1987-2006 Jeep 4.0L Inline Six - Inside The Inline
12. You are best off with the '91-and-later high-output cylinder head. It flows better than the '87-'90 "low-port" head, though not by a massive amount. We used a '00 H.O. cylinder head (casting No. 0331). These aren't the best H.O. heads from a performance standpoint, because they have smaller exhaust ports. (Reportedly, the best H.O. heads come from '91 to '98, and have the casting numbers of 7120 and 0630.) From the flow bench numbers we've seen, the differences aren't huge. Overall, if budget is a concern, all the heads can be used effectively. A dual-pattern cam with longer exhaust duration really helps the 4.0L exhale. View Related Article
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