Another factor when looking at the stock engine and it's VE when using turbo calc or any other method is to make sure you have the correct boost and temperature. The stock engine with the #6 hot side probably doesn't make 11psi at 5800rpm. It most likely is down to 9 to 10 psi and then at 6400 rpm redline it is down to 8psi on the average car.

Intake temperatures and thus manifold temperatures make a large difference also and I am not sure whether Turbo Calc takes those relationships into effect or not.

 

Yet another factor when running turbo calc to try to simulate a stock 951 would be making sure you have the correct compressor and turbine maps for the K26/6 or K26/8 turbos. Unfortunately, I don't think those maps are available. So, I am not really sure how a stock 951 could be modeled.

Jeff