Updated: Jun 18
We had a new customer come to us for help working through issues with a calibration they had gotten out of town. The Camaro had an intermittent hesitation, and was inconsistent. With a smaller cam, and through an automatic, power was in the ~330 whp range, which is fairly typical. Below, the Camaro can be seen running in the mid 12s:
Fueling is commanded at a specific AFR for each RPM range the motor will operate. Here's a sample of RPM, Target AFR, and Actual AFR: 4500 - 12.15 - 12.33 5000 - 12.40 - 12.50 5500 - 12.53 - 12.53 6000 - 12.53 - 12.80 6500 - 12.53 - ??.?? From the Power Numbers, to Time Slips, to Wide-band data, everything adds up, and makes sense; however, this does not explain the inconsistency or hesitation. Digging deeper
The fueling and airflow data provides some clues...
Long Term Fuel Trims are turned on, and are adding extra fuel during WOT. Bank 1 is adding 4% more fuel, and bank 2 is adding 5% more fuel on top of calibrated fueling. While this is not ideal, it's likely not a root cause of any issues. Calculated Airflow peaks at 52.9lb/min, which is closer to that of a 430whp car than the 330whp of airflow actually being drawn through the motor according to the Dyno. Knowing the fueling and airflow data isn't making sense, I also noted that the injector pulsewidth (how long the injectors stay open each cycle) was 12.79 ms at 6400rpm. Comparing to a similar car running the same injectors, at 6400rpm, I was only seeing 9.43ms of injector pulsewidth, and reporting 43.34lb/min of airflow at 350whp.
Breaking down the data
We're calculating more airflow than the car reasonably should, and because of that, the injectors are opening longer than they theoretically should, in order to inject the correct amount of fuel as verified by the wide-band. Fueling calculations must be wrong. We know this because: - Calculated Airflow is excessive - Injector Pulse width is excessive
- Actual fuel being injected is *Not* excessive
You may be questioning how we got the Correct amount of fuel with an Incorrect airflow calculation. Most often, this happens when a parameter is entered incorrectly. - A low "Displacement" value will cause airflow to calibrate high - A low "Injector Flow Rate" value will cause airflow to calibrate high - Too high of a "Stoich" value will cause airflow to calibrate high
The guy who calibrated this vehicle takes his job seriously, and used good data. Unfortunately, when the car was tuned, there was a mechanical issue causing the "Correct" published data to be wrong.
How did we end up injecting the correct amount of fuel?
Your ECU is a calculator that computes how much air your motor is breathing in. It then uses what it knows about your fuel choice and injector data to inject a proportional amount of fuel into your motor. As a calibrator, our job is to make sure the ECU can correctly calculate how much air is entering the motor, in order to keep your fueling on target. We do this by observing the difference between the commanded air fuel ratio and the actual air fuel ratio at your tailpipe.
If actual delivered AFR is leaner than desired AFR, we need more fuel. We satisfy this need by changing data in the ECU so that it calculates more airflow. More calculated airflow will result in the ECU calculating and delivering more fuel. This will bring us closer to our target AFR. Conversely, if we see a richer delivered AFR, that tells us that the ECU's airflow calculations are too high. We remove calculated airflow, which decreases the amount of injected fuel.
When this vehicle was being calibrated, it was showing that it was leaner than target. Because of that, the calibrator added more airflow to the ECU's calculations, thus delivering more fuel. This process was repeated until fueling targets were hit.
Below is the tuned VE table that was on the Camaro. VE stands for Volumetric Efficiency. It's a representation of a percentage of filling for a cylinder at any given manifold pressure and engine speed.
Logically speaking, an engine isn't going to be able to draw in 146% of its total displacement without some sort of forced induction. Irrational values for this table have been outlined in red. These excessively high values are why airflow and injector pulse width are calculating incorrectly.
What's the problem??
To recap, we're making the right amount of power, running 12s in the quarter mile, and the Air Fuel Ratio on our gauge is in an acceptable range. For most people, this probably looks acceptable, and nobody would know a difference.
For me, the data needs to be reasonably correct and make sense; This is why...
The data generated by tuning a vehicle gives us insight into how it is performing, and if everything is working as it should. When airflow has to be inflated by 46% in order to hit our fueling targets, this is our clue that something is wrong.
Experience shows that if this happens, and a correct tuning process has been followed with correct data, then something mechanical is at play. We're having to add Airflow to our calculations because we are not getting enough fuel.
If we are not getting enough fuel at a reasonable airflow value, that indicates that the injectors are not able to flow as much fuel as their data says they should. This can happen due to a restriction or to a loss of pressure. The flow across an injector is relative to how much pressure is pushing (and pulling) the fuel through it. Less pressure = less flow.
Following our suggestion, the Camaro's owner recorded fuel pressure at an indicated 42psi rather than the ~58psi expected by the ECU and injector data.
With the pump removed, the regulator assembly could be observed.
The O-ring on the in-tank fuel pressure regulator had blown out with the additional pressure from the new high volume fuel pump.
Fuel injectors had to stay open longer, as fuel did not flow through them as quickly at the lower resulting pressure. Should this problem have been left, the hesitation and other issues would have gotten catastrophically worse.
Repair and Retune
The Fuel Pressure Regulator (1998 Part number 217-3073) was replaced, and with the correct 58psi of fuel pressure, the Camaro is now excessively rich. We will next be re-tuning the Camaro, and removing the extra airflow (and fuel) from its calibration.
Knowing what correct data looks like is important to keeping a modified motor alive. This is why I encourage you to get HP Tuner or some sort of a data logger so that you have this type of information to review.