We’ve seen a recent uptick in customers asking for a “Stage 2+” tune designed for upgraded MPI or port injectors. The theory being power is held back due to a lack of fueling with the factory fuel injectors. Seeing as this would be easy to prove, or disprove, and potentially an additional product we could offer, we set out to test the theory.
We first conducted as series of baseline testes on a Stage 2 RS3. Then added larger port injectors and started tuning Stage 2+. Thankfully conditions were nearly identical during the tests. Stage 2+ had a slight conditional advantage, but not enough to make a real difference. Using normal pump fuels (91 and 93) unfortunately there were no gains to be had. The same held true for race fuel (100 and 104). The factory direct injection system has enough fuel to fully satisfy the needs of these tunes. Using more port injection, when unneeded, unfortunately has the drawback of actually hurting performance. Typically, we see the turbo spool slightly later, and a slight drop in overall performance.
Next we tuned using full E85. Unlike traditional gasoline, E85 requires about 30% more fuel. If we’re fuel limited, it would show up during this test. Our current Stage 2 tune is fairly maxed out in terms of what the turbo can do and even on ethanol there’s headroom left in the injectors giving us a safety buffer. Because of this we were skeptical we would see any gains. However, we continued tuning and leaning on the system more to see how much room was left.
The first thing we noticed was slower spool while using the port injectors which mirrors what we’ve seen in the past with port injectors vs direct injectors. This can be cleaned up later with more tuning. Our next hurdle was the lower RPM range. The turbo is running close to the surge line in this region and as such, there are no massive torque gains to be had. In the midrange our current stage 2 program keeps torque reasonable as to limit stress on the system. Seeing as this is a Stage 2+ tune, some may be interested in going further so we pressed on. With the turbo essentially running flat out, we were able to pick up a few HP in the mid-range. However, the gains to be had were minimal. We saw a peak increase from 518 WHP with the stock injectors to 521 WHP with the upgraded injectors. With the turbo running flat out to redline, back pressure increased significantly. As such, power from roughly 6,000 RPM to redline was less than stage 2. This proved there are no massive gains to be had in the upper RPM band. With further fine tuning, this too can be cleaned up and brought closer to the Stage 2 power levels.
Based on our testing, upgraded port injectors offer no advantage on the factory turbo on gasoline (pump/race fuels). With Ethanol, a small gain can be had in the midrange as illustrated in the attached graph. Our question to the community is this: Is this worth it? Should we make a stage 2+ tune and sell injectors given these minimal gains on ethanol?