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Car Modding, Car Tuning, and the Science of Horsepower: Running in Open Loop

Chriscamaro loves playing sports, modifying cars, and playing video games.

Open loop vs closed loop in a car

Open loop vs closed loop in a car

What Is Open Loop in a Car?

When I first learned how cars worked, particularly how the fuel gets mixed in the correct proportions with the incoming air, it became immediately apparent to me that there was room for improvement. As it turned out, this revelation wasn't anything novel because there is another way.

Now for 99% of us, the current technology is just fine. In fact, it does precisely what the engineers want it to do. It's a closed-loop feedback system that maintains a target air-fuel ratio for optimal efficiency and minimal emissions. However, for those of us who are enthusiasts, this basic fueling model is severely limiting; we want the option of an open loop.

O2 sensor

O2 sensor

Tech Talk on O2 Sensors

I remember buying my first wideband O2 sensor and the night and day difference it made for my car. Recall from my other articles that an O2 sensor is meant to measure the levels of oxygen in the exhaust, in order to determine if there's too much or too little fuel. However, the oxygen sensors used in production vehicles are of the "narrowband", not the "wideband" type. With narrowband O2 sensors, the voltage output is either high or low. There's really nothing in between. If the sensor detects something on the "rich" side of its target value (14.7:1 air to fuel), it will swing high. If it detects a lean condition, it will swing low but it does so in a step-wise fashion with no slope in between. The car's computer detects this swing from high to low to high, which it basically interprets as:

"Too much fuel!!!

OK, now there's not enough!!!

Wait, there's too much again!!!"

...and in response to these signals, it tries to adjust the amount of fuel delivered by the fuel injectors. These adjustments happen so fast that the net result is a fairly good approximation of 14.7:1 but if you look closely, it's constantly fluttering back and forth on that number, never really settling... because by design it just can't. These sensors are simple, reliable, cheap(er) and easy to handle on the electronics/programming side of things. There's also a side benefit to this back and forth behaviour. The catalyst in your catalytic converter needs a range of substances passing over it in order to catalyze the chemical reactions needed to neutralize pollutants. If the composition of the exhaust stays constant, the cat won't run at peak efficiency, so the narrowband system actually helps with the longevity and efficiency of the cat.

This is the voltage output of a narrowband O2 sensor.  Note the abrupt step at 14.7

This is the voltage output of a narrowband O2 sensor. Note the abrupt step at 14.7

Wideband O2 sensors are different. Their output is linear. There's a well-defined slope to it. This means the voltage you get on the output can be directly correlated to the air-fuel ratio (AFR). Wouldn't it be nice for us gearheads, if the computer could follow a wide band the same way it follows a narrow band, in closed loop operation?

You'd have your AFR dead nuts centered all the time, and what's more, it's not stuck on a predetermined value of 14.7. It's not commonly known that you can run a car lean with no issues, as long as you're not stomping on the gas. Some modern cars have what's called "lean-burn mode" which takes advantage of this fact but with a wideband sensor, you can have any setting you want, any time you want.

Unfortunately, the car computer isn't designed to work with such a sensor in closed loop mode. So what's the point? How can we as hotrodders benefit? The answer is open loop.

Open Loop Mode - How to Take Control!

Open loop is nothing more than what happens when you turn off closed loop. At this point, the O2 sensors become helpless, as they are no longer used in the fueling calculations. Instead, the car relies on a fixed look-up table it has in memory, which tells it how much fuel to add, given a certain set of conditions such as RPM, air temperature and air pressure.

Those 3 inputs are enough to figure out how much air mass is going into the cylinders and then you can calculate your fuel. Most production cars have a rather pathetic table with some crude defaults, which are usually super rich, and this is only meant to serve as a backup in case any of the components of the closed-loop system fail. It's also used briefly when you start the car up cold and when you're in wide-open throttle conditions (although without a wideband to tune with, the car is running blind). For a select few cars that have the support, there exist alternative operating systems that can be loaded into the car's computer, which have much larger look-up tables (VE tables actually).

The bigger tables offer the sort of resolution you could never hope to achieve with a MAF sensor alone (which is what closed loop relies on). Furthermore, in open loop, your MAF sensor is replaced by your MAP, IAT and TACHOMETER, all of which are far more reliable together than the MAF alone. MAF sensors are not particularly good at being able to measure air mass with repeatable accuracy, especially when the flow is fast and turbulent. They actually need a special metal screen to make the flow laminar (straight) or it won't work at all. In open loop you can get a very repeatable air mass measurement at any load condition, regardless of flow characteristics. Combine this with the added resolution of a VE table and you're in business!

