In designing the Dynamite fuel gun, I made a list of the following primary goals:

•  To fill a 125cc tank in under 2 seconds
•  Must automatically shut-off when the tank is full
•  Must fill the tank 100% full to the top of the rim every fill
•  The selling price must stay under $50

Other features that I wanted to incorporate into the design included:

•  A visible reservoir that allows confirmation of the fuel level at a glance
•  An easy way to fill the gun without the need of a separate fuel bottle
•  The ability to conveniently store the gun attached to the fuel bottle
•  A nozzle design that allows fueling enclosed-bodied sedans and stadium trucks
•  An ergonomic shape that feels and looks great
•  Must be ROAR, NORCA and IFMAR legal

From here, I started sketching basic designs on CAD. Looking back at some of those early designs is almost comical. What was I thinking! But finally the design came around to something that was starting to have promise. Prototypes were made and the initial testing proved that there were a lot of issues to overcome. One of the main stumbling blocks was managing the flow path of the fuel and air. A vent tube, located in the center of the nozzle, is used to extract air from the tank while fuel is dispensed around the tube. During the first tests, dispensed fuel was being drawn into the vent tube inhibiting flow. After much thought, a spreader was devised that attached to the end of the vent tube that would disperse the fuel. This improved the issue but we were having a difficult time getting the shape and size of the spreader right to balance the air vs. fuel flow. Using high-speed photography (see photo), we figured out a neat way to visually see the flow path and volume of the incoming air and outgoing dispensing fuel. By saturating an area with smoke and then by filling the fuel gun with water and dispensing the gun over the smoke filled area, we could clearly see the flow path of the air (indicated by the smoke) and fuel (indicated by the water). This allowed the optimization of the size and shape of the nozzle and the spreader to give the greatest volume of fuel flow.

Other issues that were addressed included tweaking the design to fill to the tank top of the lid every time, reducing the time to fill a tank to under 2 seconds, figuring out a way to make the gun easily storable in pit lane, designing an easy way to fill the gun without the need for a separate bottle, etc, etc. It took a lot of late night tweaking but we finally got the design right!
It took over 6 months from the first drawings to finally having the product finished and in stock, but I believe the effort was worth it. And at a selling price of less than forty bucks, this is a neat must- have item for every gas racer. Just remember the next time you use the Dynamite fuel gun-there is a lot more to the design than you'd think!