[FunnyGopher]

Larger throttle bodies (TBs) create the potential to deliver more air to the motor. They themselves generate no power. They are a support modification. The motor is requesting air, the TB helps deliver that air. The motor will not ask for more air just because a larger TB is installed.

On this car when NA, previous research has shown that the OEM TB at WOT is not a restriction on this engine, even at the top of the RPM range. The OEM TB is completely sufficient in delivering all of the air the motor is asking for. Because of this, a larger TB on this motor in an NA configuration is mostly going to contribute to throttle response. You're able to deliver more air at partial throttle when the butterfly valve in the TB is partially open. More air at partial throttle is allowing the motor to make more torque. So with less throttle pedal the car will make more torque because more air is able to be delivered. Up top in the RPM range when in WOT, there's not a single difference, because this motor doesn't ask for more air than the OEM TB can supply at WOT, so the ability to deliver more air with a bigger TB is wasted.

On a pull through boosted application like the Harrop kit, because the compression of the air is happening after the TB, getting as much air through the TB is important for allowing the charger to create higher boost pressures. You can think of pull through boosted applications like a much bigger motor asking for air. The Eaton unit that the Harrop kit uses asks for 1.32L of air for every rotation of the rotors. From my rough measurements, when using the 90mm pulley, for every crank rotation, the rotors will have rotated ~1.5 times. That's ~4L, or ~2x more air volume than the displacement of the motor for every full engine cycle. The OEM TB is sufficient at delivering the volume of air needed for a 2L motor, but a 4L motor? The TB could probably use higher flow rate for that.

With even smaller pulleys, the rotors will rotate even more times per crank rotation, in which case the OEM TB will definitely become an air restriction when those rotors are asking for multiples of air volume more than the OEM TB ever had to deliver. With the 65mm pulley, it's essentially asking for ~5.5L of air. Installing a TB that at least matches the machined throttle port on the charger will help deliver the amount of air the charger is asking for, allowing it to more easily make higher boost pressures.

TB size is not as important on a push through application like a turbo or centrifugal SC because the compression is happening before the TB, pressuring the front side of the TB and increasing the effective flow rate of the TB. The TB will open and the compressed air will just fill the intake plenum with as much air as it can very quickly. A larger TB won't hurt, but it's effects are not going to be as dramatic because the pressure has already been made.

So does all this make a huge difference in my case? Probably not. The car still hauls ass regardless. This will make a larger difference to those who opt for higher boost pressures, displacements, or RPM limits. Harrop machined their own 75mm TB for their track car, but removed the AC. The Blox 70mm TB allows you to keep the AC and still run a significantly larger TB that you can buy off the shelf. I probably can't use all the benefit right now, but that doesn't mean others can't. I'm just glad I found a solution and now maybe others can benefit from it.