC12.25x3.75 really slow it down, fun to play with.