I'm often asked the question "Why would I need to know the waste gate position if my boost control is doing what I want it to?" This is usually followed up with a comment such as "so long as the boost it what I want I don't care what the waste gate is doing" The reality is that the waste gate position can tell you many things regarding the performance of your engine and turbo system if you are just willing to listen. A very simple illustration of this point could be the waste gate valve is shut all the way. Even assuming you are achieving proper boost, the indication is that you have reached a limit of the turbo system. Of course if you have multiple waste gates on a common system or even the more common dual waste gate setup on a twin turbo engine like a V8 the position of the waste gate as compared from one side of the engine to the other can be an indication that there is a balance problem between the engine banks or turbochargers or more commonly the waste gates themselves.

Suppose as an example that the exhaust system on the vehicle develops a leak before the turbocharger on one side. You look at the boost, its normal. Look at the back pressure (assuming its monitored and usually only on one side) it is either higher or lower than normal depending on whether it's measured on the side with the leak or not. Look at the lambdas, one side is richer than the other. Now have a look at the waste gate position. In this case the side with the exhaust leak, will show a corresponding reduction in waste gate opening percentage as compared to the side with no leak. This will be due to the lack of exhaust pressure working on the face of the waste gate valve which normally tends to force it open and is indicated in the position as a more closed waste gate; and yes, by the way. this is a real scenario in which the data from the valve position was able to indicate the side of the engine to look for a problem on (center of collector had come loose and fallen out)

We all know that back pressure is a bad thing, however we don't always know the exact cause of it and often assume that whatever back pressure we have is just normal for the system we are using. It is not uncommon to find that there is a fairly significant offset in the spring tension between two waste gates of the same manufacture with the same springs installed. It becomes quite easy however to synchronize the waste gates by way of monitoring the valve positions with differing amounts of pressure set against the lift side of the diaphragm. Often times it takes either a spacer under the spring or a different spring altogether to get the valves to track each other linearly at many different lift pressures. It should be fairly obvious that when one waste gate is closed more than the other, that particular side of the engine runs less efficiently than the other side. Now this is compounded by the fact that the compressors usually feed into a common plenum. In such a case we have one turbo spinning faster than the other. The faster one has to compress the air more and therefore raises the back pressure on that side of the engine while also increasing the temperature rise of the air leaving the compressor. The slower turbo meanwhile usually is going slow enough that some of the air is still leaking backwards out of its compressor since it is at a lower airflow than the other side. Clearly balancing out the workload between sides of the engine and turbos will result in an efficiency and power increase.

In the case that the system does not achieve the boost you desire either too much or not enough, the waste gate sensor pays for itself many times over. I have had situations where the boost lines we're accidentally reversed between the lift side and close side of the waste gate during an engine swap which lead to severe over boost. The data from the gates showed them completely closed during the run. One quick check of the lines and routing on the gate revealed the problem.

On another system the car made 34 PSI boost on one run and on the next made a mere 18 psi. The data showed the waste gate position to be at approximately 35 % open on the run with boost and over 85% open on the run without boost. The solution in this case was a crew member forgetting to arm the CO2 system which controlled the boost on that vehicle.

One team was running a car all season with the waste gate sensors installed. The car used closed loop boost control and as the year went on the performance would drop more and more. This wasn't too obvious until about August when I started noticing that the waste gate position at 38 PSI was about 40% more closed than it had been at the same boost and rpm in January. Hmm - have to keep removing fuel every run too to get the lambda right, in fact, have about 15 percent less fuel than we did in January. For some reason turbos are working harder than they used to. Either we have exhaust leaks, or we have an intake leak or restriction. Well test everything you can test and can't find leaks so start looking for restrictions, can't see anything obvious, make sure throttle opens all the way - yep - check inter cooler. Aha! Inter cooler full of oil and dirt, I mean full of dirt. Oil from the turbos collected on inter cooler core and overtime enough dirt got on it that well, it was plugged enough that what would have made 48 PSI boost would only make 38!

One last example was with a team experimenting with turbine AR ratio. They had changed to a large AR and larger Turbine wheel which had significantly lowered their back pressure which was good, but on the other hand the car would not develop more than 38 PSI boost. One look at the waste gate position indicated why, both gates completely closed. Stop working on the springs in the gates and the boost control strategy, start getting the AR ratio smaller. In each of these real world examples, the time and money saved by having the data on the valve positions proved invaluable.

In closing there are several good arguments for using waste gate position sensors on your turbocharged vehicle and much more information can be gathered than simply the waste gate position. It is possible to close loop control the waste gate position signal in the MoTeC M800 and have a gate position over time (or other axis) based control. It is also feasible to have the waste gate flowed at different openings and calibrate the flow values with the valve position to have the data values in flow rate rather than simple position. Like many sensors, more information can be extracted from the sensors than the simple values that they report. And when you are at the racetrack, trying to identify a performance problem, the money you spend on the sensors can easily be recovered in lost time and round wins due to attempting the diagnosis through the guess and/or assume method.

- Shane Tecklenburg

ST Consulting