Tuning Your MAF/VE Tables

Where does the wideband O2 sensor come in? Well even though we can't use it in closed loop, we can use it to tune our VE tables. Let me explain. Normally when you tune your car in a closed loop scenario, you'll go for a road scan and record your "fuel trims." You will usually pay attention to your LTFT values, which are the long-term fuel trims. This tells you the error or how much you are off in a given cell. Running lean at a certain MAF value will produce a positive (+) error that the computer will then use to compensate for the base value that was associated with that cell (in this case by adding fuel in the amount of the error %). This way, the next time that cell is checked, it will provide the corrected number. This is the basis of closed loop. The problem is, the number never really settles because of the MAF and how sketchy it is, not to mention it gets covered in crud over time. It's basically doing the job of 3 sensors in 1, and this leads to inaccuracy. You can tune based on your LTFT values, but the next day you'll be out again. It is said that getting within a trim of +/- 3 is pretty good for a MAF-based closed loop system, but an open loop VE table can do even better than that.

In open loop, there's no compensation going on. Every time a cell is checked, it's wrong by the same amount, and the drift in that error is less because it's relying on 3 very accurate sensors that do not fluctuate much. The tachometer is based on a mechanical encoding system, the temperature is quite sluggish to change, and the manifold pressure is probably the least reliable, but even it is nothing more than a simple pressure transducer with a linear output voltage. Basically, once you tune your VE table a couple of times, you should never need to touch it again unless you change something physically in the car like the filter or the heads or something that physically alters the volumetric efficiency. This is the power of open loop - stability and a refined response at any load condition... but we're not done yet.

Open/Closed Loop Comparison - Notice How Closed Loop "Hunts"

The Best of Both Worlds - Power and Efficiency

There's another advantage we've yet to exploit. With open loop you're not locked to 14.7 anymore. You can have a different AFR in every cell in your table if you want. With a wideband O2 sensor, you no longer need to rely on LTFT values in your road scans. Those are based on the cruddy narrow band O2 sensors anyway. Instead, log your wideband O2 sensor directly and get an AFR value instead of a trim value. This you can use to populate your VE table as you drive so that you know what AFR you're getting in each and every cell. Then, after your road scan, you can adjust the cells as you like, to whatever target you like.

For example, if you're getting 14.1 AFR at 2300 RPM and 0.4g air mass, you can find the error by simple division but you can also pick your target first. Maybe you don't want 14.7 at that load point. Maybe you want 15.5. So do 14.1/15.5 (which is 0.91 or 9% on the rich side) and use the result to multiply the VE value in your table in that particular cell. This will give you an AFR of 15.5 or pretty close to it. You may need a couple of scans/tunes to zero in on it.

In any case, you may want only 12.5:1 when you're at 5500 RPM and 1.2g of air so if you're only getting 13.2:1 in that cell you can do 13.2/12.5 and multiply the answer by the VE value in THAT cell. You continue going through this exercise, which is pretty easy in Excel, and pretty soon you've got a fueling model that is far more flexible and stable than anything you could have hoped to achieve before. It isn't self-correcting like closed loop would be but that is also its strength because it's definitely going to be more consistent from cell to cell, which by itself can prevent random knock events that sometimes result from sudden changes in combustion characteristics and it will prevent that sudden surge of gas that coincides with PE (power enrichment) mode kicking in.

Think of open loop as a vacuum-actuated secondary in a carb, compared to mechanical. Perhaps not the best analog but mechanical secondaries (and PE mode) are more sudden, more aggressive and more prone to harsh transitions. In a fuel-injected car, the sudden rush of gas from a suddenly rich AFR can induce knock as well. In a large, high-resolution VE table operating in open loop, you can smoothly graduate from 15.5 to 12.5 in several small increments across half a dozen or more load cells, making it easier for the spark and torque tables to track the fuel and adjust properly.

An EPA estimated 8 inches per gallon!  Measured at the tailpipe.

An EPA estimated 8 inches per gallon! Measured at the tailpipe.

Fuel Control Is Pretty Serious Business

Fueling is very important for hotrodders and goes beyond just making sure you've got enough. I feel like people choose these values arbitrarily and tend to err on the rich side, falsely believing that if something is wrong with their tune they can just add more fuel to make it go away. While making serious power with superchargers and such means you need to run quite rich, it's not as simple as just that. You still have to find out where your engine is happiest, as running too rich can cause knock and power loss, just like running too lean can. The engine's sweet spot will also change with RPM, temperature, fuel composition and a slew of other things so it's really important to get a good tune done on your MAF or VE table, whichever the case. But with VE and open loop you have maximum control over your fuel. That freedom comes at a price, as there are no safety nets if you make a mistake but if you're careful and dial in the values, the benefits are significant and your car will thank you with many years of reliable service and plenty of burnt rubber on the asphalt to show for it.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.