i don't like the sliding scale. coz u get punished for breaking a magic number, if you know what i mean...
i think the setup you have is actually quite good. however don't limit the sections to both capacity and cylinder numbers.
maybe have something like...
Toyotas with a capacity under 1.6L N/A (3Ks, 4Ks, 5Ks, 4E, etc)
Toyotas with a capacity 1.6L and over but under 1.9L N/A (7Ks, 4As, 2Ts, 3Ts, etc)
Toyotas with a capacity 2.0L and over 4 cylinders N/A (2S, 3S, 18R, 22R etc)
Toyotas with a capacity under 2L Forced induction
Toyotas with a capacity Over 2L Forced induction
so you lose the open category and replace it with a F/I greater than 2L, and the N/A greater than 2 litre
along with the F/I under 2 litre
just a suggestion. if you don't like it feels free to shove a sock down my throat :)