We’re back with even more turbo testing.
After our initial rounds of dyno work, we knew the turbo had more to offer and wanted to see what it could truly do when pushed to its limits, unrestricted by fuel system constraints or the risk of engine failure.
So, we dusted off our PRL Civic X race car, Snow White. In addition to a full catalog of PRL components, this car features a CSS closed deck bottom end with forged rods and OEM-style forged pistons to maintain the stock compression ratio. The cylinder head remains largely stock aside from upgraded valve springs, retainers, and head studs. With the addition of port injection, the car also has more than enough fueling capacity to fully max out this turbo.
Now let’s dive into the fun part.
We spun the rollers with the 10th gen at the end of December 2025. Ambient air temperature was around 30°F, and intake air temperatures during the run were approximately 53°F. The car was filled with pump E85, and tuning was handled by Vitaliy Mikitchenko (VitViper) using MoTeC.
After dialing everything in, we achieved a maximum of 418 wheel horsepower at 6500 rpm and 384 wheel torque at 5200 rpm.
Looking at the MoTeC log from that run, we can see that maximum boost of 35 psi is achieved at 5300 rpm. This run represents essentially the maximum output the turbocharger could produce. During the pull, the wastegate was completely shut and the turbo was operating at its absolute limit.
We were very happy with these results, but there is something hidden in the log that deserves a closer look. To understand it, we need to talk about exhaust back pressure ratio, or EBP.
Understanding Exhaust Back Pressure (EBP)
What we’re looking at here is a comparison between exhaust manifold pressure (pre-turbine) and intake manifold boost pressure.
During this run we observed:
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68.1 psi pre-turbine pressure
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36.2 psi intake manifold boost
This results in an EBP ratio of 1.88.
So why does this matter, and why can it potentially be dangerous?
What this graph tells us is that the engine is trying to push more exhaust gas through the turbine than the turbine housing can physically flow. Even with the wastegate fully closed, the system has reached the airflow limit of the turbocharger.
Since back pressure increases with rpm, and this built engine is revving to 7800 rpm, significant pressure builds up in the exhaust housing at high engine speeds. This creates what are known as pumping losses, meaning the pistons must push exhaust gases out against extremely high pressure. That resistance costs horsepower and explains why gains begin to taper as the turbo approaches its efficiency limits.
Why Excessive Back Pressure Can Be Risky
High exhaust back pressure can introduce a couple of potential issues.
First, during valve overlap, excessive exhaust pressure can push exhaust gases back into the cylinder. This reduces oxygen content in the incoming air charge, raises exhaust gas temperatures, and ultimately increases the engine’s tendency to knock.
Second, the turbocharger itself is operating at the very edge of its mechanical limits. The turbine shaft is experiencing extremely high temperatures, loads, and shaft speeds. Over time, this can lead to premature bearing wear or failure. In worst-case scenarios, it can cause catastrophic turbocharger failure that could potentially damage the engine.
Now that you understand EBP and the risks associated with it, you can probably imagine why running the car “on kill” all the time is not advisable.
Yes, the turbo is technically capable of producing this level of power.
Yes, the engine and fuel system in this car are built to handle it.
But the turbocharger itself will not be particularly happy operating with an EBP ratio that high for extended periods. This limitation is largely a consequence of designing the turbo as a true drop-in, stock-location unit.
For that reason, we also wanted to identify what we feel is a safe upper limit for the turbo.
Establishing a Safe Upper Limit
In this next run, the horsepower and torque curves begin to taper slightly around 6500 rpm, producing 414 wheel horsepower at 6500 rpm and 378 wheel torque at 5100 rpm.
This was achieved by opening the wastegate slightly at 6500 rpm, allowing some exhaust gas to bypass the turbine and taper the boost pressure.
On a car without a built motor, where the fuel cut typically occurs around 7000 rpm, the losses from this slight boost taper are even less significant.
Looking at the log, peak pressures in this configuration were:
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58.2 psi exhaust manifold pressure
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35.2 psi intake manifold boost
This results in an EBP ratio of 1.65.
From there, boost tapered slightly to around 33 psi, maintaining a similar pressure ratio while reducing the overall load on the turbo.
Reducing the EBP ratio by 0.2 may not sound like much, but it is a meaningful improvement in terms of turbocharger longevity and engine safety, especially considering the tradeoff is only about 5 horsepower.
Closing Thoughts
At the end of the day, this is why we test. Every pull, every data log, and every adjustment give us a clearer picture of how our products perform and where we can confidently recommend their operating limits. This testing gave us valuable insight into the true capabilities of our drop-in turbocharger, both at its absolute limit and within a range we feel offers an excellent balance of power, efficiency, and long-term reliability.
We'll continue putting our products through this level of testing because that's how better parts are built. Stay tuned as we share more data, more development, and more behind-the-scenes looks at the engineering that drives us.
