Anybody help me.. pls

I have to scale 2500cc(120kn) engine with 1/5.

which engine i have to choice???
what is a formula....

Are you building an engine for static display or choosing one for flying a 1/5 model airplane?

By the way, you will get more responses if you post your question in either the "Engine" forum or the "Questions and Answers" forum.

If the cylinder volume of the engine is 2500 cc (cm3) this corresponds to 2500 / 2.54^3 = 152.56 cu.in.

Volume scale is linear scale cubed, i.e., 1/5 linear scale corresponds to 1/5^3=1/125 volume scale.

If your model engine is built to 1/5 linear scale its cylinder volume will be 152.56 cu.in. / 125 = 1.22 cu.in.

/Red B.

John, an engine question needs to be posted in the engine forums. Or if you are trying to find out how big an airplane can use an engine of some size then you need to ask in the correct forum that relates to the size of engine.

I'm afraid your question is not very clear. So I'm just locking this and suggest you ask in whatever other forum is most appropriate.