To estimate the spool-up point, 0% wastegate flow is assumed. Notice that the engine dP is positive at this operating point. It’s typical to have positive engine dP at low and mid-engine speeds. At high engine speeds and peak power, it’s typical for engine dP to go negative. A good target is to size a turbo for -100kPa at peak power for a setup looking for maximum drivability, such as a car used on road courses. If going for a drag setup where absolute maximum power is the goal, you want as much positive engine dP as possible which improves BSFC.
Here are the turbine map operating points for the 0.83 and 1.01 A/R turbine housings for the spool-up points.
Alright, so we have shown the improvement in performance between the Gen1 and Gen2 GTX3071R on our hypothetically turbocharged S2000. But, that’s assuming the same peak operating point which falls on the Gen1 map. Of course, with the updated compressor aero of Gen2, the new GTX3071R can flow more and therefore make more power. How much more power?
You’ll notice that the Gen1 map had a top speed line of 150k rpm. We don’t know why the Gen2 map was only plotted to 140k, but we drew in an estimated 150k speed line. With the estimated line drawn on the map and all our assumed values, we estimate the Gen2 can do about 600hp, whereas the Gen1 would top out around 550hp.
Aside from the updated compressor wheel aero, the Gen 2 GTX turbos all get new compressor housing castings. The updated castings feature fully machined compressor inlet and out geometries. The fully machined inlets without a doubt helped performance by reducing the flow losses into the compressor wheel. The fully machined compressor outlet should reduce boost leaks by providing a better surface for the hose coupling to seal against.
Another key feature of the GTX Gen2 turbos is the machined speed sensor port. So now it’s easy to add a turbo speed sensor, so that you can see exactly where you’re operating on the compressor map. That’s good information to make sure you don’t overspeed the turbo too. The other really big deal with the Gen2 turbos is the ability to get the GTX30-35 size turbos in reverse rotation which opens up a lot more packaging options. For example, a reverse rotation turbo is better suited to the S2000 in a bottom-mount location because it sits on the passenger/right side of the car. Guys putting turbos on V engines will definitely appreciate the new reverse rotation options.
So there you have it, all the tools to do some turbocharger matching. If you play around with values like turbine inlet temperature, air filter pressure drop, exhaust system pressure drop, and turbine efficiency, you can see how all these variables affect performance. For example, if you don’t have enough exhaust energy to hit your low-end performance level, one thing you can do is increase the turbine inlet temperature to get more energy into the turbine wheel and get it to spool up. How do you increase the turbine inlet temperature? You can play with ignition timing or just go with anti-lag. Have fun matching!