I believe any neckdown aft of the collector flange hurts performance/mileage/efficiency and that correct dia/length for pressure wave tuning etc is a function of the pri tubes. I would rather have (4) 1&5/8 pri tubes collected down into a 2&1/2" pipe rather than into a 2&1/4" one

live every 24 hour block of time like it's your last day on earth