Induction tract length is a well-known science and what it really comes down to is what RPM you typically run your engine at.
It's a linear curve and this makes it really easy to pick the ideal length for your RPM range. There is a formula one company in the UK (forget their name right now) and they posted up the math to figure it out but it's dead simple - assuming you know what RPM you want the most efficiency at.
I made an Excel spreadsheet covering all the bases but I can't seem to find it....
Because the curve is perfectly linear, the ideal solution, when it comes to induction tract length, is a variable length tract. I believe Yamaha came up with a cheesy solution along those lines as their intake tract length changes between two positions depending on RPM. I say cheesy because the ideal solution would be a stepper motor arrangement where the tract length changes in 255 steps according to engine RPM :smile2:
The problem with a design like that is room.
But it's a well-proven science, it's been known for years, and it's linear. Aha - found the link;
Jenvey Dynamics
"A guide figure, from the face of the trumpet to the centre of the valve head is 350mm for a 9,000 RPM engine. Other RPM are proportional i.e. for 18,000 RPM the figure is approximately 175mm."
So as you can see, the higher the RPM the shorter you want the tract length to be, and the lower the RPM the longer you want the tract length to be.
So in a perfect world, you would want the tract length to vary in steps in keeping with the RPM of the engine
or in the practical world, aim for the typical desired RPM range and make 'em that length 0
Honda engineers are not stupid. I went after power in this respect with a different Honda and measured the induction tract length to the intake valve center. They were off by 5mm........this led me to believe their engineers were aiming for a slightly different RPM range then what I had in mind but 5mm is nothing.
Like I said, it's very simple math and when Honda engineers design an engine they will run the math and then choose the best length according to their desired RPM range.
I found the file too. I'll see if I can attach it....