Sunday, March 10, 2013

Airbox Design - A Black Art?

K&N's, Pod Filters, Bellmouths...
This was the first mod on everyone's list years ago. Freer-flowing filters, bigger jets, coupled with a nice loud exhaust and suddenly you were pushing out 110 on the dyno (in the pub at least).

The reason was simple. The airboxes fitted to the average bike from way back when were usually too small, too restrictive and, in terms of power, bloody useless. Yes, they might have stopped the odd house brick from being sucked through the engine, and probably quietened it down a fair bit too, but in terms of helping the engine do its thing - hopeless.

But... today it's a different matter. Lift the tank on your average race rep now and you'll see a small suitcase mounted below. It'll be fitted with a nice big filter and the bellmouths on the carbs, or injector bodies, will have easy access to fresh, filtered air. The suitcase itself is probably fed cool air from the front of the bike through a couple of sizeable plastic tubes. The manufacturers cottoned on, and now know how to make an engine breathe!

The '83 GSX750
It boasts Twin Swirl Combustion Chambers, but struggles to get a smooth, cool, filtered supply of air into them. Not a wonder with this stuck in the way!

Initial glance and you might not see a problem, but looking at it from above you can see that carbs two and three have an open supply of air, whereas one and four are pointing straight at the back of the airbox - basically restricted.

So junk the airbox, fit four separate filters and everything's fixed... or is it? Well it brings with it a new set of problems. CV carbs are often hard to set up when used with pod filters. After a re-jet you might get them to fuel nicely at idle, but then suffer somewhere else in the range. Some bikes just aren't suited to them at all, and will never run right.
Then you need to think about what's happening at higher RPM and higher vehicle speed. Sat in the garage, idling nicely, you blip the throttle and it sounds great. Out on the road, cracking on a pace, and now it has to suck turbulent air travelling past at high speed, already warmed by a hot engine. Not ideal conditions, and leading to an air-starved engine.

What kind of air does the engine want?
1) Cool
2) Filtered
3) Smooth-flowing
4) Unrestricted
5) And a large supply of it

(Is this a beer ad?)

But all this is a big ask on an older bike. Why? There's no bloody room, the carbs are already wider than that frame gap! Two cylinders would be so much easier.

The frame tubes dictate how big, and what shape, it can be and it's in two pieces because it's so difficult to fit! To improve on the original is going to be hard, but impossible? Maybe, maybe not.

Amazingly, the inlet tube on top is standard. It doesn't attach to anything and just sucks air from under the seat! Times have definitely changed.

Is there a plan?
Hopefully. Time to start designing a new box from aluminium. The original box no longer seals properly when pushed together anyway (from years of abuse); so whether I use K&N's, or make a new box, the old one has to go.
How big?
If I remember correctly, I once read an article by John Robinson, from Performance Bikes, stating that the airbox should be ten times the capacity of the engine to ensure a ready supply of filtered air. That leaves me with the challenge of fitting a 7.5L container somewhere between frame, engine, carbs and suspension. Happy days!

Who's got a number for K&N?