Tuning turbocharged engines tends to scare off a lot of novice tuners. Even if you're perfectly competent tuning naturally aspirated engines, the thought of tuning a turbocharged engine can seem very daunting. Read on to learn about the techniques required and let us show you that there really is nothing to be afraid of!
Turbocharged vs N/A Tuning
Whether you’re tuning a turbocharged or naturally aspirated (N/A) engine, what you’re trying to do is to optimise the fuel delivery to suit the mass of air entering the engine, and to optimise the ignition timing so you’re creating a spark at the right point in the engine cycle to achieve maximum torque - this is referred to as MBT timing. This means fundamentally you're doing the same thing if you're tuning a turbocharged engine, or a naturally aspirated one - There are few extra considerations for the former though.
While the tuning process is essentially the same, turbocharged engines usually have a narrower tuning window in terms of what we need to provide to keep them running reliably while producing good power. Since turbocharged engines will normally be making more power than a comparable N/A engine, engine damage can occur more quickly if the fuel or ignition isn't correct. Understanding what the engine wants then is the first step to creating a reliable tune and reducing your stress load on the dyno.
One of the concepts we need to start with is what boost pressure means. In many ways boost pressure as a number isn’t actually telling us that much and the far more important aspect is that boost pressure results in airflow - It’s really the airflow that’s the key to what the engine needs in terms of fuel and ignition. What this means is that saying we’re running 20 psi of boost isn’t that helpful because the amount of airflow and hence power we can produce with a larger frame turbocharger like a Garrett GTX55 is completely different to the same boost pressure on a modestly sized turbo like a Borg Warner EFR 7163.
Forget for a moment that you’re usually using boost pressure on your load axis on a standalone ECU. The important point to understand is that as you move past atmospheric pressure of 100 kPa (which is about the maximum we can reach with a naturally aspirated engine), up into positive boost, the turbocharger is simply being used to force more air into your cylinders. This has two effects on our tuning requirements and we'll start with the fuel delivery.
As we combust more fuel and air in the cylinder, we end up creating more heat. We need to be mindful of this heat or we risk damaging our engine or creating conditions where knock is more likely to occur. While it might sound a little counter-intuitive, adding a little extra fuel by way of a richer air-fuel ratio target will mean that some of the additional fuel will pass through the combustion chamber unburned, and this will have the effect of cooling our combustion charge temperature. With this in mind, as the airflow and boost pressure increases, you'll want to move towards a richer air-fuel ratio. Of course, there is a limit to how rich you can go before power starts to taper off or we run into issues with a rich misfire.
Secondly, moving further into positive boost also affects the ignition timing and the trend we see here is that more boost requires us to retard the timing, or in other words, start the spark event later in the engine cycle. You might be assuming that we need to retard the timing to prevent knock, and while that can be a consideration, even before the engine starts suffering from knock we will see this trend in our timing table. The reason for this is that all other things being equal, as we combust more fuel and air in the cylinder, the combustion process happens faster. This means we don't need as much ignition advance to reach MBT.
Since I’ve just mentioned knock, this is another consideration for us, and just about every turbocharged engine running on pump fuel will be what I refer to as knock limited. This simply means that as you advance the timing towards MBT, you’ll end up suffering from knock before MBT is actually reached. Knock can quickly destroy our engine, so if that is the case you’ll need to stop and retard the timing a little to build in a safety buffer.
AFR Target Map Explained
Now that we've covered the basics, we'll get a little further into the details and we'll start by considering the AFR target map which breaks down the operating regions of the engine into a few different zones, and each of these zones will have different air-fuel ratio requirement. On the vertical axis, we have boost pressure and on the horizontal we have RPM. With a naturally aspirated engine, as previously mentioned, you can’t access the area above 100 kPa. When you’re dealing with turbocharged engines however things are quite different.
The red line on the above graph shows an approximation of what you could expect a typical boost curve to look like. How far you move into the boosted region, of course, is going to depend on your turbo, wastegate spring pressure and your boost aims. An area in which you will spend a large amount of time is the transition region, where you're likely to be at perhaps 25-40% throttle. This could be for example when you’re driving up a slight hill or using a little throttle to pass a car on the open road. As its name implies, you'll also transition through this region as the turbo starts to spool up when you go to wide-open throttle. When selecting our air-fuel ratio targets, we want to consider the amount of load being applied on the engine and then we can select AFR targets that are suitable based on this.
For a typical engine running on pump gas, we could expect to tune for a stoichiometric Afr in the idle and cruise zones. In the transition zone, however, there is more airflow into the engine and we will need to tune with a richer AFR - Somewhere in the region of 14.0:1 would be pretty typical. As the boost builds and we run through the medium boost and into the high boost regions we’d progressively richen the mixture from around 12.0-12.5:1 down to perhaps 11.5:1. Note that these are guidelines only and ultimately we need to test and find what the engine wants.
Using the Dyno
When I'm tuning on the dyno I try and use the dyno to replicate the way the engine will perform on the road. For example, if you go to wide-open throttle on the road then naturally the car will be accelerating and the engine rpm will be increasing rapidly. Under WOT conditions then it would make sense to tune the fuel and ignition by performing ramp runs on the dyno. However, if we're using around 20-30% throttle we're more likely to see the engine rpm remain stable or increase slowly. In this case, we would tune these areas under steady-state conditions on the dyno, where the dyno will apply a variable amount of load to keep our rpm consistent as we change the throttle position.
