Chevy/GM Cylinder Heads - Breathing On A Budget

Making Gm's Vortec Heads Work Better For Less

Mike Petralia Sep 1, 2002 0 Comment(s)
Sucp_0209_09_z Chevy_gm_cylinder_heads Hole_deburring 1/17

Be sure to deburr the area around the spark plug hole after polishing.

Intake Testing Notes
Test 1: Stock outboard Vortec port as shipped (i.e. cylinder No. 1, 2, 7, or 8).
Test 2: Stock inboard Vortec port as shipped (i.e. cylinder No. 2, 4, 5, or 6).
Test 3: Performed 35/45/60-degree valve job and 75-degree throat cut on outboard port.
Test 4: Same as #3, added back-cut valve, outboard port.
Test 5/1: Same as #4 with hand-blended throat area above valve seat, outboard port.
Test 6/2: Same as #5/1 with fully blended valve bowl up to guide boss on inboard port.
Test 7/3: Inboard port with same porting as #6/2 but also blended intake port opening.
Test 8/4: Outboard port with same porting as #6/2, but with reshaped valve guide boss, totally smoothed port walls, and filled rocker stud hole with clay.
Test 9: Same as #6/2 with just rocker arm stud hole filled on inboard port.
Test 10: Same as #8/4, but using Manley valve.

Exhaust Testing Notes
Test 1: Stock outboard (i.e. cylinder No. 1, 2, 7, or 8) Vortec port as shipped.
Test 2: Stock inboard (i.e. cylinder No. 2, 4, 5, or 6) Vortec port as shipped.
Test 3: Performed valve job with radiused cutter and throat cut, outboard port.
Test 4: Same as #3, added back-cut valve, outboard port.
Test 5: Performed valve job, but no throat cut or back cut on stock valve, inboard port.
Test 6: Performed valve job, but no throat cut, added back cut valve, outboard port.
Test 7: Same as #6 but without back cut, outboard port.Test 8: Same as #6, inboard port.
Test 9: Fully ported with back cut valve, outboard port.
Test 10: Same as #9 with Manley valve.

Sucp_0209_10_z Chevy_gm_cylinder_heads Exhaust_port_polish 2/17

Once you've got the intake ports done, you can clean up and polish the exhaust. These ports are short and easy to work on, but take your time and don't enlarge them. Just straighten them out and clean them up.

Explaining The Valve Job
Discussing valve jobs can be confusing even when you're talking to a pro. Most of the confusion revolves around the words "above" and "below" when used in relation to the valve. The problem is that when the heads are on the motor, the valve cuts, throat cuts, and valve pocket are all "above" the valve. But on the valve seat machine they're "below" the valve because the head sits upside down. For accurate reference here, we'll refer to everything as if it was on the engine. So the pockets, valve guide boss, valve seat, and throat cuts are all "above" the valve.

The typical three-angle valve job is what we're all used to hearing about. In actuality, only one angle, the 45-degree cut, really contacts the valve, and the rest are there to smooth airflow around it. These additional cuts are especially helpful at lower lifts. There are also what are called "back cuts" on the valves that help improve flow, but they don't come in direct contact with the cylinder head.

The first angle, which is actually exposed to the chamber and therefore the only one that's actually "below" the valve, is a 35-37-degree radius cut that smoothes the transition from the rough chamber wall to the valve seat. Next is the 45-degree valve seat. This is the area that creates the seal and is also responsible for taking heat away from the valves; this cut is usually wider on the exhaust to absorb more heat. The next cut is a 60-degree angle that smoothes the transition from the rough pocket wall to the 45-degree valve seat. Finally, there's the 75-degree throat cut, which is not always performed but can help airflow if you're won't be hand-blending the bowl area above the seat. This angle offers the smoothest transition for the air entering the bowl.

Sources

Standard Abrasives
Simi Valley, CA 93063
805-520-5800
Mondello Technical School
Paso Robles, CA 93446

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