Big engine, little frame

I keep hearing locals and others talking about putting a 900/1000cc engine into a frame for a 600.
I dont get the logic in this other than a smaller bikes frame might be tighter steering or lighter.
My understanding was that a bikes frame is made by the manufacturer to handle that engine and its power output.
What percentage of a bikes weight would be taken up by the engine alone?
Am I missing something here?

its been done for decades mate. Many many decades. Smaller bike frames are generally lighter with quicker steering geometry (not always, just generally). This is the attraction.

Chris Dowde squeezed a gixxer 750 motor into a GSX400 frame back in the mid-80's, for Rob Phillis. That bike was awesome in the superbikes series of the time. Back in the late 80's they jammed FZR1000 motors into 750 frames for a lighter better-handling bike.

I have squeezed a GPz1100-based motor into a z750 frame for Period 5. The z750 frame has better bracing and steering geometry compared to the 1100 frame. So this bike steers like a 750 and is much lighter. This type of mod is not uncommon at all.....whether for track or street use.

Not sure what 900/1000 motor would fit into a 600 frame though ? Maybe a CBR9/1000 into a CBR600 frame ? This could work - it all depends on clearances and chain run alignment. A 600 frame will easily cope with the extra power - may need some bracing etc if you hot the motor up ?

There are also a few examples of ZX9 motors being jammed into ZX750 frames. This is a pretty easy mod as well - at the end of the day, most frames are capable of dealing with extra hp, as long as suspension etc is set up properly.

its been done for decades mate. Many many decades. Smaller bike frames are generally lighter with quicker steering geometry (not always, just generally). This is the attraction.

Chris Dowde squeezed a gixxer 750 motor into a GSX400 frame back in the mid-80's, for Rob Phillis. That bike was awesome in the superbikes series of the time. Back in the late 80's they jammed FZR1000 motors into 750 frames for a lighter better-handling bike.

I have squeezed a GPz1100-based motor into a z750 frame for Period 5. The z750 frame has better bracing and steering geometry compared to the 1100 frame. So this bike steers like a 750 and is much lighter. This type of mod is not uncommon at all.....whether for track or street use.

Not sure what 900/1000 motor would fit into a 600 frame though ? Maybe a CBR9/1000 into a CBR600 frame ? This could work - it all depends on clearances and chain run alignment. A 600 frame will easily cope with the extra power - may need some bracing etc if you hot the motor up ?

There are also a few examples of ZX9 motors being jammed into ZX750 frames. This is a pretty easy mod as well - at the end of the day, most frames are capable of dealing with extra hp, as long as suspension etc is set up properly.

Makes sense. Thanks