Actually there is a trick to install big TB's on stock ITR manifolds.
Sepang drag racers have been using it for quite a while already, but I guess no one noticed.
The TB plate/plenum extender/other names I don't remember ATM. One metal billet piece, bored to the wanted TB bore, with a relocated IACV port (or ports, if the plenum has more than one).
That's where the leak is coming from, IMO.
Based from my experience tuning 2 s2 manifolds, the flow is much better. If you compare fuelling when idling with the stock TB, you will notice that to get a stoich idle, it needs more fuel, so it's probably due to the fact that the plenum is bigger on the inside. The runners are slightly longer and bigger too, giving a bit of top end power, a B20b previously using ITR intake swapping to S2 got 10hp (tuned a bit after swapping, like I said in some other posts, this kinds of mod usually need tuning to get the power gain, if you plonk in without tuning.. I don't think you'll see much difference)
For me, in both cases, cars ran a little leaner after swapping intake, and needed a little bump in fuelling to compensate.
I don't think there's any disadvantages to using bigger TB's, because I believe that the runner size and length ultimately determines how much air gets into the combustion chamber, and it doesn't matter so much because it's not a venturi (with 70mm, I doubt anyone will see a venturi effect), unlike the runners.. It's like having cars travelling on a three lane road at karak, but due to the construction, only one lane is open. Cars still move, but the volume that gets past the same as a normal one lane road.
Practical wise, I don't like too big TB's because they're so hard to drive, the power delivery isn't linear at all, which makes it hard to drive in the rain, or for controlling traction when cornering.
Just like the Quad Setup a while back, the car 'jumps' with the slightest throttle.. how to drive in traffic? (O_o)
