For background: these ninja can be run all day at the track (WOT 90% of the time) in temps in the high 90's and I've never seen one overheat as long as there's not a ton of idle time and there's not a mechanical issue going on. And from experience, the engine runs best under these conditions. It's uniformly hot, response is great, and generally it feels best. But we're talking heat that rolls off the bike and into your helmet when you coast into the pits and makes you sweat like a mad man. The whole bike literally gets hot. I've rarely gotten the bike this hot on street, so the difference in how the bike runs when it gets to this temp is pretty noticeable.
If you actually do proceed forward with the bigger radiator idea and nothing else, you're going to experience similar symptoms to what happens when you run a stock bike in sub-40 degree weather; that is, it's going to get to 180 (or whatever) when the thermostat opens, and then it's going to cool below that, and then close, and then open, and then close, etc etc and you'll never get the engine up to its happy hot temp. This is because the cooling system is cheap and dumb; until the thermostat opens, the cooling system is relying on a tiny hole in the thermostat to flow enough coolant to get uniform engine temps without flowing so much coolant that the radiator cools it down big time. Unlike your car, there is no bypass in this system... all coolant that flows through the engine also has to flow through the radiator. Not ideal because what actually happens is the engine lazily comes up to temperature, or if it's winter, the engine never quite gets to full operating temperature. This is why many winter riders cover 1/3-1/2 of their radiator with cardboard to reduce the effectiveness of the radiator.
To solve this cold weather issue,
there's a product called the thermobob. It's a clever replacement thermostat housing that adds a 1/4" bypass line to the system without removing the thermostat itself from good coolant flow (which can add delay to the thermostat function). What this means is that when the thermostat has not yet reached its full open temp, there's a 1/4" bypass line looping a useful flow rate of coolant through the engine, giving you quick warm ups, very consistent engine temps, ridding you of any cold weather symptoms. This should have come on the bike from the factory!!!!
If you simply MUST have a larger radiator for vanity, I'd get the thermobob. Otherwise... I wouldn't waste my money on the radiator. The stock radiator is not the prettiest thing, but cooling is one area that if you don't know what you're doing, you can make a very simple system into a pile of crap. I'm not saying your bike won't run with a bigger radiator, because it will, but if you data log some temps before and after throughout a few seasons, you'll see that a bigger radiator with more cooling capacity will not be helping your case.
(yes, I used a thermobob and yes, I logged some temps when I had and EFI conversion kit and could use a laptop to log data straight from the ECU. The guy selling thermobobs is not lying to sell a product. His data is real)
EDIT: without thermobob. Add a larger radiator and expect the delta T to be even worse.
with thermobob. Look at the consistency. This would allow you to run a larger radiator without issue, just like it allows winter use on the stock radiator without issue
