The combination of massive tb and massive intake runners in the FAST intake cut down on air intake velocity. Slow moving air will produce less power.

And in all honesty it's much more complicated than that and I truly don't understand/know the theory.

 

if that's what you believe then so be it, I know of a fast 102/102 combo on a 53 with a mild cam and it doesn't seem to have any lack of

 

ive got the fast lsxrt 102mm and a ported and polished stock tb and it makes more power than stock intake and tb did on my 2012 5.3. maybe you need a good tuner. i had area47 do mine.

 

IMO. Its a tuning issue.

 

On my 404 it made almost 70rwhp over stock 78mm setup with a LSXrt and FAST 92mm.

Bust i lost low end TQ and the HP gain was from 5200 - 6400rpm...

 

I recently swapped a ported LSXRT and 92TB from the factory Gen 3 truck intake/TB on my 383. I don't have any dyno numbers, however its a massive difference above 4500 rpms. I don't know if I gained as much as gnd111 ^ , but it feels like a completely different engine.

I didn't loose anything down low either. The motor has a smallish cam (224/228) and small runner heads (210), so this might be the reason why I didn't loose any low end.