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Reference Manual


Chapter 1: Engines: Oldsmobile V8s: Differences between 5A and 6A/7A heads

 

Three types of cylinder heads were installed from the factory on the 307s with codes of 5A, 6A, and 7A. The 6A heads are fairly rare, but they are out there on some mid-'80s 307s (all with roller cams). Because they are so hard to find and rarely used, they will not be covered in this manual.

Rumor has it the 6A heads were used on Canadian 307's and did not have the AIR bosses drilled.

The 5A head is of a conventional Oldsmobile design, with large rectangular ports (about 1.3" wide by 2.0" tall). The combustion chambers are 67 cc and valve sizes are 1.75" intake and 1.50" exhaust. By comparison, the 7A heads have square intake ports (about 1.3" square) with 64 cc combustion chambers. Valve sizes are the same as the 5A heads'. The chart below outlines the differences between the two heads:

  Head Code: 5A Head Code: 7A
Combustion Chamber Size 67 cubic centimeters 64 cubic centimeters
Port Size 1.30" x 2.03" 1.30" x 1.30"
Valve Size, Intake 1.75" diameter 1.75" diameter
Valve Size, Exhaust 1.50" diameter 1.50" diameter

On paper, the 7A heads may seem like junk, good only as boat anchors. And from a racing standpoint, that is largely true. However, they do offer characteristics that the 5A heads just cannot give.

The intake ports on the 7A heads are "swirled", and smaller than conventional ports, which equates to lower overall flow, but higher velocities than normal ports offer. Higher velocities results in higher torque output and increased atomization of fuel, especially when cold. In fact, the 7A 307 engines are rated at 10 more lb*ft than the 5A engines are. Driveability is markedly better with 7A heads than 5A heads -- it almost feels as smooth as fuel-injection!

Below is a table with pictures that shows the differences between the 5A and 7A heads. Click on a picture for a larger version!

Head Code: 5A   Head Code: 7A
Intake Ports
Exhaust Ports
Combustion Chambers
Pictures provided by Jason Adcock   Pictures provided by Todd Daenzer

 

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