Derestricting air-boxes
Engines need to breathe; here’s how to improve their respiration rate, according to Steve Cooper.
Newton’s Third Law says that for every action, there is an equal and opposite reaction. The statement means that in every movement, there’s a pair of forces acting on the two interacting objects. When a piston descends on its induction stroke it creates a vacuum which then pulls in air to occupy the volume displaced. Simplistically a 250cc piston going down drags in an equivalent volume of air and fuel but of course as we know air is hugely compressible. Get more air in along with the requisite fuel and you get a bigger bang; ultimately this is the basis of forced induction i.e. turbos or super chargers. Every motorcycle induction system is a compromise between available space, engine needs and noise emissions. Modern bikes breathe so much better simply because manufacturers have invested huge amounts of R&D on getting more air into their engines to achieve ever leaner burning to comply with the law. Many of the CMM eligible machines have rudimentary air intakes which can substantially compromise engine efficiency; not necessarily fuel consumption but certainly in terms of power output and delivery. You fettle the porting, adjust compression ratios, revise carburettor jetting or fit a freer flowing exhaust but all will have only limited effects if the engine cannot draw in the necessary amount of air it needs for optimal running. Standalone pod filters are an option, ditto less restrictive air filters or you can even run your bike with no air filtration at all; each has its own pros and cons. However there’s another option which retains the basic original design of the bike along with efficient air filtration while providing the engine with all the air it needs. Our CMM Project Yamaha RD350 has had its jetting majorly revised to overcome flat-spots and the manifold issues attributable to modern petrol. The results have been remarkable; low and mid throttle response improved but the top end felt as though there was more to come. Plug chops showed the engine to be running a little rich and the obvious route would have been to jet down accordingly. However, those who know better than me suggested the less obvious route of allowing the RD the air it needs rather than restricting its supply of fuel. What follows is a pragmatic approach to providing the engine with the air it actually requires rather than jetting down to the strictures of the inlet system.
Okay if pretty much any fool can punch holes in an air-box the obvious question must be, what effect has it had? The answer is – simply astounding. It’s hard to credit but the bike is just so much crisper throughout the entire rev range and, crucially, the top-end is now so much freer and cleaner. Plug checks show the mixture is correct and bike now pulls even better. Perhaps, best of all, the bike still looks unmodified so it won’t offend the Originality Police! When you grasp that our classic motorcycles were made at a time when the manufacturers were still learning their game you can appreciate that, 40 years on, it’s entirely possible to improve on what’s there. Okay so drilling holes in air-boxes may upset some folk but the RD was purchased with the prime intention of riding it not restoring it. Liberating the latent potential of an RD can’t be a bad thing surely? Leaning off the mixture rather than jetting down has liberated the aging hooligan within both bike and rider!