Category Archives: Troubleshooting

Supporting Modifications and Your Turbocharger’s Efficiency

Occasionally, we have the opportunity to help our customer troubleshoot situations where their expectations are not being met after installing one of our turbochargers.  This sounds bad initially, but hear me out.

For a moment, consider that a turbocharger is a lot like a big barrel of water.  At the top, there is an inlet where water can fill the barrel.  At the bottom there is an outlet where water can exit the barrel.  If the inlet at the top of the barrel is closed, it will be impossible to achieve constant, high volume flow out the exit.  Now consider the opposite, where the exit is barely open but the inlet is entirely open and a hose is pouring water in the opening.  It won’t take long before water is overflowing out the opening and there is still very little water coming out the exit.

Turbochargers work on similar principles, only more complex.   Their ability to flow is dependent on the engines ability to move the gasses that are injected into the cylinder for combustion.   For example, if one were to use our Steam STX 71 with a factory exhaust, it is likely that they would be disappointed on the dyno.  The factory exhaust is simply not capable of moving exhaust fast enough for the turbo to spool quickly or make power because the engine can only combust as fast as it can evacuate the exhaust gasses.  Therefore, the more gasses you can create, the more horsepower and torque you will make.  This process is contingent on the ability to move gasses through the system and out the tailpipes.

Sometimes when a customer might state that a turbo is spooling slower than expected, we discover later that there is an exhaust leak somewhere.   Since a turbocharger’s job is to leverage wasted heat from the engine by means of capturing it in the exhaust path, it makes sense that if that heat is leaking off before it reaches the turbo, the turbo will spool slower and produce less power overall.  This one takes us back to our early days in school where we learn about the Law of Conservation of Energy.

Another culprit of a let-down on the dyno can be the wastegate.  The wastegate is called that for a reason.  It’s sole purpose is to waste energy.  This is a good thing when it is tuned properly, and a very bad thing when it is not.  The wastegate’s job is to open when the turbocharger has pressurized the intake manifold to the proper pressure (usually measured in psi).  Let’s assume you are striving to reach 20psi with your tune.  When the turbocharger achieves this, the wastegate on the hotside of the turbo opens up to let exhaust gasses that would normally drive the turbine, escape around the turbine instead of passing through it.  This makes it possible for the turbo to flow at a desired rate without overboosting the engine.  If the wastegate is opening to early, or the spring is insufficient for the tune, exhaust gases that are necessary to make power will be lost along with the power they would have supplied to the engine.

Intercoolers.  We love them and we hate them.  We love them because they cool the air charge going to our engine, lowering the risk of detonation, and giving us much more power than we would have if they were hotter.   Hot intake gasses are less dense, and because of this, supply less oxygen per cubic liter than colder intake gasses.  A warmer intake charge also means that your tuner will have to compensate for those temperatures in power-robbing timing settings.   A good top-mount intercooler will be sufficient for most, however many opt for a front mount intercooler.  There are advantages and disadvantages to both.  Your build and your pocketbook will largely determine what works best for you.  A front mount intercooler means a lot more air is necessary to pressurize the intake system.  This results in power later, but it usually results in lower intake temperatures too.  Also, more piping, means more connections and more connections means more opportunity for leaks.  If you find that your intake is leaking, you’ll know it by a massive loss in power and it taking longer to reach boost.  The air that would have been used to combust a greater amount of fuel is being dumped to atmosphere.  That means less power to the wheels.

Supporting modifications influence your turbocharger’s efficiency.  The fewer you have, the less power you will make.  It is important to consider what other items will be necessary for you to achieve your goals when bolting on a device that is intended to push your vehicles power well beyond what the factory intended.

Troubleshooting Slow Turbo Spool

When our customers experience slow turbo spool, we take it personally.   We want our customers to have a great experience with our turbos.  There are a few things that are absolute musts before you get your car tuned if you expect a good tune result.

Vehicle preparation:

  1. Ensure you have the minimum set of modifications
    • 3″ high flowing turbo back exhaust
    • upgraded fuel injectors and pump
    • upgraded intercooler (recommended)
    • big MAF intake (if your tune is not speed density)
  2. Have a healthy motor
  3. Ensure there are no leaks
    • in the intake
    • in the exhaust pre-turbo

#1 is really about hitting your best possible numbers.  #2 is more about the longevity of your build.  #3 will really kill the spool and responsiveness of the turbo.

Here is a representative result of what we would expect to see of a STX 71 on a built EJ257 (AVCS not working).  This car had all of the required supporting mods.  Versus the stock STI turbo, the VF39, there is hardly any trade off in terms of power and torque, and a something like 130 WHP in upside, and power until redline.

STX 71 vs VF39 turbo

In this particular case, the turbo and accessories were later upgraded to an even larger front mount intercooler, external wastegate, and our production Steam STX 71 turbo.  Improving items from #1 did indeed improve the peak HP by around 30 whp, but the owner was understandably concerned about how late the power came on.

Proto STX 71 vs Prod STX 71 (exhaust leak)

The most reasonable explanation for the huge amount of lag, is #3, or in other words, an intake leak, or a pre-turbo exhaust leak.

As a the turbocharger is the heart of the power, and a decent investment cost-wise, it is easy to point the finger at the turbo when power delivery is not within expectations.  After expressing concern that the turbo could be the cause for the late spool, the owner eventually checked for leaks and found this, a huge up-pipe exhaust leak.  An up-pipe leak is perhaps one of the most common leaks for Subaru turbochargers.  This has a lot to do with the flange design.  Notice that the leak is out the weakest side of the flange, the side with the largest distance between studs.   For this reason, we encourage customers to triple check the installation to be sure that they do not have leaks if they are experiencing lag.   It’s a common initial response to not want to check for leaks; however, going in in for a tune with a leaking turbo system will be disappointing.  We’ve had tuners tell us how common this scenario is.  Based on their feedback, it is our recommendation to check every new installation for leaks before going in for a tune.


The moral of the story is that, we want you to have great results with your turbo, but leaks and insufficient supporting mods both have the ability to decrease any turbocharger’s efficiency and output.  Nobody wants that.

If the turbocharger isn’t making the power you would have hoped, check to see if their are some gaps in the supporting mods.

If you are experiencing delayed boost, for example, 5000+ RPM peak torque on a STX 67 or 71, check for exhaust leaks, and pressure test the intake.