Because most ECUs are tuned to keep the car rich, especially under WOT conditions. The excess fuel acts as a combustion chamber coolant, which keeps detonation at bay, which reduces warrantee claims. It only makes sense. To make more power, you actually dial fuel
out, which increases combustion temperatures and increases power.
Hondas with mods, like I said, need fuel enrichment because their ECU doesn't know how much air is actually coming into the engine. You put a CAI on a Honda so that it sniffs better, then put a header on it to improve scavenging, and the Volumetric Efficiency goes off of the charts. The Honda ECU, meantime, is only sending the stock amount of fuel, which makes it lean, and power goes up quite a bit, relatively speaking. Then, at some magical but undetermined point, they start going too lean, and you need to start adding fuel again, which is pretty easy to do with a black box Toys like this sort of work on Hondas, because the ECU isn't too bright to start with.
Because we have Mass Air Flow sensors, and the ECU actually monitors what goes into the engine and adjusts fuel accordingly, toys like this don't generally work. When they do work, the O2 sensor tells the ECU that something's wrong, and you have trouble. Additionally, the Nissan ECU learns pretty well about it's environment. You don't want to interfere with that.
You said he races...what does he race? Bikes? Chickens? The fact that some monkey can pilot a 600 HP car in a straight line for 1400 feet says nothing about his ability to tune an engine. No offense to you or your friend, but you can't be so naive about those from whom you take your advice.
It's a gasoline injection system, as opposed to a diesel injection system. Please narrow your question down, or at least make it multiple choice.
