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.
- 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)
- Have a healthy motor
- 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.
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.
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.