Previously we released a write up comparing the STX67 to the stock FA20 Turbo. We now have been able to fully push the FA20 with an STX71. Throwing caution to the wind, we pushed the motor well into un-happy territory. We found some very interesting results. We then backed the car off to a safe level and let it go. Lets go over what modifications the car had done to it:
TGV and EGR deletes
COBB Big SF intake
COBB Flex-Fuel tuned
Lets pause for a minute and talk output and numbers on our machine. We keep weather and compensations at their factory settings. We run a Mustang AWD500 dynamometer. Our facility is located approximately 4670 feet above MSL, in Woods Cross UT. Generally we see cars make around 15-16% lower output than a dynojet. In stock form the FA20 WRX generates the following on our dyno:
203 awhp 207 awtq
This average was the result of 3 stock cars, 2 manual and one CVT. We averaged their 9 pulls. It is common to see the COBB OTS cars generate around 230awhp, and 245awtq. This was the exact case with the stock turbo FA20 WRX. That was until we set into tuning. After full calibrations on both 91 octane and E85 our results for the stock turbo were as follows:
91 : 298 AWHP
E65 : 353 AWHP
Below 3400RPM Stock FA Turbo Shines
Above 3400RPM to 5400 STX67
3900RPM and above STX71
This is great as it shows exactly how well the STX67 performs. You can see the 71 does take a bit longer to spool, but then it’s performance up top is spectacular. Both of their spools are closer to the stock unit, yet they both generates a lot more horsepower. It also helps get a a good idea of how well these upgraded turbos will drive.
Finally onto the numbers. How much more horsepower did the STX 67 generate over the stock unit, and how much above that did the 71 do? Well on E65 here was our output on each:
Stock: 353 AWHP
STX67: 442 AWHP
STX71: 501 AWHP
What was the power restriction? Well, it wasn’t the fuel system!
In our data gathering, we made sure to note the fuel system head room remaining. With a DI system, you only have 1/4 of the time to inject fuel. This is a lot less than the 100% of the time a port injection system can inject. With that in mind, we knew the fuel system would limit us in output. We are happy to report that even in the mid range (peak torque) we still had almost a MS of fuel remaining! We had even more fuel remaining up top. There are several things that limit horsepower on this platform; the fuel system itself doesn’t appear to be one. Our biggest fear was that the fuel system would be closer in size to the Mazda Speed line. Which, would have been very limiting.
There is a lot more we learned from this build. With limitations being somewhat where we expected them to be, and others showing their faces. We can confidently say the fuel system is not a limiting factor on this platform. COBB’s flex-fuel is solid on the FA20. If you were on the fence on running it, don’t be. The R/D that goes into the COBB line is great. Hopefully we can get one soon with the next big restriction removed. We won’t share what exactly that was here, but know it’s something that you can already buy to alleviate the issue. IAG’s motor did very well. It is a bit louder and had more “Slap” than their EJ25 motors, but that did not affect performance in the least. This was a very well done build. Hopefully this helps answer some of the questions people had regarding the STX turbo line.