It's simple really; power depends on a perfect burn. Get your mixture right and the air will ensure the fuel burns to produce maximum horsepower.
Skimp on air and unburnt fuel will go to waste with the exhaust gases, while too much air means precious energy (that's fuel, that is) will be left out of the cylinder. Fuel also helps cool the valves, cylinders and pistons so a very lean mixture will wear and melt engine internals. For complete combustion (Stoichiometric's the word to impress your mates), the fuel/air ratio must be around 14.7:1 - that's 14.7 pounds of air to a pound of petrol. However, complete does not mean optimum. Optimum combustion produces maximum power and is achieved with a slightly rich mixture (up to 10% air deficiency), while a slightly lean mixture (up to 10% air surplus) will give greatest fuel economy. Optimum fuel ratio usually sits between 12.8:1 and 13.2:1.
Got that? Now we all know how manufacturers compete fiercely to build the most powerful bike in the class, but when it comes to fuelling, their hands are tied by noise and emissions regulations. So some models might be set up to produce a flat spot at around 5000rpm to cheat the drive-by tests. Other manufacturers keep noise down by running an excessively rich mixture - this lowers the temperature in the combustion chamber for a less aggressive, quieter bang. Fuelling can also be set up lean-ish, with the resulting cleaner burn reducing emissions. In short, a bike's set up depends on how the bigwigs choose to deal with market regulations.
Then, fitting an aftermarket race can is sometimes enough to upset a balanced intake/exhaust set-up, while a full race system or top-end tune will certainly require tweaking the intake. Just how you go about it depends on the fuel system...
If all's well the fuel system flows and mixes accurate proportions of air and fuel before delivering it in the correct volume via the intake valve into the combustion chamber. Until recently, carburettors ruled on production bikes.
The main feature of a carburettor is its 'venturi' - a tube-like chamber through which air flows and mixes with fuel, ready for combustion. A smaller tube fitted with a nozzle (the 'jet') connects the venturi to a section of the fuel reservoir called the 'float chamber'. Air rushing through the venturi creates an area of low pressure that combines with the higher atmospheric pressure in the float chamber to draw fuel up and into the venturi. As it enters the fast-moving airflow the fuel vaporises and disperses. The faster the airflow, the more fuel will be drawn into the venturi - with the size of the jet determining the amount of fuel released for a given mixture. And here you have the basic operating principle of a carb.
Guide to fuel
Now then, nothing's quite that simple in real life and sure enough, one jet is not sufficient to ensure the correct carburation at varying engine speeds so different circuits - designed to override one another at different speeds - are engineered into the system.
The pilot circuit operates low speed idling (up to 3000rpm), the main jet's in charge up top while the needle circuit is the daddy and controls the greatest proportion of the powerband.
Stock needles and jets can be adjusted, or aftermarket jet kits are available and include a pilot screw (air flow) and pilot jet (fuel flow) to tweak the low-down power curve, a main jet for top end and a tapered needle to tweak midrange (although the needle controls the bulk of the powerband on a constant velocity (CV) carb, but more on this later).
Mess with jets and the float level might also need altering to strengthen the mixture. The float supplies a constant level of fuel in the chamber and it works like an er, toilet (snigger). Ok the cistern, if you like. An arm (the tang) links a valve to the float that 'floats' on the fuel's surface.
As the fuel level goes down so does the float, which opens a valve that connects the reservoir to the chamber so more fuel is released. As the fuel level rises again so does the float, obstructing the valve. Some tuners bend the tang to adjust the level.
Throttle response will feel crisper with a smoother and optimised power curve, however response can be further improved on a CV carb by fitting a weaker piston spring and tweaking the lift hole that pressurises the area above the piston, causing it to 'lift'. Confused? The throttle works like this - an obstruction in the venturi stops the airflow and thus the engine. As you twist the throttle grip you gradually dislodge the obstruction, and this controls speed. Most bikes have either slide or CV carburettors. On a slide carb the twist grip is linked by cable to a 'slide' that acts like a guillotine, controlling the venturi's airflow. But whack the throttle open and the sudden surge in airflow can cause the engine to run super-lean and hesitate. A CV carb comprises a piston placed ahead of a throttle plate (or butterfly valve). While the plate is controlled directly by the twist grip, the piston is controlled by the difference in air pressure between the venturi and the atmosphere. As the engine speed picks up so does the speed of the air flow, causing a reduction in pressure that sucks up the piston against the resistance provided by the spring, maintaining a constant airspeed through the venturi and across the jet. In short, it's a mechanical device to help the cack-handed throttle jockeys among us, while racers prefer the added feel of a slide.
