| With your GT28R setup, the biggest limitation with the turbo is the small turbine, which generates the most amount of backpressure in the exhaust tract (vs. the manifold itself). Since you are only looking at making ~450RWHP on pump fuel, the exhaust manifolds aren't your biggest bottleneck. The best thing you could do that would give the best performance increase would be to go with some higher lift cams (with similar to stock duration - slightly more duration/overlap) and make the best of your bottom end and midrange. To add to this, your compressor is limited to 23.4psi as indicated by the compressor flow map. We have confirmed on Tyler's TT that you can't produce any more than this pressure (which confirms the accuracy of the map), so your setup isn't consistent with a "high horsepower" configuration, but rather, a great bottom end and midrange TORQUE producer. (Dont take this as bad, torque is WAY funner than HP). You want to optimize what you can get out of the 23psi that your turbos can produce, and to do this, camshafts would be YOUR best choice, performance-wise. Now for other vehicles that use larger, free-er flowing turbine sections, this may not be the case. As an example, if you had the GT28RS, I would acutally suggest upgrading the manifolds first simply because the turbine section of those turbochargers is very free-flowing. Combining the free-flowing characteristics of that turbo with a free-flowing exhaust, on top of the fact that these turbos dont get to their peak until higher RPM, a manifold upgrade would prove more beneficial over a camshaft upgrade.
Hope this helps...
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