Car Modding, Car Tuning and the Science of Horsepower: Volumetric Efficiency
What is Volumetric Efficiency?
Tuning your car gives it a sense of character. It's putting your signature on the car so that a part of yourself becomes imprinted on it. Every sound, every tenth of a second on the strip, every screech of the tires is unique and tells people, "I did that!" It gives you pride in doing a good job and if your vehicle outperforms a factory copy of itself or an inherently superior model, it's like no other feeling in the world.
A well-balanced car requires a well-balanced tune and when it comes to performance vehicles, tuning the Volumetric Efficiency tables is arguably the best place to start. So what is volumetric efficiency and why is it so important to tuners?
Volumetric efficiency is a way of measuring how much air is actually going into your engine when compared to how much air SHOULD be going into it under the ideal "academic" conditions. Picture it this way: Say you own a 2013 Camaro SS. It's got a 6.2L V8 in it. This engine displacement tells you how much volume is displaced by the engine. That is to say, when each of the 8 cylinders travels from top bottom dead center to top dead center, it sweeps out a cylindrical volume of air, the sum total of which gives you the 6.2L.
In a perfect world, under operating conditions, the engine would be pulling that much air for a given cycle but because air is a highly compressible fluid and because the intake tract of your car isn't perfect, there are flow restrictions and pressure head losses that cause the pressure to drop below ambient as the air enters the cylinders. Of course the geometry of the cylinders cannot change, so the slightly lower pressure of the air means lower air mass, less fuel added, and ultimately less power. The mass of air actually going into the engine can be compared to the theoretical amount and an efficiency can be calculated, the volumetric efficiency.
Simply put, the volumetric efficiency is the ratio of air going into the engine to the ratio of air that the cylinder geometry can accommodate at standard pressure. A volumetric efficiency of 90% therefore means that only enough air to fill 5.58L is actually going into the engine but that 5.58L is being spread out over the full 6.2L, resulting in a drop in pressure and air mass.
Why is Volumetric Efficiency Important?
Inside every car's computer there is a 3D table, like an Excel surface graph, where RPMs are on the X-Axis, manifold absolute pressure is on the Y-Axis and the volumetric efficiency values are on the Z-Axis. This basically "maps" out all possible combinations of engine speed and air pressure in the manifold, which together are a way of expressing mass flow. It's a less direct way of measuring mass flow than say a mass airflow or MAF sensor, but it has a number of distinct advantages and disadvantages. Whereas a MAF sensor uses a wire suspended in the air stream to give a direct air mass reading, the VE measurement relies on the MAP sensor and the crankshaft position sensor to provide 2 of the most variable readings that factor into an mass flow calculation. There are others like temperature but this isn't included in the table.
In some vehicles, the VE table is a backup, in case the MAF fails, so that the car will still run. In other cars, there is no MAF so the VE is the only means of measuring mass flow. MAF sensors are more forgiving because they are not as sensitive to changes in the environment. The also have a better resolution than VE tables, which have a limited number of cells to lock in at. They do however tend to get fouled up and the readings will skew over time. They are also less reliable at high flow rates because the sensor relies on laminar air flow to function properly and this cannot always be maintained when the engine is consuming a lot of air, as it would under full-throttle or boosted conditions. Finally, the mesh screen in front of the MAF sensor meant to promote laminar airflow is itself a restriction and can be a bottleneck for extreme modders.
The great thing about the volumetric efficiency method of measuring mass flow is that once you tune it in, it's very very robust as long as the car and the environment don't change too much. It's quick to lock in at a given value and nowadays, with some tuning companies offering upgraded operating systems, you can obtain VE tables with much higher resolutions, bringing the precision closer to MAF sensors.
One final note, which is perhaps most important of all. The calculated VE value is subsequently used in a ton of other calculations that affect drive-ability and performance, even for MAF equipped cars so it's critical to have a good VE tune, even if it's not being used directly.
The Best Available Tuning Software
How Do I Tune My Volumetric Efficiency Tables Then?
As mentioned previously, the VE table is a 3D table. It's a guess as to what the efficiencies are at every RPM and every manifold pressure. A value of 90% would be expressed as 0.90. These are programmed from the factory based on carefully controlled flow testing but once on the road in God knows what part of the world, that tune is not ideal anymore. You have to dial it in again. If the number in the table is lower than reality, the computer will command less fuel than required, the O2 sensor will read lean and raise the LTFT fuel trim to compensate and your trims will be out of whack. Since the fuel trims only function properly in closed loop, this means havok if you're running at wide open throttle and your VE table is no good. You could be running rich or lean and have an expensive explosion because the computer can only guess how much fuel it needs without the O2 sensor input.
The first step to a VE tune is to DISABLE power enrichment. Depending on the tuning software you use, this could be an enable/disable toggle or something like setting the PE ENABLE throttle position to 100%. Basically you want to disable PE because it is this mode of operation that tells the computer to stop using closed loop. You want to use closed loop while tuning the VE because your O2 sensor is the only way you have of telling how far off the VE table values are from reality. I know PE disables closed loop for a reason but we have to rely on closed loop for the duration of the tune so make sure you disable PE mode so that you can get an LTFT trim value at any operating point, including wide open throttle.
The second step is to physically unplug your MAF sensor, if your car has one. Be aware that this will cause your "Service Engine Soon" MIL light to illuminate, because the car will think the MAF sensor is malfunctioning. This is ok. The car will probably also run like crap. This is also ok (for the duration of the tune only). You'll want to reverse these 2 changes after you're done tuning though because the car won't run too well at first without the MAF and you should not push the car too hard without PE mode to enrich the fuel ratio.
