"The stock 4.0 HO throttle body has a bore of 60mm that tapers down to 56mm below the throttle butterfly. Removing this taper to make the whole TB 60mm increases flow to ~428cfm @ 1.5inHg. A TB that's bored out to 62mm flows ~461cfm @ 1.5inHg for a 3hp increase on a stock 4.0 engine.
'87-'90 Renix 4.0 owners need not feel left out. The stock Renix throttle body has a bore of 51mm (2.0") and flows a pitiful ~295cfm @ 1.5inHg. When bored out to 58mm, flow increases by 34% to ~396cfm @ 1.5inHg resulting in an 8hp gain. Alternatively, you can remove the Renix TB and replace it with a TB (with taper removed) from the HO engine. Joe Attardo's HO TB to Renix intake manifold conversion article explains how it can be done."
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