Can someone with the knowledge explain to this dude how the air/fuel ratio relates to engine knock? and how they differ? What causes the other? What's worse? What can be corrected by the ECU? His searches couldn't find what he was looking for. Thanks!
Knock is mainly caused by too much spark advance and overly hot in-cylinder conditions. A richer AFR can help reduce knock by adding "extra gas" to cool the in-cylinder temps.
Extra gas does not cool the engine.. that is a myth. It's because running rich slows the burn, effectivly retarding the timing.
Oh really? And where did you get this info? So when compairing exhaust temps of a map set at 9:1 and 12:1 there would be no difference? I would recommend you seak some good reading from some books Fong.
http://www.innovatemotorsports.com/resources/tuning-videos.php the video with the guy by the supra. the "tuning basics" video. It says it part way through when they start talking about turbo cars. I typed it almost word for word.
True but... Fuel is the only way to cool the cylinders (minus the fact of having a cool air charge) Granted you can't just run an overly rich afr to avoid knock if you have excess cylinder temp, but running a "richer" than stoic afr will help combat knock that is related to excessive cylinder temperatures, and is the only way to make peak power. That being said, afr is important to knock, but you are correct in saying that people attempt to "flood" a motor to "tune out" knock, this is wrong. Lean AFR does not always equal knock but it can lead to it very quickly. There is a reason that most turbocharged vehicles run in the 10.5-11.5:1 afr's on 92 octane and leaner on race (read more stable) gasolines. Even N/A vehicles are seldomly leaner than 13.5:1 under peak load. AFR has nothing to do with power outputs on a true standalone vehicle. The issue with most vehicles is that they are using some sort of trickery in regards to the ecu. By slowing down the airflow signal (the most common way of fooling the ecu to adding fuel) you are also adding cylinder timing, this is where the power increase is, not the "leaner afr". Of course if your car is stupid rich and is pre-igniting or you are not getting a complete burn within the cylinder, then you will gain power by leaning it out. Every engine burns gasoline differently and there is not a direct chart as far as a comparision, but there is a direct relationship between true detonation and afr. FYI, I have also seen many knock sensors pick up "knock" due to an overly rich condition. In the end you need the proper AFR for your motor and fuel type to achieve maximum power output. Make your power thru proper ignition timing, not just scewing the afr. This is easier said that done without using a standalone ecu in many applications. Most of the time you have to adjust the fuel signal to get the proper ignition timing for your application. Something that requires a chassis dynomometer to read torque output to tell you when you've reached the point you want to be (peak cylinder pressure/proper burn). I could go on and on, but in reality, Ben does a great job on covering afr in this video without getting into specifics as he always does. I haven't worked with him directly, but I have worked with one of his co-workers thru the efi classes (101 and advanced) great classes if you want more info on tuning theory, application, and practices. kickin...what are you looking for exactly?
Good explanation, Josh. The reason I posted this was because I couldn't pinpoint engine reliability relating to pinging/knocking vs lean AFRs. If I understand correctly, knocking can be temporarily handled by the ECU by pulling timing. In the case when timing gets pulled, will the occupants of the car notice a weaker power output? When running lean, the engine doesn't pull timing but instead detects a lean AFR from the front O2 sensor and corrects it by spitting out more fuel. Is this correct?
some of that is not correct. When the knock sensor picks up knock, it will pull timing, and the afr is adjusted by the front o2 sensor. the thing is, that once the ecu goes open loop, the front o2 sensors is just along for the ride. In closed loop there is a target afr that the ecu is shooting for and it adjusts the fuel trims to get to that spot. Once you mash the gas the the ecu crosses over, you are running what the ecu *THINKS* is correct. Thats why you have to pay attention to making changes in air flow through the motor. if you add a header and cai and don't get the ecu tuned for it, the ecu will injector the amount og gas that it thinks that it needs for a certain load point. Next time you are out at uni, ask to see my protuner software, and I can show ya a little more. Russ
^ this also explains how my car is acting this summer also, after i put on the header and uppipe the car was a lot more sensitive to higher boost on 92octane, than it was with the stock manifold and uppipe. this is hard to compensate for without a good em unit, so i had had to settle with turning down the boost and running 98 octane.
Russ, I would very much like to see you at Firestone then. Thank you. The last time I was logged using the Datalogger software Justin totally helped me with, I was running well with the Cobb AP and stock airbox... but I was at 90% injector duty cycle. I would really like to see what it is today. I totally forgot about open vs closed loop. Would open loop allow the fuel injectors to spit at a 90% injector duty cycle on a stock WRX or STi? Or is it less?
