This is from Fastman's site:
STANDARD MODIFIED HEMI TB FEATURES:
-Front Side Taper Boring: The opening of the TB is Radiused and Taper Bored to almost 85mm leading back to the stock bore.
-Half Shaft: Modified Factory Shaft. The top of the shaft is removed promoting even more airflow at wide open throttle.
-(Optional) Backside Bore: I can also offer an optional Backside Taper Bore. The back side is taper bored to match the manifold opening very closely.
The issue I have with these throttle bodies is that they still maintain the OEM sized butterfly valve, and the only part of this thing that is almost 85mm
is the opening neck, then it's tapered back to the stock 80mm
opening. Even using the venturi effect you increase the velocity of the air flow, and at least he cuts off one side of the axial shaft, but you MAY move more air through this throttlebody, but it will be at a lower velocity compared to the OEM as there's now a longer run length and a faster closure radius. The most important thing to look at is VELOCITY of your flow in a naturally aspirated set up as you need to be able to flow the air into the combustion chamber in the very brief period of the intake valve being open, as well as clear the chamber while both intake and exhaust valves overlap.
If you want some true performance, you want to get yourself a true 85mm throttlebody, either cast or modified to an 85mm butterfly valve. These ported ones are a money grab for people who think that bigger is better, the most part they come with negligable gains.
If you are set on the fastman then pick up his 84mm, which at least has a 84mm bore, not the stock 80mm. The price is comparable to the 85mm BBK; again, more reason to go to the BBK since it's a larger bore for a very similar price.