We have gone over the basic function of an engine, timing, fuel and knocking. Now we’ll start getting deeper into MAF sensors and modifying their signal.
The ECU knows the position of the crank by the crank sensor. It knows the amount of air coming in by the MAF(mass airflow) sensor which measures air density. The MAP(manifold absolute pressure) sensor measures the air pressure. These sensors combined help the ECU determine what changes in timing and fuel to make.
Like most engine sensors, the knock sensor is very important. This sensor is responsible for detecting knock or random combustion events which generate a pinging/knocking sound like I mentioned before; marbles hitting. You can mimic this sound by tapping the engine block with a screw driver lightly, the knock sensor would pick it up.
When you have a condition that is too lean and or there is too much timing you will tend to experience knock and that is when the sensor sends a signal to the ECU. The ECU then makes timing adjustments such as retarding the timing in order to not repeat the knock it just picked up. On a Maxima, the ECU does not do a good enough job at reducing timing when knock is present, this is why it is important to do something about timing once you start to modify your Maxima for more power. Side note, when you hear a near stock VQ35 knocking, it is likely that there is oil present in the intake manifold and in the combustion due to poor PCV valve design(a catch can is needed).
There are devices that can be programmed to make adjustments to the timing once knock is detected. One system is the J&S Safeguard which is an old school device yet extremely important in the my turbo 97. Once any sign of knock is detected you can choose to reduce certain amount of timing degree’s depending on the strength of the knock. Devices like these do a much better job than the factory ECU which only reduces a small amount of timing when knock is detected. Nowadays standalone ECU’s handle this much better as well as newer factory ECU’s.
The common MAF has a hot wire suspended in the air inside the intake airflow path. The temperature of the wire affects how much current flows through it. When the wire is hot there is more resistance and therefore less current flows (electrical current moves easier in the cold). The wire is cooled down as air flows through it, reducing resistance and increasing how much current flows through. The resistance measured is converted to a voltage value between 0 and 5.0 which is then sent to the ECU.
You may have heard of the MAF size being increased or swapping to a different one. This is because the MAF is calibrated for a specific air flow and once you start to exceed that the MAF will no longer read additional air.
There are ways to solve this for example once the MAF has maxed out and its sending the ECU 5v, you can then use the MAP sensor to adjust your fuel and timing. Another way could be to increase the size of the housing where the sensors wire is. You can also put a MAF that is designed to read higher amount of air. For my 1997 I use a Nissan Z32 MAF because it is designed to read more air than the factory 4th generation Maxima. The Z32 MAF will read roughly 550WHP worth of air. At about 18psi with a 66mm turbo I see the MAF reading 5v.
Engine tuning is controlling the output of the engine by adjusting air, fuel and timing.
Air Fuel Controllers (AFC)
The most common method of making adjustments to the fuel is by using an air fuel controller (AFC). You may have heard of piggyback. This is referring to a device that is connected between the cars ECU and the engine. The piggyback device can be programmed to adjust the signals going to and from the factory ECU. For example if the ECU is sending 50% duty cycle to the injectors, the device could modify that and add or subtract duty cycle. This method of tuning allows you to keep the factory ECU while still being able to tune.
With the common AFC you are limited to being able to control only the MAF signal. By modifying the MAF signal the ECU is receiving, you are able to control the amount of fuel being delivered. This is a great way to make small adjustments to a lightly modified Maxima. One side effect of controlling fuel this way is that when you remove fuel you are increasing timing and when you add fuel you are retarding timing (less timing advance). Technically you can advance timing this way on an all motor Maxima and gain a few HP, as long as the higher timing is not causing knocking; for example ~200WHP/WTQ on an all motor automatic 97.
Advanced Air Fuel Controllers
More advanced air fuel controllers are those that not only can modify the MAF signal but they can modify nearly all engine related signals. For example an Emanage Ultimate can modify fuel injector duty cycle and duration, ignition timing, throttle position, and can read and monitor many inputs. The next level after this would be using a standalone ECU.
Standalone ECU’s are computers that replace the car’s factory ECU. This is generally the ideal method of tuning as it gives you total control while the air fuel controllers only modify the factory ECU’s input and output. Standalone’s tend to be far more expensive than air fuel control so generally you see more air fuel controllers used on home built cars.
In the next article I will go over logging and examples.
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