Cylinder head

The stock cylinder head is cast iron and weighs 60lb. The valve head diameter is 1.91" intake/1.50"exhaust.

Year............Casting No

1987-90.........2686

1991-95.........7120

1996-99.........0630

2000-01.........0331

Cylinder head flow figures (cfm) at 28inH2O pressure drop are:

Non-HO head #2686

Valve lift (in)..... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6

Intake flow.... .....122.0 ..168.0 ..186.0 ..189.0 ..192.0

Exhaust flow.........88.0 ...114.0 ..130.0 ..134.0 ..138.0

 

Ported non-HO head #2686

Valve lift (in)..... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6

Intake flow..........124.0 ..183.0 ..197.0 ..207.0 ..216.0

Exhaust flow.........87.0 ...113.0 ..138.0 ..153.0 ..159.0

HO head #7120 & #0630

Valve lift (in)... 0.1 ... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6

Intake flow.... ...66.0 ...128.0 ..179.0 ..206.0 ..209.0 ..209.0

Exhaust flow.......55.0 ...100.0 ..120.0 ..136.0.. 141.0 ..141.2

HO head #0331

Valve lift (in)..... 0.1 ... 0.2 ... 0.3 ... 0.4 ... 0.5 ... 0.6

Intake flow..........59.4....122.6...171.1...201.1...214.3...218.4

Exhaust flow.........47.1....93.4....123.3...140.9...147.0...149.7

Ported big valve 2.02/1.60 HO head

Valve lift (in)... 0.1 ... 0.2 ... 0.3 ... 0.4 ... 0.5

Intake flow........73.9....142.4...197.8...229.8...247.0

Exhaust flow.......65.3....114.0...135.9...146.3...157.1

 

The early '87-'90 non-HO heads have low intake ports that flow rather poorly.
The later HO heads havehigher intake ports that flow more air by allowing a straighter shot into the cylinders.  The '91-'95 HOheads with casting no.7120 have the highest intake and exhaust port airflows, especially at lower valve lifts where it is most important, and are the best for performance. The '96-'99 0630 heads are almost identical except that they don't have a port for the coolant temp. gauge sending unit.  The '00 and laterHO heads with casting no.0331 have smaller exhaust ports to produce a faster warm-up of the catalytic converter and improve emissions, but performance also suffers because the ports don't flow as well as those of the 7120 and 0630 castings.The exhaust ports flow relatively poorly compared to the intake ports on all the 4.0 heads so this engine would perform better with a dual pattern cam that has more exhaust valve opening duration and lift than the intake. The intake ports of the HO head are rectangular and measure 1.625" x 1.375", while the exhaust ports are elliptical and measure only 1.375" x 1.250". The port cross-sectional areas are 2.234 and 1.350 sq.inches respectively.  For good street performance, it is recommended that the port c/s area should beabout 85% of the valve head area.  Therefore it should be 2.435 sq. in. for the intake ports and 1.502 sq.in. for the exhaust ports.  The stock port c/s areas therefore need to be enlarged by 9.0% for the intakeand 11.3% for the exhaust.  The ports should be matched to the intake and exhaust manifold runners tosmoothen airflow and prevent power-robbing turbulence.