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Essentially the 1j head flows a bit better, the 2j head for example has a bit more restriction around 5th and 6th cylinder due to casting shapes and the 1j also has slightly bigger exhaust valves (yet 2j's intake valves are slightly bigger) but you also gain slightly higher compression and 500cc more displacement using the 1j head on the 2j block apparently.
That info is bassed on stock heads though, which realistically are still good for large power.
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