Vigo
Solar Addict
Well, looking at alt fans they are 'centrifugal impellers' which means at a low enough speed they barely do anything, and at some point they will start to work, and eventually they may 'cavitate' if spun too fast but i don't know if an alternator really hits those speeds in practice or not. But i would guess the fan performance is on a bell curve and there are places on the steep part of the curve where doubling the fan speed gets you way more than double the airflow.
From memory i think most automotive alternators have a max speed rating of 12000-14000rpm or so, which is why when you put it on an engine that spins to 6-7000rpm, you can't 'pulley' it enough to make it have max output and cooling at idle, or it will kill itself when engine is at redline. If you put it on an engine with a 1000-4000rpm rev range like a typical 'small engine' then you actually could pulley it to make max output at lower rpms and not have it die if you rev the engine all the way up. But then you have to have a small engine which could actually make that power at that lower rpm, and most engines smaller than 400cc probably can't max out even a standard car alternator at much less than the 3600rpm they're governed to from the factory.
Now i do have a couple of Ninja250 engines around here..
From memory i think most automotive alternators have a max speed rating of 12000-14000rpm or so, which is why when you put it on an engine that spins to 6-7000rpm, you can't 'pulley' it enough to make it have max output and cooling at idle, or it will kill itself when engine is at redline. If you put it on an engine with a 1000-4000rpm rev range like a typical 'small engine' then you actually could pulley it to make max output at lower rpms and not have it die if you rev the engine all the way up. But then you have to have a small engine which could actually make that power at that lower rpm, and most engines smaller than 400cc probably can't max out even a standard car alternator at much less than the 3600rpm they're governed to from the factory.
Now i do have a couple of Ninja250 engines around here..