Of the various reasons behind the creation of this monthly column, one was to provide a forum in which theoretical notions could be broken down into practical elements and applied directly to circle track engines and related components. In that context, we've dug into a range of topics and attempted to expose ways each might relate to contemporary engine builders and racers. That continues to be our charter. At the moment, Circle Track's "Project GREEN" is becoming aplatform for the reduction of more theory to practice, including data streams that further validate aspects of how certain engine parts affect both the amount and placement of torque and power in an engine's rpm range. This month, we'll share some thoughts on comparing carburetion to EFI.
In prior Enginology columns, we've discussed the importance of fuel atomization. The notion that smaller fuel droplets effectively require less time to combust than larger ones suggests atomization efficiency is important not only to optimizing power but with respect to combustion efficiency as it relates to on-track fuel economy. The combustion process can also be completed in less time, as a function of smaller droplets, suggesting less initial ignition spark timing is required and resulting in a reduction of pre-TDC combustion pressure and lost net torque.
Carburetors are not the best way to provide small, uniformly-distributed fuel droplets. This was the reason for previously suggesting mechanical ways for further reduction of droplet size along the inlet path and in the combustion space, post-carburetor. In addition, since carburetor atomization efficiency is tied to airflow rate, the lower the engine speed, the less efficient their ability to produce well-atomized fuel. Of course, the smaller the droplets, the easier it becomes to maintain their suspension in air, compared to larger fractions of fuel.
The fact that electronic fuel injection provides well-atomized fuel independent of engine speed is largely responsible for improved power, virtually throughout the rpm range, all else being equal. Such differences are particularly present below the engine speed at which peak torque is produced and where the atomization efficiency of a carburetor suffers from insufficient airflow rate.
To illustrate this point, we've included one particular data comparison from the first Project GREEN ("One More Lap to Go" series of stories) engine dynamometer session at Mast Motorsports. As you review and absorb the information, it's critical that you understand the data represents a back-to-back comparison between a carbureted version of the project's mule engine and a configuration using GM's LS EFI system, both performed on their CT525 crate engine. The data supports the theoretical aspects of how each system impacts fuel atomization and power produced, clearly favoring the EFI systems and its spray delivery of fuel.
While the ensuing editorial material on this series of dyno tests will deal in much more detail than this column has space to discuss, the objective here is to provide an illustration (with test results) in support of the theory discussed that's applicable to other engines in the circle track community.
If you subscribe to the notion that overall on-track torque is more important than peak power, the argument in favor of EFI is even stronger. Note the improvement in torque between 3,000 and 5,500 rpm. Again, this was an otherwise stock crate engine, not one benefitting from any tailoring of parts to a particular track layout or setup. Given parameters specific to either or both these requirements, chances are good that further gains would be possible.
There are a few more related topics to discuss, as well. For example, lower and mid-rpm throttle response is likely with EFI. Stated another way, such equipped engines tend to accelerate through their rpm ranges more quickly and waste less fuel in the process. Based on a more uniform and consistent burn (cycle-to-cycle and cylinder-to-cylinder), the development of combustion pressure in an EFI engine is also characteristic of how engines perform when provided air/fuel charges that are well mixed. Also keep in mind that as atomization efficiency improves, well-blended air/fuel charges can negotiate changes in induction path direction (and inlet pressure variations associated with unsteady, non-homogeneous flow) with minimal air/fuel separation.
Perhaps a more obscure issue relates to how well an engine's torque output (curve) can approximate its volumetric efficiency curve when using EFI. In theory, minus any pumping losses, torque curves should closely match v.e. curves, primarily because if an engine is doing a good job of ingesting air, the potential for achieving equally high levels of output is present. However, if the fuel being delivered during a given operational range of rpm is poorly atomized, air/fuel charges will reflect this inefficiency, resulting in less than optimum power. Such inefficiency also encourages problems associated with excessive air and fuel separation, largely mechanical, along inlet paths and in the combustion space. EFI can have a direct and favorable impact on these conditions, as witnessed (again) by the example data provided.
Of course, there's always the on-track fuel economy issue, becoming increasingly important as dictated by fuel costs. Bottom-line, if it's possible to make more power with the same amount of fuel delivered to the combustion space (compared to carburetion), less is wasted in conjunction with increased on-track power; clearly a healthy combination. EFI is one approach to accomplishing that objective.
Now all this isn't to say carburetors are unacceptable. For many reasons that include rules requirements, convenience, cost, and prior experience at the racer level, carburetors are woven into the fabric of circle-track racing and will likely remain so for a long time. But if you look back over their history in motorsports, much time and expense have been devoted to improving their ability to supply air/fuel charges with features that include good atomization efficiency.
Aside from companion problems related to how wet flow is handled downstream of any "mixing" device or system (carburetors or EFI), the probability of raising net atomization efficiency is greater if the wet flow begins with well-mixed air/fuel charges based on improvements in mixture quality as a function of enhanced atomization. This month, we've had an opportunity to link theory with supporting data. That's a nice and meaningful package.