Quote:
Originally Posted by Dougal
It's due to the different exhaust wheel size.
The exhaust scroll creates a vortex, the exhaust wheel interrupts that vortex to create torque on the shaft.
In a free vortex the further you go in, the higher the gas velocity. So bigger wheels interrupt the vortex further out where the gas velocity is lower, that's why you need a smaller A/R ratio with a bigger wheel.
My engine originally came with an IHI turbo which is equivalent to a T3 with a 0.42 A/R exhaust housing.
I'm now using a T25 with a 0.49 A/R housing and they spool up at almost exactly the same point.
Multiply the A/R by the wheel radius and you get what I call the effective flow area. It's a handy number for comparing the spool up point of different turbos.
Remember A/R is usually in inches, so you need to know the wheel radius in inches. A T3 usually has a 60mm (2.36") exhaust wheel.
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Thanks Dougal .. I didn't pay much atention to trim numbers in the wheels that I selected .. and actually should be interesting see those numbers for my KKK that it's a .38 AR in the turbine side ..