As Conrad says, LOW compression, at least for a "performance" application. They'd be good on a "milieage" engine or a "torque" engine not intended for revs and where 87 octane is desired (truck). -8s are "ideal" for a 461 (400 with a 4.25" stroke..). -4s with a bunch milled off, are better for the 400. It has recently been "reported" of chronic head gasket issues with a pair of -8s milled "too far" (.075", and we stay with .060" MAX), on a 400.
Good buy!
Jim
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