By Chris Beardsley:
Unlike port fuel injection systems, carburetors have a unique advantage while operating on boosted engines without an intercooler.
In carbureted applications, the air charge from the supercharger is significantly warmer than ambient air. When warmer air is forced through a carburetor, the *vaporization process is enhanced as the cool fuel mixes with it. Ever try starting your carbureted engine in the dead of winter? Now compare that to a hot August afternoon. The warmer air of the supercharger blowing through the carburetor amplifies the vaporization process. The result of superior vaporization is a cooler, denser air charge under pressure.
The warmer air mixing through the carburetor does something else just before it cools. The heat acting on the fuel causes the fuel particles to disperse—a chemical explosive process that sends fuel in every direction with violent force. When this occurs at the entrance to the plenum, each intake runner is filled with a more evenly balanced mixture of fuel and air that enters the cylinders. Naturally, cylinder-to-cylinder distribution affects horsepower. For these reasons, the ample performance of carburetors incorporated in boosted projects without an intercooler is evident. Even common pump fuels generate impressive power, and increasing ignition timing can further the power potential using these principles.
“But intercooling is better,” I hear you say. “What if I add one of those?”
While intercoolers have their place in boosted performance, for most carbureted applications adding an intercooler works against you. It looks fantastic and its associated plumbing enriches any engine compartment.
But, by directing the air charge through an intercooler to feed the carburetor, we lose its natural heat and associated benefits during the vaporization process. The intake air charge lacks the density, the distribution or the balanced fuel mixture of a simple direct flow system.
“Yet, with the intercooler the air is colder, right?” Yes, technically the air charge might be a few degrees colder. Air reaching the carburetor without intercooling typically reflects intake temperatures of 60-70 degrees below the throttle blades compared to intercooled temperatures that may be 10 degrees cooler. But they enter the carburetor around 100 degrees cooler. Relinquishing this amount of heat weakens the vaporization process and the distribution. The slightly cooler air temperature does not recover the loss of these other consequential factors.
In the case of operating a supercharger with a carburetor, run the air straight in and enjoy its inherent hidden benefits, including the cost saving and complexity of the intercooler.
*Author correction: The earlier use of the word atomization has been replaced by vaporization.