The third step is to connect your OBDII scanner and go for a drive. Make sure to log all the usual suspects: RPM, A/F ratio, STFT, LTFT, Fuel Trim Cell, Spark Advance, Knock, MAP, IAT, vehicle speed, current gear, ECT, O2 voltage, throttle position and any others you feel like checking within the limits of your bandwidth.
Go for a nice long drive and let the car's coolant heat up to nominal. Begin logging and start out as gentle as you can. Treat the car like a fragile egg. Try to hit every possible speed while keeping throttle activity nice and low. Then you'll want to very gradually become more and more aggressive with the pedal until, at the end of your drive, you're stomping on it like a maniac. What this does is populate every cell in the VE table with a fuel trim value. Since the table is RPM vs MAP, going gentle will fill out all the RPMs in the low MAP rows like 0 - 30 kPa. As you become a more aggressive driver, the MAP rows will switch to 40, 50, 60, 70, 80, 90 and perhaps even 100 kPa. While in each row you want the engine speed to hit as many values as possible. Remember, to hit a new row you need to step on the gas pedal with a different speed. Stepping on it quickly will allow a rush of air that will cause the MAP to hit a higher value. You should have a live gauge on your laptop to monitor MAP so you can hold it to a given range of 10 kPa, say 40-50 and keep it there until your tach needle has gone through several hundred RPMs. The longer your road scan and the more unique values you hit, the less interpolation you will need to do and the better your tune will be.
For a VE tune AND boost you need to replace your MAP sensor
Analyzing the Data
This is the hard part, or at least the most time consuming. Back at home, dump your road scan into excel and isolate the columns for MAP, RPM and LTFT. Now you need to sort all the data so that you have all the MAP values sorted in order first. That's easy. But then you have to go through every row where the MAP falls between 30 and 40 for example and look at the RPMs. You now have to sort all the RPMs so they are grouped according to the resolution of your VE table. For example, if your VE table only has cells for every 400 RPM, you need to make sure you group together everything between 1200 and 1600 RPM, which also belongs to your 30-40 kPa row. This may take some Excel skills but once you have both axes sorted, the worst is over.
When you're done sorting you should be able to find all logged entries that correspond to each cell in your VE table. You won't have all of them simply because your scan won't have picked up all of them but whatever it did pick up, you should have sorted so that you can say, "Here's all the data points for 30-40 kPa and 1200-1600 RPM", for example. Then you'll want to average all the data points that fit into each cell. This unfortunately has to be done because the VE table only has so much resolution and can't usually tell between 1 kPa or 10 RPMs so just take the whole cell's worth and calculate the average.
Now make a new table, just like your VE table, with the same axes but you're putting the averaged LTFT values in there. Some will be + and some -. You will see holes in the table because you don't have data for those cells. Simply use Excel to do linear interpolation (find the middle value between 2 cells with a missing data point in between) on all the holes until they are all filled in. As an example, if you have 1, a gap and then 3, the interpolated value would be 2 because 2 is halfway between 1 and 3. Keep doing this until your whole table is full.
Next, for every whole number LTFT value, like 1, 2, 3, -3, etc. you want to apply a VE correction to your original VE table of 0.001. For positive LTFT, you apply a positive correction. For negative LTFT, you apply a negative correction. If at 4000RPM and 90 kPa you have an average LTFT of 4.6, you will find that cell in your original VE table and increase it by 0.0046, which is about 1/2 a percent. This is somewhat low-balling the correction but better to do a couple of scans and zero in than to overshoot. What this does is adjust the fuel delivery so that lean areas get more fuel and rich areas get less, all without relying on the O2 sensor. This makes the car self-sufficient and it will run better in all respects whether the car relies on the VE directly or not. You'll get less knock and lower your risk of damage if you run mods like boost or nitrous.
Completing the Volumetric Efficiency Tune
Now that you have a new volumetric efficiency table, graph the table in Excel and look for rough areas. The graph should be fairly smooth. Rough areas can indicate either a bad tune or mechanical problems. You don't want the computer to make severe adjustments moving from one cell to another so try to smooth it out a bit. This is more art than science and takes some practice but can yield very good results.
When you're happy, overwrite the old VE table in your bin file or whatever file your software uses to upload to the car and then do a PCM write, making sure to restore PE mode if you're done tuning. If supported, you should also clear the fuel trim memory because you have a new tune and you don't want it using values from the old tune. Before you road scan your car again you will need to teach the car the new values which involves a couple hours of varied driving, similar to what you did during the last road scan. Then you can repeat the whole process or call it a day if you're satisfied. You will know when you're done if the LTFTs in all cells settle between +/- 3. For some, they're not happy with +/- 3 but shoot for +/- 1. It's up to you.
Experimenting With Volumetric Efficiency
Volumetric Efficiency is the foundation of a good performance tune so it's imperative that you understand what it means and how to keep it under control. Some tuning companies offer widgets or tools to compute all the VE values for you based on your road scan columns but the above procedure will always work, if you don't have access to these tools. If your car is or looks to make a lot of horsepower, sooner or later you're going to consider using your VE tables instead of MAF tables. I encourage you to read the automotive forums and learn from the experiences of others who have VE tuned their cars. Their wisdom can make life a lot easier and less expensive for you.
If you're just tinkering with your daily driver, you can do a VE tune for fun, just to learn something or see how it affects the car but it's not necessary and the MAF is usually the preferred method of controlling fuel. Since the MAF only becomes a liability at high rates of flow, you shouldn't damage or remove it unless you fall into that category because it provides redundancy in case of a sensor failure and some cars will run in "limp" mode when the MAF is disconnected, hurting the car in other ways. As always, use good judgement and I'll see you on the track!