X2 on the 4 barrel. a lot of it depends on the driving style, and a solid fuel carburetor (for me) has worked more efficient than an air bleed caruretor. most thermoquads, afb's, avs', quadrajunks, are solid fuel metering systems. holleys are air bleed setups. kind of hyppocritical since i run holleys....but i loved my TQ, it actually ran harder than my 4175 spreadbore holley, but i couldnt seem to get the damn crossovers that are glued to the bottom of the main body to seal up so it would always bypass fuel. It definately put my BBD's and holley 2210's to shame, for being user friendly, driveability, and mileage (i gained 3 mpg) i never looked back