Is there such thing as a healthy 460? IMHO the 460 was Ford's worst engine they produced... You'll have to do a lot of work to any 460 I've seen to make it a good runner, while getting ANY kind of mileage. All of the 460s I've worked with were 8.5:1 compression, cammed down, and would get out-run by decent built 302's... I know the 460 has potential since it has a decent chamber and a lot of cubes, but it'll take a lot of work.
There is such a thing. I know a lot of people don't like the 460, (but plenty do) and they do suck gas, but I've seen many go the distance in truck use. On the other hand, I've heard of others needing a rebuild in 40,000 miles. I don't know how they were maintained.
What they do have is torque. The '51 Mercury is a heavy car. Plus, I have the engine on hand and I know it is sound. If it doesn't work out, I'll do something different. My preference will be to keep the flathead. However it goes, there will be major bucks and time in this car before I'm done, and even then the average 4-door sedan from Japan, Inc will be able to outrun it.
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If you look around, it seems like most of these cars are chopped with Corvette engines in them. Seems like a travesty to me.