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What is Engine Tuning? Logging

We have gone over sensors and the type of devices used to tune in part two of this series. Now we’ll take a look at some examples, i’d like to show you what I do to tune.

I would like to note that this is a general explanation of what the tuning process is about and not specific to a tuning device. The same steps can be applied to most systems; the concept is the same.

Logging and Tuning AFR

o2 Sensors

Oxygen (o2) sensors are a critical component to the entire tuning process. They read how much oxygen you have in your exhaust, that is the air to fuel ratio (AFR). 14.7:1 is called stoichiometric and this is considered the ideal mixture of air and fuel in order to have an efficient and complete combustion; 14.7 parts of air to 1 part fuel.

Richer means there is more fuel and Leaner means there is more air and less fuel.

The leaner the mixture is the higher the combustion temperature and therefore more prone to causing damage. Richer means cooler which is why most turbo VQ’s run at an AFR of 11.5:1; not only in VQ’s but most turbo engines in general.

11.5 AFR is only an estimation on turbo, supercharged, and nitrous cars. This is why tuning your engine is ideal instead of using generic tunes from others car.


Logging is the most important part of tuning in my opinion. Without it theres nothing you can do but guess what the engine is doing.

First I recommend a track or Dyno for safety. You want to get in a gear that is long but not your highest gear. On a 4 speed auto, 3rd gear is ideal and 2nd gear is long enough to be usable to tune with lightly modified Maxima’s.

Closed Loop and Open Loop

Most of the time the car will be in closed loop below 3k RPM, that means the ECU will be reading the o2’s and making adjustments so that the output AFR is 14.7 for a clean and complete burn. Above 3k RPM and above around 30% throttle the ECU goes into open loop and no longer is using the o2 sensors for feedback. Once in open loop mode the ECU is making adjustments based on tables in the software, usually called Maps.

The MAF and TPS(throttle position sensor) among other sensors are used during open loop to determine the fuel and timing (the o2’s are not used). The 3k RPM and 30% throttle are just estimates and not exact values used by the ECU, there are other factors that determine if the ECU operates in open and closed loop such as engine start up, faulty readings from the o2’s, high coolant temperatures, etc…

On automatic transmissions, the car is ran with over drive off and revved to above 3k before going wide open throttle (WOT) so that it does not downshift; if you go WOT below 3k rpm the transmission will downshift.

Logging and Adjustments

To provide context, I will go over some of the logs from my own Maxima when it ran 11.3 and 11.2 in the 1/4 mile.
For reference below are some details about the car and a video from this day:

1997 VQ35(VQ30 Timing) Automatic.
66mm Turbo at 20-22psi.
Nitrous for launching.
Using an Emanage Ultimate.
The 11.3 run was using all 4 gears.
The 11.2 only 1st and 3rd gear were used; 2nd and 4th gear failed before this run, however the logs still are relevant to our topic. We’ll use the 3rd gear portion of the logs.

Once you’re cruising at your desired RPM such as 3k, you hit record and do a WOT run till redline or the maximum RPM you plan to take the engine to on a regular basis.

Here we have the log to the 11.3 1/4 mile run:

In this log I have selected 6,300 RPM in 3rd gear (the red vertical line). On the right you can see the values for that moment which shows an AFR of 10.5 and 20psi. You want to look at the AFR curve and make sure it is within your desired target AFR, lets use 11.2 as our target.

When your intent is to make adjustments you usually have your tables open such as the fuel table for example. The table shows RPM on top and PSI on the left. So I look at 20psi-21psi on the vertical axis then 6300 RPM on the horizontal axis and where they intercept is the cell that is being used to make adjustments to the fuel. Here we see 22.2%, this means its adding 22.2% to the duty cycle the ECU is giving out to the injectors. Remember that with a piggyback system you are modifying the ECU’s output, its not creating the outputs like a standalone.

So at this moment I am thinking that 10.5 is too rich and I want to make it leaner. I decrease the percentage from 22.5% to 21% in that cell so that it provides less fuel. I look at the rest of the AFR curve and make sure that there are no other areas that are too far off my target; too rich or too lean. You’re ready to do another run after you save your map and export it to the ECU.

Here we have the log to the 11.2 1/4 mile run:

For this run you can now see that at the same gear, RPM and PSI we now have 11.0 AFR. So we have successfully made the AFR leaner by .5. In the case of the Emanage you can see the adjustment value being applied highlighted in blue in the right labeled (I/J Adjustment Map) which is generally the value you entered on your table.

When entering values in the cells keep in mind that the RPM’s and PSI are constantly moving, so the computer may not be using the exact cell you have entered a value for. Instead, it uses the average of the surrounding values (interpolation). For example if you are between 6,000 and 6,300 RPM, the software will use the interpolated value thats between those two cells.

Now that you have seen the change in AFR due to your map change, you can repeat the process till you have achieved your target AFR.

For part 4 and the final part of this series I will touch on tuning the ignition timing.

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