yet another r1 related airbox question...! right i'm in the process of trying to fabricate (read bodge ) an aluminium airbox for my 05 r1 engine and was wondering will a 75mm/3" inlet be sufficient..?
also anybody know what kind of volume i should be aiming for, or is this not critical (within reason)..?

The volume of an airbox should (accourding to general opinion) be equal to the volume of the engine or, in the case of a forced induction set-up equal to the "equivalent" engine size.....

75mm inlet isn't too small (after all if you look at the K+N conical filters they often have a neck narrower than that for 200Bhp+ filters).....

You generally also want to leave at least a choke diameter between the top of the inlet trumpet and the "top" of the airbox. Anything less than that and you may in fact create more power (even if the flow is less linear) by removing the trumpets...

FWIW, I'm going for a 100mm diameter plenum with a total capacity of 1600cc for my blown 1098 ST1100....

I found the inlet diameter to my air box needed to be 75mm diameter or bigger. Above this size had no noticeable effect. Smaller made the fuelling go off.

As for volume, if you don't want to tune for a specific engine speed (e.g.to fill out a flat spot in the torque curve) make it as big as you can.

The box volume, inlet pipe diameter and length all make the box resonant at a certain engine speed. This makes the engine breath really well. Below this speed it will be slightly worse. Above will be as normal. I've posted details on here before if you want to search.

2 other tips:

Make sure the wall of the airbox is at least 1.5 time the diameter of the throttle body away from the inlet to the throttle body or it will restrict breathing.

Make sure you have a "trumpet" or rounded edge to your throttle body inlets and your air box inlet. It restricts airflow at high revs if you don't.

I wanted to have the same volume as the original airbox when I made mine. I taped off all the holes of the original airbox, filled it with water and measured how much water went in. I don't know exactly what I ended up with, but it was a lot more then 1600cc.

Great info, decided to do the same thing for my ZX9, and then enclose the bonnet to try and reduce noise as much as possible

thanks for the info, now i need a bit more advice

the airbox i'm making will be approx 1.5l volume, and the pipe i've got made up for the inlet is 3" dia, but to reduce the height of the airbox i'd like to ovalise the end of the inlet where it is welded on to the end, will this then restrict the flow even tho the area is still the same..?

thanks.

If you squash the pipe to make it oval then you reduce the open area of the pipe. Also round pipes with the same open area as other shapes flow more air, so a squashed oval pipe will flow less air on two counts.

1.5 litres? That's small. I went for a bigger box than Yamaha supplied, not smaller. First, I'd advise a bigger box. Second, if yoy're going to have a small air box, then you should have a very short inlet pipe to reduce resonance. Get it wrong and you'll introduce peaks and troughs into your torque curve.