Notes on tuning an EFI system

The other night I answered some questions on a forum about tuning an EFI system. I thought I did a pretty good job in my response so I posted it here.

“The crackling and popping of the exhaust “usually” comes from a leaky exhaust(what I was told by my tuner).”

I don’t want to be offensive or a to be a jerk, but your tuner is wrong. The afterfires still happen with no leaks in┬áthe exhaust system, even if you cut the fuel completely like what I do they still happen. I do not have a very high opinion of tuners and I have yet to meet one that’s not a flake. I’m not saying their aren’t good guys out there, just that I haven’t met one. But the reason for afterfires in this situation is because some fuel still lingers in the intake on a quick transition from on to off throttle, and there is enough air that gets past the idle valve, or there is already enough in the system to cause the afterfire. I believe this is more pronounced on engines with a wet flow intake manifold.

With most cars that have a lot of drama out the exhaust, this is done totally through tuning. If I recall correctly, the 458 Ferrari recently had a new software update. A lot of the owners were disappointed because the new tune reduced the “drama”. They did not fix any leaks.

“I am having a problem seeing how raw fuel on the sensor gives a lean number to the ECU.”

From my understanding, it’s not that the o2 sensor reads raw fuel, it’s that it doesn’t take into account that raw fuel is even present. Let’s say you have a cylinder that goes out and a ton of raw fuel enters the exhaust. Well, there was also a lot of air that entered the exhaust that was supposed to burn that fuel, but it didn’t. The o2 sensor doesn’t read the raw fuel, but it does read all that fresh air. The result is that your o2 gauge spikes lean and the computer compensates by adding more fuel… which probably isn’t helping at that point.

With a big cam that has a lot of overlap, there is so much turbulence in the intake and incomplete combustion going on at idle that the o2 sensor just doesn’t get a consistant sample of exhaust. Not only does this cause a jumpy o2 reading, but the readings you do get are just not accurate. The engine might read lean because it can’t combust the fuel due to conditions at idle not being quite right. Most people see this and then add even more fuel which hurts combustion further resulting in an even more lean reading. It’s a vicious cycle…

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