It all depends on about a jillion different variables. Motor, rear gear, car weight, purpose of the car etc. I just put a 13" from Dynamic Converter in my car. 400ci with a little more than a Ram Air III cam, Turbo 400 and 3.55 gears. It makes plenty of power down low so I don't need a high stall speed converter. I also don't drag race. The best thing you can do is call a major mfg and tell them what you have and what you'd like to do with it. Thats what they are there for. Coan, Continental, Dynamic, B&M, TCI all have web sites with general guidelines and there is a ton of info in the archives here and all over the web.
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frittering and wasting the hours in an off hand way....
1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core.
