I've always heard that an automatic has ~20% drivetrain loss. I assume that is through the TQ, tranny and rear end. That's 80 HP on a 400 horse engine and seems high. I also question if it is always 20%, no matter the HP of the engine. A 700HP engine would be losing 140 HP in the same theoretical driveline, which seems insanely high.
Whatever the driveline losses are, I would expect the majority of that to be in the torque converter and then the rear end (once TH350 is in direct drive). So if you have a very loose TC and a Ford 9" rear end, you're going to be wasting more power than a lockup converter and a lightweight rear end.
BTW, what is the idle vacuum? My 400 makes 8-10" at 750 RPM with XE274 cam. Lumpy idle.
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I could explain all this to the girl at the parts store, but she'd probably call the asylum.
White '67 LeMans 407/TH350/Ford 3.89... RIP
Red '67 LeMans. 407/TH400/Ford 3.25
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