4.2 vs 4.6 as a DD?

[Cheromaniac]

and why should the exhaust not be enlarged?

You can enlarge the exhaust ports a bit but not so much to match the gasket bore. The reason is to prevent exhaust gas reversion from the header back into the combustion chambers. Otherwise the fresh intake charge would be diluted with spent exhaust gases and the result would be a power loss.

[sly-jeeper]

yep just like a two stroke!