What you have to realise is that because EngMod is a 1D simulation it cannot predict changes is flow regimes due to duct geometry.
Making the entry area 1.2 instead of 1.5 will change the volume, and this is what the sim "sees" but this change will dramatically affect the flow thru the duct into the port.
Its the same with say port stagger,this dramatically affects the scavenging regime, all the sim sees is the high ports affect on blowdown and calculates this change.
What you have to do is look at what is being done in the fastest engines on the planet and translate that experience to the project at hand.
This is why the sim is overpredicting the power of the GP125, it knows nothing about the duct shortcomings, even though the numbers may be similar to a RSA.
Neels hates fudge factors, but what we need to pull the power down,is a factor that represents a " % of shit design " that we cant modify out of our projects.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
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