The E36 Throttle body is a modest 63.83mm (internal) bore from the factory, while the intake manifold has a 70mm opening that it mates up to. This means that the TB is not set up to flow as much air as it could...which inspired companies like Dinan, Riot Racing, and VAC Motorsports to develop a bored out throttle body which would allow more air to pass through the throttle body at WOT, thus reducing the resistance and increasing throttle response and peak horsepower.
Since the engine is essentially a big air pump, as we know, increasing the flow of air and reducing resistance in the intake tract will usually increase the overall horsepower. This is why mods like the M50 manifold, Samco intake boot, ASC Throttle body delete, 3.5" HFM, and Cold Air Intakes are commonly seen on e36's in an effort to squeeze out every last bit of power from our S52's. It can be broken down simply... Larger tubing, larger intake runners, smoother curves = less resistance, more power.
It just makes sense to me that there would be an increase in power from adding a BBTB, since the whole idea of most of the NA Mods is to lower the restriction of the 3" intake system. I've heard many people argue that the 3.5" intake setup is totally unnecessary for an NA M3. I've also heard that the TB is "not the bottleneck". However, If you believe that the 3.5" HFM, 3.5" CAI, or samco boot does anything, then you have to believe that this will too because it operates off of the exact same principle. The 3.5" Intake is 88.9mm inner diameter (3.5"), which smoothly shrinks down to 76.2mm (3") in the Bimmerworld boot, then goes through a throttle body which has a 63.83mm (2.51") inner diameter? Sounds like a bottleneck to me... Even at 68mm, the TB is still the smallest inner diameter in the intake tract!
Note: Dinan, Jim Conforti, and Active Autowerke all must believe that the 3.5" HFM makes a difference... since they all make a tune specifically for it AND recognize the power gains it yields.
The last thing to consider is throttle response. The idea here is that being a larger bore, at the same throttle position the BBTB will pass more air. This means that when you're at 25% throttle, the different throttle bodies will each pass different amounts of air and cause the engine to accelerate more or less. This GREATLY affects the way the throttle feels in the e36, there is no 2 ways about it. The BBTB pulls your neck harder when you hit the gas and makes the car seem to accelerate much more effortlessly at low-mid throttle. Throttle response is mostly based on feeling, though, so this cannot really be described. Drive an e36 with a BBTB and you will understand.
The point of this mod is to increase the airflow into the engine as efficiently as possible. West coast riots has an amazing product, and is worth every cent in my opinion. They also include a free installation guide that I have based mine off of.
1. Flat head screwdriver.
2. 10mm Socket
3. Socket Wrench
4. Socket Wrench Extension
Remove Air intake boot
Remove Throttle Body Plate
Remove all of the hoses connected to the throttle body
Put the new throttle body on