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Just curious, I know everyone says that the pre-96 NPI heads are junk and not worth messing with, but can anyone tell me exactly what is different about them versus the 96+ heads? Thanks.

Mike
 

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94/5 versus later NPI head differences

The early NPI heads were referred to as "swirl port" heads which means there was a protrusion (about the size of your thumb) in the intake port in order to promote air swirl into the chamber... This was good for air fuel mixing but terrible for port flow... If you look down the port the protrusion blocks off about 40% of the intake valve face.....
 

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the 96-98 heads do still have the swirl dam.... there are more prominent reasons.. i don't remember.. but it is on the forum somewhere
 

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jamesD said:
the 96-98 heads do still have the swirl dam.... there are more prominent reasons.. i don't remember.. but it is on the forum somewhere

The swirl dam in the combustion chamber is not what I was referring to... The major problem with the pre-96 heads is the protrusion in the port (upstream of the valve) that blocks off a major portion of the valve area and chokes the port.
 

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High-Mileage 4.6L Thrasher
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Here's what fastford351 is talking about.
Picture's worth how many words?
'94 on the left,'97 on the right.

JL
 
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