Ah, the elusive fuel system "tune-up". Just how much fuel should you stuff down your motor's gullet anyway? The more fuel you burn, the more power is made. But if excess fuel is present, efficient combustion will be impossible due to insufficient oxygen. Performance can also suffer if there is not enough fuel to go with the available air, and damage can occur from being horribly lean. If things are extremely lean, the motor won't even make enough heat to hurt itself or color the plugs! In that case, tuning the motor by reading the heat evidence on the spark plugs can leave you thinking the motor is still too rich and cause you to correct in the wrong direction.
You can easily control how much fuel is going into a mechanically injected motor at wide open throttle with jets, pills, bypasses, etc. All the "knobs" are there to get pretty close if you know where to set them. When putting together a new combination, the question is, "Where do I start on the main jet?" Without getting into the gory calculations, let's explore some of the things a fuel injection expert would consider when calculating your fuel needs.
There is a certain ratio of air to fuel required in order for combustion to happen as it should. The air/fuel ratio (AFR) to achieve best torque in an alcohol motor is generally around 5 parts of air to 1 part of fuel (5:1). The AFR for best horsepower is a bit leaner at approximately 5.4:1. Which ratio should you shoot for with your setup? Actually, the answer is BOTH! The "best torque" AFR is what gets your car moving off the starting line and accelerating at its best rate. Down the track when the car is moving along pretty well, the "best horsepower" AFR is most desirable and will result in the best top speed. So how do you tune for two different air fuel ratios on one pass? Well, that's the subject of another article, but high-speed bypasses or "lean-outs" can be setup for this purpose.
Now that you know roughly what ratio of air to fuel you need, you need to know how much air is entering the cylinders to calculate how much fuel should go with it. Think hard about your setup, because it makes a difference.
Air consumption can be quite an educated guess for a normally-aspirated setup. Factors like runner volume, head flow, camshaft profile, rocker ratio, intake and exhaust tract length, air density, and barometric pressure all figure into how much air the motor will ingest. In the case of a normally aspirated motor, the ambient air must jump into the cylinders on its own. As the piston moves downward and creates a vacuum, the air rushes in to fill it. Volumetric Efficiency (VE) is a term used to describe how full the cylinders are. A motor with a VE of 100% has cylinders that are completely full of air at atmospheric pressure with the piston at Bottom Dead Center (BDC). Because the air/fuel mixture has inertia and wants to keep moving, and because the intake valve is still open at BDC, the mixture will continue to enter the cylinder even when the piston stops moving down and after it begins upward. Joyfully, if things haven't gotten too far behind (i.e., a vacuum), the cylinders can end up over full and have a VE of slightly more than 100%.
By comparison, determining how much air enters a supercharged motor is a piece of cake! The blower is a big compressor that force-feeds air into the cylinders. If the rotor tips have seals and things are fairly fresh, you can figure how much air is coming out of the bottom of the blower every time the engine turns over one revolution. In fact, you can almost forget how big the motor is entirely. If you just make sure that the mixture rushing out of the bottom of the supercharger is the right air/fuel mixture, you're almost there. The engine size or volume beneath the blower determines how much boost is created. But almost regardless of boost, that magic ratio of air and fuel must still be achieved. More boost just means that more of the magic 5:1 mixture is crammed into each cylinder. In the case of a blown motor, things like head porting, camshaft profile, and runner size aren't nearly as critical as a normally-aspirated situation. The brute force approach of cramming the mixture through the runners and past the valves makes all the little subtleties of induction efficiency even more subtle. In fact, in many blown applications, the biggest bottleneck is not getting more mixture into the cylinders, but getting the burned gasses out! An exhaust valve originally sized for a normally-aspirated setup is no longer big enough to empty an engine with a VE of up to 300%.
So about that air/fuel ratio...how do you get anything useful out of that? It isn't very convenient or sensible to measure air in gallons or fuel in cubic feet, so one must convert to some common unit. Pounds are most often used for the purpose of calculating a tune-up. Pumps are generally flow-checked and rated in gallons per minute, and you can easily figure what a gallon of fuel weighs to derive a "pounds per minute" figure. Cubic feet per minute (CFM) of airflow into the motor is fairly easy to come up with and after correcting this for the volumetric efficiency, all you need to know is what a cubic foot of air actually weighs. The ambient air temperature, barometric pressure and humidity must all be factored in to determine this weight. Only then can you make an apples-to-apples comparison of pounds-per-minute of both air and fuel going into the motor at a particular RPM.
For every pump and nozzle combination, there is a main jet that will return just the right amount of fuel back to the tank, leaving the rest to be injected into the motor. Whether the motor is normally aspirated or supercharged, an accurate fuel pump flow number must be known. Guessing or using the manufacturer�s rated figures can cause trouble. A pump that is worn and in need of service can fall far short of expected output. An inflated pump flow figure will result in a lean calculated tune-up. On the other hand, a blower that is worn and not sealing well can't deliver the air that it once did. Fortunately, in that case, the tune-up will be rich, as ingested air will fall short of expectations.
Although methanol isn't nearly as load-sensitive as nitromethane, the optimal AFR can change slightly when the load on the motor is increased or decreased. Different gears, tire sizes, or converter stall speed can make the same engine appear to be running slightly richer or leaner than before. Generally, as the load on the motor is increased, a bit more fuel can be burned.
There are a lot of things to consider when contemplating a tune-up. A great many things affect the air and fuel flow into an engine and your engine's appetite for a combustible mixture at wide open throttle. Whether you change a seemingly minor drive train or engine component or you end up in vastly different atmospheric conditions than you're used to, be sure to consider how the change will affect your tune-up.