So with the prices of 20B LIM/UIM w/ TB being super high now and while the LIM is not a plug and play option for an FD3S I've been looking for other options. Projay or custom made seems to be the only options for 3/4 rotors. Projay has 3 intake manifolds now. The new one Sniper, cast, and the billet. I'm not building a drag race car more of a time attack, What is the optimal TB size, What is too big? I know there is people making over 1000HP on a stock TB but for the price its almost better to buy a Projay. WIll 100mm TB cause idle and or low RPM driving issues?

 

My 20b did not come with LIM and UIM, so I arrived at the same conclusion as you. I have the ProJay billet. 105mm TB. I don’t personally have it running yet, but know of 2 others that have it running fine. I’d recommend going with a drive by wire TB instead of the one it comes with. Adapters are available from a few places. They aren’t cheap though.

And if for some reason you have an issue, you can always make an adapter to reduce it.

 

I'm going to run a GM 90mm DBW while it's NA, and probably the 102 once I go FI.

If you have the 20b lower and plenum, Full Function Engineering has adapters for the GM throttle bodies now.

 

jump to 5:05 in this vid; in summary, he switched to the smaller Hellcat DBW TB (90 - 92mm?) and was much happier with it:

 

Yup. DBW is great. Here is an adapter for the ProJay intake. https://www.raceonly.com.au/product/...m-dbw-adaptor/