I can think of two reasons someone may consider doing the ZXR900 conversion:

1) They have killed their stock ZXR750 engine somehow and need a replacement.

  • This was my situation.
  • ZXR750 engines (1991-1995) are very difficult to find in Australia due to the ZXR being a very popular race bike here.
  • Compatible ZX9 engines (1994-1996) are easier (than the 750) to find in Australia, often found sitting around in wrecked ZX9's. However since this conversion has become more popular, good ZX9 engines are becoming harder to find.
  • In Australia either engine sells for approximately the same price due to demand.

2) They want more power!
<ul><li>The existing engine probably runs fine, it would be a waste to replace the engine completely.</li></ul>Alternatives
A common alternative to fitting the ZX9 engine is known as the “793 conversion”; fitting a 1996 ZX7 top end to a 1989 ZXR750 bottom end; resulting in a 793cc displacement (when combining the ZX7’s wider bore with the ZXR’s longer stroke).
I am unsure of the modifications required for the top-forward engine mount after fitting the ZX7 head.

Factors
The easiest/best options would depend on the model of bike you’re starting with, and the model of engine(s) available.

For example; mounting a 1994-1996 ZX9 engine into a 1993-1995 ZXR750 frame is almost a direct bolt-in. Only requiring a small amount of modification (clearing room for a few slightly larger components) to the frame.
Where-as mounting the same ZX9 engine into a 1996 ZX7 frame requires mounting adapters to reposition the mount points for the later model frame.

This post is a work in progress and will be updated over time, as I discover more specific details about other conversions on different model frames & engines.<p></p>