injector size

[lnirenberg]

I have a Mass-flo efi on my 61 bel air 409 (overbored to 421) with about 510 hp at the crank using 42# injectors. The motor has the large port iron heads with hardened valve seats with about 10-10.5:1 compression that I run on 93 pump gas. The ecu is a fox mustang unit that is sold as "self learning". I was having trouble getting the right amount of fuel across the rev range--enough fuel at WOT the car runs rich at idle turn down the pressure to get the right af ratio at idle and it runs lean at WOT and knocks. To try to fix this I had a tuner install a Moates Qthorse so he could fine tune the af but its still running lean at WOT. I ran the injector size through an on-line calculator and I should be fine but the tuner is recommending larger injectors in the 55# range. Any help would be appreciated.