|
I LOVE this.
from what i understand there are three base maps - timing, idle, and fueling.
the fueling map is three dimensional and uses rpm and airflow to determine how much fuel to deliver.
typically, the lower-rpm end of the map delivers a stoichiometric air fuel ratio (14.7:1) which gets richer (more gas) as rpms increase (to cool engine, avoid knock, etc.).
the o2 sensors are the only true indicator of afr that the computer gets, but the narrow band o2 sensors are only really creating long term fuel trims for the low-rpm area of the map as they do not work outside of a small range centred around 14.7:1.
at low rpm the stock porsche engine is designed to have a pulse-tuned intake and backpressure from the exhaust to improve cylinder scavenging and fill. on an engine with intake and exhaust modified to increase airflow at high rpm, some of these low rpm benefits will be reduced. this is why most folks see a reduction in low to mid-rpm torque but with an overall increase in power in the upper rpm ranges. to the computer this means the engine is drawing less air at low rpm. the o2 sensors should detect this and reduce fuel as a response. when the rpms increase and the engine is breathing more freely the computer doesn’t know it and you should actually go into a lean condition. the leaner condition at wot is supported by the comments from topless.
another possibility is that the 02 sensors are no longer working properly. in this scenario the computer doesn’t detect the reduced airflow at lower rpm and continues to fuel as planned, resulting in a rich condition. this should transition to a less rich condition as rpms increase.
oddly, you are getting more airflow than expected, but that is at idle which is a different map.
otherwise, the computer is looking at throttle position as well as maf output, so one presumes that any wild fluctuations in maf output would get detected and ignored. a maf that is consistently reading off due to steady-state turbulence might be a culprit. my understanding is that there is also a maf calibration map, so any consistent maf inaccuracies should be addressed by the computer as determined by low-rpm o2 sensor feedback.
if the maf is metering more air than is actually flowing (ie, located in a high pressure area of the intake such as the outside of a bend) and, as rpms increase, the inaccuracies of the maf readings increase in a greater than linear manner such that more air is metered than is actually flowing beyond what has been accounted for in the maf calibration map, the mixture will get rich and the computer would not know it. in this case the best solution would be to simulate the 996 intake geometry and locate the maf as close as possible to the air box.
regardless, we are talking a 996 engine with 996 tune. presumably Porsche isn’t leaving any power on the table with this combo. i find it hard to believe that intake and exhaust modifications can have such a drastic effect on afr. i think the real test would be to compare your wideband o2 results to a stock 996, or even get your hands on the base map for a 996 to see what the afrs are supposed to be from Porsche. my thinking is that you’ll find that porsche wants a 10:1 at wot to deal with vagaries in fuel octane, etc. it’s all really moot because, regardless of the cause of bad afrs, if you can improve them …
insite, I am watching this and will be next in line; you have proven results from jaay and the guy topless posted about, and now, hopefully you!
|
Threaded Mode
