It's certainly a balancing act, yet we do it every time we make a pass down the racetrack in a drag race application. We've done the research and selected our octane level and we've predicted what we feel is the best distillation curve for our application. Yet when all is said and done the car goes out and just doesn't run down the race track like it should.
To understand superheating and how it relates to our program we have to realize that this event is created on purpose in direct injection engines to cause the fuel to vaporize almost instantaneously. As we talked about earlier, EFI systems don't have the advantage of pre-emulsed fuel prior to introduction to the cylinder. So they only have milliseconds prior to the combustion process to take a solid fuel column and turn it into a vapor. Hence superheating of fuel was introduced.
This superheated fuel would expand greatly when introduced into a lower pressure cavity and as the temperature of the fuel was increased vaporization would increase accordingly. The downside to this concept is simple. The hotter fuel would increase ambient air temperature in the chamber increasing molecule size and decreasing total cylinder fill percentage. But at the correct percentage, the increase in vaporization and resultant effect of more complete combustion would more than offset this volumetric efficiency loss up to a point.
Now in a modern day carbureted engine, the fuel is atomized quite well, however, if the engine doesn't appear to be accelerating through the first few moments of the pass, superheating may be the answer. Now, you can't simulate the phenomenon the way the GDI systems do, but you can make a few changes to your program to come close.
The first area to look is of course fuel selection. Have you selected a fuel that has too high an octane rating and the fuel is resisting combustion as a result of lower than anticipated cylinder pressure? Are you required by the sanctioning body to use a particular brand of fuel? These are oftentimes the two biggest problems in a program.
The second area is the fuels distillation curve. The base properties of the fuel may vaporize at 70-80 degrees but some of the higher calories may require more temperature to vaporize and won't become burnable in the time allowed to perform the task. As a result this more resistant fuel burns out the pipes or doesn't burn at all.
The first of a couple of quick fixes for this is of course to select a fuel that offers lower overall temperatures to vaporize, hence an increase in burn capability. A second simple option is increasing your sparkplug tip temperature or the tip length to increase the fuel's temperature as it comes into the chamber. This is a great fine-tuning tool.
But if you're program is a little further out or you just really need to find out where you're at, start increasing the engine staging temperature and/or intake manifold temperature.
Bethlehem Sparkplug Company developed this spark plus in 1917 to offset the poor performance characteristics of the fuels of that era.
Every engine program has a particular temperature curve that it will want to operate at for a specific load and a specific fuel. Finding that curve is not real difficult. We all know that in a typical program, staging your vehicle at 130 degrees is too cold. Staging at 220 is too hot. So start moving your baseline up or down and map the changes in acceleration. If you see the car picking up on the front half, keep increasing the stage temp until you
see it lose on the back half. Now its time to start moving timing, plug heat range and/or tip length to recover that back half E.T.
Either way, superheating and temperature curves are fine-tuning tools that everyone needs to be aware of to optimize their program.