I was fortunate to spend a lot of time with Jim Allen, Rod Geraghty, Howard Power, Alessandro Rossi, and Ginnaty Kalistratov. Those people were involved in engine design and manufacturing. In my opinion, they were the most intelligent RC engine people I ever met.
Howard Power was a boat racer and a Physics Professor for the University of California. Howard also wrote a monthly article for RC Modeler. In one article, he explained the physics of a model engine carburetor. It was difficult to get through the article withou falling into a comatose state.
Basically, the amount of fuel charge going into the engine is limited by the engine's bore and stroke. A good tuned pipe boosts the amount, but not much.
I does not matter how much nitro content is in the fuel. You can't put more fuel in the engine than the engine can hold.
Whatever the carb bore is, you can't get more fuel into the engine by making the carb bore bigger.
It is the air, drawn into the carburetor, that creates a vacuum and draws fuel through the spray bar. The air, moving fast, creates the vacuum. A bigger bore means slower moving air and less vaccum. That is why big carb bores at higher altitudes have so much trouble. Thin air, moving slowly cannot draw enough fuel for the engine to run well.
A big bore carburetor can work well at sea level and low altitudes. However, if you want to run above 1000 feet altitude, a big bore carb will cause some trouble.
However, a medium sized carb bore will work well at all altitudes. The air, moving quickly through the carb will draw all the fuel you need, unless your spray bar is too small.
From what I learned from, Jim Allen, Howard Power, Rod Geraghty, Alessandro Rossi, Gennaty Kalistratov, putting a carb with a 0.40 bore or larger causes more trouble than it helps. I recommend a carb bore at least 0.335" to 0.395". These will work well at sea level and higher altitudes.
Al Hobbs
