Well, color me excited to come across a bunch of reading to do about a turbocharged Kubota 3cyl as i have B6100, b7100, b8200, and an appropriately sized turbo sitting around (among many INAPPROPRIATELY sized ones).
I tripled the wheel horsepower of a minivan by turbocharging it on the stock bottom end. I'm an ASE Master Tech, teach automotive at a community college, and have turbocharged multiple vehicles and cracked my share of piston ringlands along the way.
Maybe if i'd said that originally someone would have acknowledged what i said originally in post #11. You don't have to gain 'peak' power to gain power with a turbo. You don't have to add fuel. You will be able to burn your existing amount of fuel better than you currently can in certain situations. It will basically help you not 'bog under load'. Any time you lose rpm and your exhaust smoke darkens under load, that's when a turbo would do something for you on stock fuel.
By extension it also stands to be mentioned that anytime you can see your exhaust darker than the surrounding air, you ALREADY have more fuel than you're burning, which could be burned with the addition of a turbo. You also have to consider that it takes more fuel to make power under load at low or mid rpms, then it does to have the engine sit at the governor rpm with no load. If you just rev your engine to max rpm with no load, you are only giving it enough fuel to spin the engine at that speed (but not put out much power to the end of the crankshaft). There is much more fuel available then what you're using in that situation. The only time you are actually using all the fuel the stock injection pump will deliver is when you are LOADED at max rpm to the exact right extent that you are staying at max rpm, and not bogging from the load, and your exhaust is still 'clean' (not dark). That's balancing on the head of a pin (ie basically never happens)! At all other times, there is more 'stock fuel' available than what you're using, and you could use some of it if you had a turbo. So all this stuff about adding fuel i think is tinged with some misunderstanding.