Then comes fuel injection. Imagine the throttle body as the equivalent of a carb's venturi - but rather than fuel being drawn into the intake tract by the airflow, it's pumped out from a gallery of pressurised fuel (the 'fuel rail') by electrically controlled valves with a nozzle (the 'injectors').
Kawasaki built the first fuel injected production motorcycle in 1980, the Z1000-H1, and later bikes like Ducati's 851/888 and Honda's RC45 and VFR800 had a pretty successful time of it. But adapting the technology to small, high-revving engines proved tricky so the system never became mainstream with motorcycles, especially in-line fours... until 1998, when Suzuki led the electronic fuel injection (EFI) craze with its GSX-R750. Now, carbs are old hat.
Quite simply, in a world that places increasing demands on engine performance and emissions controls, fuel injection is more flexible and offers greater potential and in theory, a more precise metering of the air/fuel mixture. After all, you won't find carbs in Formula One, the pinnacle of engine development, will you? The snatchy power delivery of Honda's VTR1000 SP-1 shows how hard EFI still is to get right, but increasingly refined systems are being developed and implemented to the latest generation of production bikes.
EFI works like this: an on-board computer (the 'engine control unit', or ECU) gathers information from a number of sensors (for throttle position, crank position, cam position, air temperature, engine cooling temperature, intake air pressure, atmospheric pressure). It then refers to a table of injector settings in its memory (the 'fuel map') before sending a signal to the injectors telling them how much fuel to deliver to each cylinder.
Bikes get indirect injection where fuel is pumped into the throttle body's intake tract rather than directly into the combustion chamber. Direct injection for petrol engines is an engineer's nightmare as it leaves little time for the fuel to 'atomise' (it's actually forming a mist of tiny droplets) and disperse thoroughly in the airstream. Italian firm Bimota collapsed after it failed to make direct injection work properly on its 500cc, two-stroke V-Due bikes.
The injectors' position varies from right up close to the intake valve (this works best at lower revs) to as far back as upstream of the bellmouth entry to each intake, the latter allowing more time for the fuel to atomise at stupidly high revs (see CBR600RR injector caption). The twistgrip controls a throttle plate housed in the throttle body's intake tract, with a throttle sensor nearby. Suzuki developed the Dual Throttle Valve system which includes a secondary plate controlled by the ECU that acts like a CV carb - ie it more evenly matches the airflow with the demands of the engine. Triumph's Daytona 600 and Kawasaki's ZX-636R and Z1000 use variants of this system.
EFI allows you to make precise and specific fuel adjustments to every point in the bike's operating range by tweaking the fuel map. You can adjust the ECU's existing map, or override it with a new map using a tool like a Power Commander - a chip into which you download a fuel map and then plug it straight into the bike's ECU. A ready-made map to suit your bike and any mods (exhaust, cams, gasflow, airfilter) can be downloaded from a website or CD-Rom. If you're not that clever with computers get your supplier to do it; if you're a whiz then adjust the map at your pleasure with a computer and the supplied software package and cable - it's a good idea to check any changes on a dyno. Or you can take your bike to a PC tuning centre for a custom map.
AIRFLOW
Getting as much air as possible into the cylinder is the bane of the tuner's life and imposes a limit on engine performance - whatever the fuel system. Which is why ram air systems were invented to pick up maximum air pressure from the bike's forward motion, and why forcing air into the cylinder with a turbo or replacing some of it with nitrous is the only way to ensure ballistic gains. It's also why OE airfilters on modern bikes are not restrictive - truth is, most work exceptionally well.
There are two sorts of filters: foam filters that can be cleaned, and paper filters that need replacing when soiled (oo-er). Stock filters are increasingly made of foam, like the K&Ns and Pipercrosses. These aftermarket free-flowing filters have the capacity to flow more air, but if the engine can only suck in the amount of air the stock filter already flows it will be pointless. Oh but there is a benefit - the filter can get dirtier without restricting airflow.
But more air can also mean the flow is more turbulent and slower. This can create problems with carburettors as reduced air velocity through the venturi and across the jet means the flow will struggle to draw and carry enough fuel into the cylinder (see chart below).
Airflow determines engine characteristics so to 'detune' a sportsbike engine for a less sporty model, manufacturers fit smaller carbs or throttle bodies (Honda's Hornet 600 got the 1997 CBR600's motor with 34mm carbs rather than the CBR's 36mm). The smaller bore of the intake tract speeds up the airflow so at lower revs the engine will suck in more air/fuel and boost midrange. But this also restricts ultimate air capacity, thereby reducing the quantity of mixture the cylinders will draw in at higher revs, effectively sacrificing peak power for driveability.
