I never checked my mileage when I had the 1 bbl and stock intake/exhaust. But if I had to guess, I'd say it's a little worse with 4 bbl. The performance improvement however, is well worth it. I hated that craptacular Rochester 1 bbl.
-David

Tradin' up dimes for nickels...
'`'`'`'`'`'`
My \'65 Chevy C-20