SR20 Big Turbo – Which Size is Best?
Compressed by jpeg-recompress
In this installment of the turbo selection series we look at SR20 big turbo to see which is the optimum Garrett for 30 PSI of boost
SR20 Big Turbo – Step 1, Volumetric Efficiency
As always, we need to know the volumetric efficiency of the SR20. I’m going to use dyno numbers for an NA SR20 because with our big turbo upgrades we will be getting close to NA VE numbers.
Using the dynopgraph from here we get these numbers. Plugging RPM and power into the VE calculator we get the VE numbers.
| RPM | Power | VE |
| 3000 | 68 | 0.78 |
| 5000 | 122 | 0.84 |
| 7500 | 110 | 0.5 |
Step 2 – Turbo Calculator
We now have the numbers we need to use the turbo calculator. We’ll target 7PSI at 3000RPM and 30 PSI from 5000 until redline.
| RPM | Boost Target (PSI) | VE | lbs/min | Pressure Ratio | Power (Wheel) |
| 3000 | 7 | 0.78 | 9.4 | 1.47 | 98 |
| 5000 | 30 | 0.84 | 35 | 3 | 362 |
| 7500 | 30 | 0.75 | 46.4 | 3 | 485 |
| 7500 | 30 | 0.82 (Alternative number) | 50.7 | 3 | 530 |
The table above has two VE numbers for 7500RPM to give us an idea of where we would be if we can make the engine breather better.
SR20 Big Turbo – Garrett
Looking at the G-Series Gen 1 turbos (Gen 2 don’t look like they suit our needs)
G25-660
The G25-660 looks a very good fit but we have to make some sacrifices low down, it might be hard to get 7PSI of boost at 3000RPM without getting surge. But apart from that it puts us just where we want to be. And if we make VE improvements in the future, there is headroom to make the most of the changes.
GTX3076R Gen 2
The GTX looks good also but being an older gen turbo the efficiency is generally lower versus the G25. Again we might have to make sacrifices down low with our boost but apart from that it looks good. It also has a bit of headroom for VE improvements, slightly more than the G25.
SR20 Big Turbo Choice – Conclusion
All in all the newer tech of the G25-660 Gen 1 look the choice.
But if our budget doesn’t stretch there, the GTX3076R Gen 2 should also do the job.
The older gen turbos (GT3076 for example) don’t work over such a big range as these newer turbo so not really an option.
