Velocity stacks.

[Katman]

Anyone have any theories as to why Honda would use two different length velocity stacks on the NC35? - 35mm for the rears versus 75mm for the fronts.

(Apart from the fact that all four couldn't be 75mm as they would get in each others way).

[vifferman]

I've heard it's to do with the airflow within the airbox (Helmholtz resonance, and all that). The RC46 is the same, and if you get them the wrong way round, the engine doesn't run quite as well.

[Morcs]

Anyone have any theories as to why Honda would use two different length velocity stacks on the NC35? - 35mm for the rears versus 75mm for the fronts.

(Apart from the fact that all four couldn't be 75mm as they would get in each others way).

Could have originated from them not being able to fit, and is countered by how the airflow is controlled. Have a look a the airbox, and how the air would flow and feed into the stacks.

[vifferman]

Could have originated from them not being able to fit.

Nup.
It's nothing to do with fitment - all to do with airflow.

[Katman]

Cheers for that.

Yep, looking at the shape of the airbox and position of the intake it makes sense that the rear carbs would be fed at a faster rate.

[xwhatsit]

Well, looking at how harmonics work etc. (from music as well as things like exhausts), harmonics will coincide if you simply double or halve the frequency. There are other harmonics too, but those are the once that work together the best.

So, if for some reason you can't have the harmonic that you get with a 75mm long intake trumpet (because there's no room for it), if you halve the trumpet to 35mm (taking into account differences in the shape of the bell etc., 35mm might very well work out to work), and you halve the harmonic, then that might be what Viffermon is talking about.