The Science Behind DaVinci's FirePower Booster
The modified "G” 48mm carburetor is the correct choice for all V-Twin Engines when the correct booster adjusts the correct airflow. The booster controls the amount of airflow needed for an engine. So a “G” carb with the correct booster will work great on all Harley V-twins. Engines that are making over 110 hp really need two carbs, because one is not big enough at the top end.
All cubic inch engines respond best to our modifications. Large cubic inch engines, as well as Sportsters, respond with great power and a smooth ride. My wife’s 2001 1200xl Sportster has a modified “G” and it is still the best ride around. Her bike has no flaws and it always runs perfectly. The only other mod on her bike is gutted Vance and Hines Drag pipes. It has great response and a lot of power down low and up top. An 88 V-twin with carb and pipes of any combination and a 160 lb. rider cannot keep up. Granted, she only weighs 115 lbs. but the bike is heavy with extras. It still burns my ass, so I have a 95ci kit from George Brice that I am about to fire up. I hope she doesn’t find out that her bike would really respond to an ignition kit, a front belt gear, and a header. Then I’ll have to get nitrous. Then she will probably just take off her windshield. I could go on but you know what I’m going through. The reason her bike runs so well is because of the combination of a correct booster and carb. This rule applies to any engine, large or small. The combination is what carburetor “voodoo magic” is all about. The two main components you need are a carb and a booster. Just add air and fuel and you’re done.
The booster size calibrates the cfm rating of a “G” carb by restricting the airflow due to size. A bigger booster restricts airflow and increases signal (response). A smaller booster adds more air, which is needed for larger engines. Both boosters supply the correct A/F ratio according to the amount of air that passes through them. The fuel is controlled by the inside diameter of the booster. For example, the airflow through the booster is 100 times faster than the stock perpendicular brass tube just off idle. There is no more or less air but the velocity with the booster carb as it transitions is 100 times faster than the stock perpendicular tube.
The stock S&S carb has no means of accelerating the air at low and mid-range. This limits the adaptability to application and narrows the tuning tolerance. The booster design is a small venturi inside a larger one. Or a smaller carb inside a larger one. This design began in the 1920s and reached a frenzy in the late 60s with the Quadrajet Carburetor. The Quadrajet was designed by three different engineering groups within GM who challenged each other’s talent, making the Quadrajet probably the most over-engineered carb ever produced. The unique design utilized a triple staged booster. The result was a universal carb with only two cfm ratings: 750 and 850. It was used on every GM car and truck produced for almost 15 years. It was adaptable to all makes and sizes. All you need to do was just bolt it on and let the engine tell the carb what it needs. What does the engine tell a carb? I need more gas, or less gas at any given rpm. Deliver the right amount when needed.
Think of the booster as the ears that are listening. Would you rather listen to a sound from a responsive speaker design, akin to the booster? Or would you rather listen to a sawed off pipe, akin to the stock perpendicular brass tube that is not facing the sound but rather has its head turned 90 degrees. It is always advantageous to use a larger carb with a larger booster to gain signal, but of course there are limits. But to have no booster at all is not an option.
The “G” is adaptable to all V twins 883 to 145 ci. All will start and run perfectly with the right booster combination. Of course, the “G” carb is too small or too big for this range. Carbs are dumb and blind as well. The carb can hear but can’t see a thing. So it doesn't know it is on a big inch engine or a small inch one. And it wouldn't change if it could see the engine because it only knows how to hear. A carb in tune with itself will act like this on any engine. If a little bit of air is flowing through the carb, the carb delivers a stoichiometric amount of fuel to balance the ratio for combustion. If a little more is needed, it delivers more fuel and so on and so on. With the right booster kit, your carb will become more efficient and will make for a smoother ride.
Dan DaVinci 10-22-05