Great discussion guys, a couple of thoughts to enrich the conversation. For background, early in my career I was a powertrain engineer at Ford Motor Company, so I have enough tuning experience to have an educated point of you, but I do not know the specifics of how Porsche calibrated the 986/996, and the logic on their program. I’m also posting a copy of the dyno runs that are shown in the video, for easier view ability.
First, the car does have a “tune” from Fabspeed. They built the tune for the higher flow exhaust system, but it has not been updated for the intake system. Even the first tune from Fabspeed took several iterations for them to find additional horsepower, and I did not settle on it until we verified it on the dyno. They were able to extract an additional 5 hp from the tune but no more. Remember, if you are tuning a turbo engine, you can increase the boost and find significant gains in horsepower. On the other hand, if you are tuning naturally aspirated you can only adjust the spark table and the air fuel ratio. Nothing more.
1) Typically, the spark table is advanced to the point where the knock sensors detect early pre-detonation and then the spark is backed off slightly. I am assuming that is what is happening in the Porsche calibration but I am not certain. There is no easy way to measure this on a typical Dyno set up.
2) The other variable is the air fuel ratio. When an engine is operating under normal conditions the O2 sensors are used to keep the air fuel ratio at the stoichiometric ratio, 14.7:1. This is done primarily for emissions, but it also aids in fuel economy. This condition is considered a “closed loop” operation meaning the ECU is constantly adjusting the fuel flow based on the signals from the O2 sensors to keep the engine operating at this ideal stoichiometric condition. However, when you move to wide-open throttle (WOT), the engine typically goes into a closed loop parameter, whereas the fuel flow rate is simply a mathematical relationship (called a transfer function) where the ECU looks at the airflow meter in the intake system and calculates the right amount of fuel to inject into the cylinders. In some cases, these transfer functions are fixed, and can be adjusted by the tuner, in other situations these transfer functions will be automatically modified overtime based on the performance of the engine, this is not managed by the tuner but managed by the logic within the ECU. It is my assumption that given the age of this car, that it is a fixed transfer function. You will note at the bottom of the chart that the air fuel ratio is monitored during the runs through the output of the exhaust system through a wideband sensor. So, we know if the tune is generally correct for the engine, or a further programming needs to be done. In general, a leaner engine (ie higher A/F ratio) delivers the best horsepower. However, in reality, programmers need to richen the ratio (lower A/F ratios) to cool down the cylinder and prevent the engine from pre-detonation. At these WOT limits it’s a tradeoff between spark advance, and air fuel ratio, to uncover the best overall performance. Keeping in mind that the knock detectors can automatically pull spark out at any point if it detects pre-detonation. Cutting through all of the gibberish, if you look at the chart the car is running near an AF ratio of 12.0 for all three runs. The exhaust-only tune is running slightly leaner by 0.5, but that small delta is likely only making a few hp difference, at most.
This means… that a custom tune might be able to find a few horsepower at most, but no significant gains. I did inquire on the cost/timing of a new tune from Fabspeed, but learned that many tuners are now getting out of the business as the EPA is putting pressure on larger aftermarket shops as these tunes have not been tested by the EPA and are therefore not compliant. I think we will see fewer tuning shops in existence in the future. Once a shop starts to pick up business momentum the EPA will come in and quickly shut them down. Furthermore, to properly tune a car you want to bring it to an shop that has a dyno and can make fine-tuning adjustments on the car based on multiple, often dozens, of dyno pulls to optimized all of the variables. Keep in mind every car is slightly different due to the manufacturing tolerances, and the best tune for a specific car is not necessarily the best tune for another car even if it is exactly the same make model and year. And, once you start changing the variables such as intake and exhaust, the only way to give it a proper tune is with 6-8 eight hours of dyno time back-and-forth with the programmer. This is not realistic for the majority of people putting together a hobby car as the car and the programmer are in two different cities and do not have the luxury of dyno testing as they optimize the tune.
All of this being said, I do not think we will see significant gains on the intake system. If somebody has better data to prove me wrong, I’m open to seeing it. As I was expecting, and even hoping, to see more significant improvements from the intake mods.
Oh, BTW.
Husker Boxster: butt dynos are historically bad. You mentioned that you thought you felt a little mid-range grunt, but I think you looked at the chart incorrectly. The dyno shows that with the intake system it actually lost up to 20hp in the mid-range.
Blue 62: the car does not adjust each individual cylinder; it looks at the left bank and the right bank and treats each bank as a family. It does not have information to adjust an individual cylinder. And while you could say that it optimizers every stroke, in reality it adjust the parameters more gradually, and may take 30 or 100 strokes to bring the spark or air fuel ratio back in line with the program. If you look at some of my commentary above, at wide open throttle it is likely an closed loop program and is doing nothing more than dumping in a predefined amount of fuel based on the transfer function, running spark off of the spark table, and pulling spark out if it detects detonation on either bank of cylinders.
There may be some Porsche tuners in the community that can add some further detail to how Porsche manages their programming on their ECU. If you have first-hand experience here, and can add some color to the above, I would welcome the insights, as some of my insights are based on Fords programming, and it may be slightly different for Porsche. But I doubt it will be much different as every engine is still bound by the same laws of physics!
