A number of BECs (Bike Engine Cars) use sausage filters, socks or unfiltered trumpets.
Sticking with my theory that I probably can't design things better than Yamaha manged and the fact that the engine would of used an airbox whilst in the bike, I thought I would stick with the same idea and use an airbox.
Using an airbox will have the added benefit of reducing the induction noise and it will allow me to direct cool air into the throttle bodies rather than picking up warm air from the engine bay.
The question is what sort of airbox, what size and what size airduct ?
Airbox Design - The Theory
I found a number of useful forum posts and links to some interesting reads. If you want a really detailed analysis on airbox design and fluid dynamics give this a go Airbox Design v1 Resonance and Airbox Design v2 pressure
This link Resonant Air boxes - Theory & Application in also provides an insightful look at motorbike airbox characteristics and their effect on noise and power.
I won't pretend I understand that much of what those article talk about but one key thing I did pickup was this quote :
A modern engine with valve overlap will naturally have a dip in the torque at about a third to a half the red line rpm. If the air box is tuned to have minimum resistance to air flow at this rpm, the dip in the torque curve will be partially filled in by the ease of pulling air into the engine.
Another worthwhile read is formula1-dictionary explanation of airboxes. It explains the effects of Ram Air charging, intake design. It also mentions carbon fibre as a good material choice due to it's heat handling capabilities.
I have saved a collection of images of designs I like
R1 Air Duct Size Calculation
So how do we determine the correct size air intake to use ?
The following quote was taken from a website that I failed to grab the link from;
Yes, the usual equation is D=(CIDxVExRPM)/(IVx1130). Assuming very good preparation for your engine, that works out to:
D=(97x0.9x9000)/203,400=3.86" diameter (~98mm)
If the head work is not first class, use 0.85 for the VE instead of 0.9. The 180 in the second term is intake velocity in feet per second. 180 is a good number, but if you prefer another, go ahead and use it.
D = (61.0237 * 0.85 * 11500) / 203,400 = 2.933in (74.50mm)
This tells me a 75mm minimum intake size should be used. I will probably use a 100mm if engine bay space permits.
What I have picked up along the way are these key things;
- An airbox should be as large as possible. 10-20x engine capacity
- More free air space between throttle bodies intake stacks and filter the better
- Use a free flowing oil based air filter where possible
- Construct from materials that reflect heat.
- The intake airduct should source 100% or as close to 100% of it's air from ambient air outside the car.
- The intake ideally should route to the front of vehicle rather than side exit
- The intake should be a short as possible.
- Adding a 90 degree bend to the intake is the equivalent of adding 48 inches in length to a 3 inch diameter tube and 64 inches to a 4 inch.
In summary, I have found more information pertaining to the importance of the intake design than the airbox design but the general rule of thumb is larger the better. I also believe the use of exposed filters, socks etc.. could be detrimental to engine performance so an airbox is the way forward for me.