DFI is especially effective when combined with boost, which is exactly why we've seen Chevy combine it with the Cobalt SS' turbo engine and Mazda combine it with the turbocharged MZR engine featured in the Mazdaspeed3 (as well as the MS6 and CX-7). To get a better grasp of what tuning challenges and potential advantages DFI offer power-hungry Cobalt SS and MS3 owners, I turned to Trey Cobb of Cobb Tuning, owner of one of the leading tuning companies in the country.
Cobb Tuning has developed its famous AccessPORT ECU tuning tool for the Mazdaspeed3 (as well as a whole host of bolt-on performance parts), so you can be sure the company has learned a thing or two about DFI. When I asked Trey about the challenges of cracking the MS3's ECU such that the AccessPORT can be used to reflash the fuel and ignition timing maps (along with many other features), he said, "Very few DFI-specific challenges came up during initial development of the AccessPORT. There are some unique aspects to the ECU programming on DFI motors, but overall the higher level aspects to the engine management (fuel tables, ignition advance, camshaft advance, boost control) are virtually identical to what we normally see in non-DFI applications. The greater challenge is actually coming from the more sophisticated torque control logic that is increasingly being implemented by the OEM's [as a form of traction control]." Impressively, the AccessPORT for the MS3 does allow gear-specific tuning of throttle position and boost levels such that the OE torque control system is tunable.
When I asked Trey about any limitations to potential power output with DFI engines, he felt that, "The high-pressure (DFI) fuel pump on the MS3 engine is a limitation. The factory components also tend to have a fair degree of variance and have been subject to manufacturer recalls. We've found that the factory pump may be suitable for higher power levels on some cars, but not on others. The manufacturing variations seem to have a greater impact on the pump's ability to maintain the necessary fuel rail pressure, so much so that on some cars the factory pump is only just barely able to keep up with factory power levels. This is where having the ability to datalog your fuel rail pressures [available through the AccessPORT] is critical to verifying accurate and safe fuel delivery." There are, however, aftermarket fuel pumps available for the MS3 and replacing the OE unit is not overly difficult (though you do have to be extra careful when dealing with a fuel system with more than 1,000 psi of pressure), but they are substantially more expensive than non-DFI fuel pumps.
When I asked Trey if he thinks we'll see some really highly tuned DFI engines pumping out big horsepower numbers, he responded, "Certainly, and with respect to the Mazdaspeed3, the next hurdle to overcome is with the camshafts. The four-cylinder MZR engine is not designed to operate at a very high engine rpm range, due in part to the limitations of direct injection. There simply isn't as much time to inject fuel per cycle as there is with non-DFI setups, thus lowering your maximum rpm ceiling. The camshaft profile for the MZR motor was designed specifically for lower to midrange power output, and the engines are tremendously efficient in that range. Even with how well air fills the cylinder and manifolds, the engine's volumentric efficiency [torque] decreases significantly at higher (6000-plus) rpm. Horsepower is dependent on rpm, so to get the horsepower numbers people are looking for, a solution needs to become available to increase the MZR's VE up in the higher rpm range. That will involve, I suspect, different camshafts and potentially higher flowing injectors in addition to the normal supporting modifications (fuel pump, turbo, manifold, intercooler and so on). The limitation of time to inject fuel at high rpm will still exist, as that's inherent to the nature of DFI systems."