To break this down a little further, we’re going to use the dyno in steady-state mode to tune the cruise and transition areas of the map, but once we get to WOT we will switch to using the dyno to perform ramp runs. This gives us the most realistic operating conditions which ensure that once we’re out on the road or track, the AFR will match what we saw on the dyno and the ignition timing will be optimal. This technique also helps to limit the amount of heat being generated since we don’t need to sit at WOT and high boost in steady-state for extended periods. Ultimately it’s the cruise and transition areas we’re going to spend the most time using in a street car so it makes sense to focus our efforts here.
If we do our job properly then this will give us an engine that is smooth to drive, feels more responsive to throttle input, and as a bonus, will have better fuel economy. While we can’t ignore the other areas of the fuel map, the high rpm, low load region, for example, is one that we’ll only access on a gear change or a throttle lift, and due to this the accuracy is less important and if anything, it’s best to be a little rich.
With a turbocharged engine, you have the ability to run at different boost levels depending on what you’re doing with your boost control. For example with the above drawn in graphs, let’s pretend that the bottom line is your minimum boost pressure. This could be your wastegate spring pressure, meaning the boost pressure physically can’t be lowered below this level at full throttle. From here, you can electronically or pneumatically increase the boost pressure. On the dyno we will always start our tuning with the minimum boost pressure we can achieve. We can then dial in the fuel and ignition before slowly increasing the boost. This allows us to start with minimal load and stress on the engine, plus we can start to extrapolate the trends in the fuel and ignition tables out into the higher boost areas. This means that when we do raise the boost it’s likely that our tune will already be very close and only need minor adjustments.
Common Boost Control Problems
While we'd love for every tuning session to be smooth sailing, that is rarely the case, unfortunately, and there are a couple of common problems we'll face when tuning turbocharged engines in particular. Understanding these problems will allow you to quickly notice when they’re happening, and you’ll have an understanding of what the problem is, as well as potential solutions:
The most common issue I see is a boost control system that’s plumbed up incorrectly. This goes for both electronic and pneumatic boost control systems and it can waste a lot of time on the dyno. In some instances, you’ll find you can’t raise the boost when you try to, but in other instances, you’re likely to have excessive boost pressure which can potentially damage your engine. This is why it’s always important to make sure your ECU is set up with an overboost cut before tuning. Fortunately, the solution here is easy - Read the manual that came with your wastegate or boost controller and ensure the plumbing is correct.
The second issue you’re likely to see, particularly with stock turbochargers, is a situation where the boost tends to drop off at high rpm. This may result in you not being able to maintain your desired boost target and is usually the result of a turbo that’s starting to become restrictive at high rpm. While fitting a stiffer spring might help out in this case, the ultimate solution is to consider a more appropriately sized turbo. Of course, there are no free lunches and generally, as we free up the hot side of the turbocharger we end up sacrificing boost threshold and boost response.
The last, and potentially most dangerous situation you may come across is where the boost pressure rises exponentially as engine rpm increases and you can’t control it. Understandably this could quickly destroy your engine if it’s allowed to go on unchecked. There are a variety of potential culprits here, but assuming the boost control system is plumbed properly, the usual reason is a wastegate that’s too small or one that is positioned in such a way that the exhaust gas has trouble flowing into it.
Heat is one of the main aspects of tuning a turbocharged engine that scares tuners off. With proper heat management, there is really nothing to be scared of here. It can, however, be easy to be so focused on optimising the fuel and ignition, that you forget to keep an eye on various temperatures. It is important to get into the habit of doing so and the following are parameters you’ll want to keep an eye on:
- Engine coolant temperature - Particularly as you move into the higher RPM regions during steady-state tuning the engine coolant temp can rise quickly, requiring you to come back to idle and allow it to stabilise before continuing.
- Intake air temperature - While you’ll almost certainly have an intercooler fitted to a turbocharged engine, under high load in steady-state, it can become heat soaked. This can also occur after multiple back-to-back ramp runs too. Particularly on a dyno, it’s difficult to replicate real-world airflow and the intercooler may not be able to reject heat as well as it does at high speed on the road or track. Having a spray bottle of water handy that you can spray onto the intercooler core between runs is a great way to help get more consistent and realistic air temperatures on the dyno.
- Oil temperature - This is usually less of a concern and many aftermarket ECUs won’t have a sensor for this. If you can monitor it, do so as steady-state tuning under high load and put a lot of heat into the oil.
To keep the above temperatures under control, if you bring the engine up to a certain cell where you are in positive boost pressure and see that a change needs to be made, there is no need to keep the engine in that cell to make the change. Instead, you can back off the throttle, make the change and then come back to the cell to check whether you’re now on target. This is a much safer way to tune although it will take a little longer. The same goes for knock. Instead of holding the engine in that cell where it is experiencing knock while you slowly retard the timing, you’re best to immediately back off the throttle, make the change and then return to that cell to see if the change has worked.
Hopefully, that gives you a good insight into the world of turbocharged engine tuning. Start to learn more about how you can tune an NA or turbocharged car here with your free EFI tuning lessons.