I want to put a bigger engine in my Ninja

This question comes up quite a bit. If you'd like to try, go ahead. It's not generally perceived to be a good idea. If you have success, please report back to the board with pictures, and be prepared to answer some very pointed questions.

Posts usually follow one of two themes:

1. I'm going to put a bigger engine in my 250 frame.

2. I'm going to mod the hell out of my engine.

To which the replies are:

1. Look at the frames of the bikes that the donor engine would come from. Notice that they have a twin spar frame? Now glance at the frame of the 250... See that it's different? A swap isn't going to happen. Besides that, what do you think an engine with triple the power would do to the stock 250 frame? It would crumple the thing like tissue paper, and all your work shoehorning a Yamahonsuzaki 600cc SuperScreamer engine into the frame will be destroyed (if you're lucky) or you'll be killed (if less lucky).

2. Every few months or so someone comes along and wants to supercharge/turbocharge/nuclear power their EX250 engine. They're sure they can do it and go 800 mph. No problem, because they're smart.

Well, Mike Norman at G Force built race motors adhering to production AFM rules, meaning stock exhaust, airbox, and carbs. These developed enough horsepower to easily overwhelm the rather fragile EX250 bottom end. After destroying a large number of cranks and bearings, he eventually (nearly) perfected the process. But they were still small bombs waiting to go off. There is an inherent limit to what you can pump through a Ninja 250, and from hundreds, if not thousands, of dyno runs, we know that number is in the high 30s of rear wheel HP.

A super/turbo/nuke EX would exceed that limit easily, and for that very reason the admins poo-poo those who suggest it. It should also be noted that the (non-super/turbo/atomic) improvements from G Force cost thousands of dollars; it wasn't a simple matter of bolting on a few parts or overboring the engine.

If you can't be happy with the 250 in stock form, you're not going to be happy with the bike at all. Sell it and buy what you really want. It'll be MUCH cheaper, more reliable, and a lot less stressful.

Besides, there is a much easier way

Aside from the fact that the poster may be a cheap bastard, the main reason this question probably gets asked is that he wants a powerful engine in a lightweight bike.

Guess what? It's already been done. Here are some dry weight numbers: