Maxed-out injectors, maxed-out fuel pumps, maxed-out mass-air-flow meters and maxed-out turbochargers are realities on the road to increased performance with the VR38DETT. During the upgrade process, it doesn't take long to encounter these obstacles. With just a few basic upgrades (boost up, drop-in panel filters and a capable exhaust system), an additional 10-to-20 percent more power can be produced. This elevagted power level pushes the injectors, fuel pumps, mass-air-flow meters and turbochargers to the limit.
With our Project R35 GT-R, the simple addition of a GReddy 94mm titainum exhaust, HKS drop-in panel filters and a COBB AccessPort exposed the limits of the factory injectors, pumps and air-flow meters.
In looking at the datalogs, the Cobb AccessPort reported a 97- to 98-percent injector duty cycle. In reality, this equates to the injectors being at 100-percent duty cycle as the injector doesn’t have any time to close when duty cycles exceed 90 percent. Despite running at a 50psi base fuel pressure (that increases the effective flow to 620 cc/min), the factory 570 cc/min injectors simply lack the flow capacity to support over 600 flywheel horsepower at the richer air-fuel ratios required on pump gas.
With the R35’s injectors essentially running out of controllable range, we decided to upgrade to higher capacity units. While Fuel Injector Clinic offers new injectors with adapter plugs for a no-splice installation, we wanted our upgraded injectors to appear stock. Fortunately, Fuel Injector Clinic was able to modify our stock injectors to flow 900 cc/min. This flow rating is enough to support 900-plus flywheel horsepower at richer pump gas air-fuel ratios. At the power level that maxes out the factory injectors, these 900 cc/min injectors will only be at a 63-percent duty cycle.
PUMP IT UP
As for the R35 GT-R fuel pumps, the factory rates the stock pumps at 130 lph each. We sent the stock pumps to RC Engineering for flow testing. RC Engineering verified the stock pumps’ delivery capacity. One pump checked in at 130 lph while the second delivered 143 lph at 75psi of line pressure with a supply voltage of 14.0 volts. In theory, that should be enough flow to support 700 whp. In reality, a two-pump system has some efficiency loss. As a result, most GT-R tuners recommend upgrading the fuel pumps when power levels exceed 650 flywheel horsepower (550whp).
Looking for a simple drop-in solution, we sourced a pair of HKS 195-lph upgraded pumps. The HKS 195-lph pumps are advertised to provide 50-percent more fuel flow than the factory pumps. The additional fuel supply should be able to support flywheel horsepower levels over 900 horsepower.
TO THE LIMIT MAFs
While the factory injectors and fuel pumps actually limit how much fuel gets to the engine, the mass-air-flow sensors serve a monitoring role. The mass-air-flow meters are designed to measure the amount of air going into the engine and report this to the ECU. In the case of the GT-R, a 0.0-to-4.9-volt signal is generated by the sensor and sent to the ECU. Higher voltages result at higher airflows as a non-linear, logarithmic function. Being nonlinear, you don’t have twice the airflow at 4.0 volts versus 2.0 volts. On the R35 GT-R, the sensors will read 4.9-volts at any flow level above 600 flywheel horsepower. Hence the factory mass-air-sensors simply can’t accurately report the airflow into the engine at power levels above 600 flywheel horsepower. Fortunately, the aftermarket offers larger-diameter intake tubes that rescale and extend the range of airflow that can be monitored.
BIG INTAKE SOLUTION
Gotboost Performance offers its 3.0-inch intake system for the R35 to extend the range of the mass-air-flow sensors by about 45 percent. After installation of the intake system, a Cobb AccessPort or other engine management system must be recalibrated to work with the larger intake. This process allows airflows to be measured beyond the 850-flywheel-horsepower range. In addition to providing critical air-flow information to the ECU at higher horsepower levels, the Gotboost 3.0-inch intakes also reduce restrictions to the compressor inlets of the turbochargers. As a result, horsepower, torque and response gains can all be realized.
To verify the math and ensure that the injectors, pumps and mass-air-flow meters were responding correctly, we decided to conduct the dyno testing in three phases. First, we would establish the baseline with the factory pumps and injectors in place. Second, we would install the upgraded pumps and larger injectors and optimize the tune for this configuration. Third, we would install the larger intakes and recalibrate the maps for peak performance. During each run, we would datalog the ECU parameters to record injector duty cycle, air-fuel ratios and boost levels.