Finally got a set of plastic stacks bored, tapered, and bell-mouthed. I severely underestimated the amount of effort this was going to take but I am ecstatic to have it done!
There is a very dramatic difference between the "T" spec stacks and what I have now. I step bored them in 1mm steps from 39 to 44 over the length of the funnels, then reamed with a monster taper ream and finally machined a radius on the top.
Before and after of the top of the trumpet:
The bottom of the stacks where they meet the throttle bodies had quite a bit of core shift that would be inconsequential with the 911T setup but had to be corrected. I centered the mill on the throttle bore location to remedy this.
Side by side before/after:
The 39mm bottom bore has about 60% more flow capacity than the stock T stack. More important to my setup, the cross sectional area is 17.5% greater than the S/RS setup - which should support my horsepower goals and provide sufficient flow for the 3.2 Motor with 39mm intake ports.
