[blykins]

Tied to FIA rules (.060” max overbore, factory heads, flat tappet cam, factory rockers, etc.), I can make 450 hp at 7000 and 390 lbft of torque. That is not a “max effort” build by any means. 11:1 compression, non aggressive cam lobes, easy stuff.

Given modern heads, roller cam, etc., 500 hp would be easy with a 289.

RPM is a friend for small displacement engines.

If you’re focused on streetable pieces, a 289 with aftermarket heads, dual plane intake, 9-9.5:1, small hydraulic roller, will make about 310-320 hp on the dyno here.