what is the difference between 18' and 23' degree heads?
TECH Veteran
Joined: Dec 2004
Posts: 4,012
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From: Beaumont, TX
Hot Rod Mag article comparing 13, 18, and 23 degree: http://www.hotrod.com/techarticles/48778/
Info about Trick Flow's 18-degree vs 23-degree heads:
http://www.trickflow.com/articles/18..._body_wlft.asp
NASCAR 18-degree and 23-degree cylinder heads descriptions: http://www.musclemotorparts.com/chevyheads.htm
Standard small-block Chevy heads position the valves at a 23-degree angle to the deck. In 1989, Chevy introduced the 18-degree Bow Tie aluminum head that shifted the valve angle vertically while simultaneously raising the intake port to take advantage of this angle. The advantage of the valve angle change is improved airflow due to placing the valve in a more “upright” position relative to the piston. Raising the port also necessitated a dedicated intake manifold, requiring a sizable cash investment to switch to these heads.
http://www.trickflow.com/articles/18..._body_wlft.asp
So what's the big deal about 18 degrees? The shallower valve angle allows the cylinder head designer to use a smaller volume combustion chamber to help boost the engine's compression ratio without going to large-dome pistons. The combustion chamber on the Trick Flow 18 degree head is 56cc versus the 64cc chamber on most high performance 23 degree heads.
Another big advantage of the 18 degree valve angle is valve location. The valves in a conventional 23 degree head are .275 in. from the cylinder bore centerline. When the valves are close to maximum lift, they become shrouded by the combustion chamber and the cylinder walls. That hurts the engine's ability to breathe--and that hurts horsepower.
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The one drawback to 18 degree heads is that they can't use standard 23 degree head components. For example, the 18 degree head's intake manifold mounting flange is angled at five degrees (standard is 10 degrees) to accommodate the raised intake runners. That requires a special 18 degree intake.
The 18 degree heads are also taller than their 23 degree cousins, and require longer valves and special head studs. Eighteen degree pistons with domes and valve reliefs matched to the combustion chamber shape and valve locations are needed. And then there's the big kicker—18 degree heads must use shaft mount, offset rocker arms and mechanical roller cams with offset lifters. These components are readily available, but often cost more than 23 degree stuff. Nobody said going fast was gonna be cheap.
Another big advantage of the 18 degree valve angle is valve location. The valves in a conventional 23 degree head are .275 in. from the cylinder bore centerline. When the valves are close to maximum lift, they become shrouded by the combustion chamber and the cylinder walls. That hurts the engine's ability to breathe--and that hurts horsepower.
...
The one drawback to 18 degree heads is that they can't use standard 23 degree head components. For example, the 18 degree head's intake manifold mounting flange is angled at five degrees (standard is 10 degrees) to accommodate the raised intake runners. That requires a special 18 degree intake.
The 18 degree heads are also taller than their 23 degree cousins, and require longer valves and special head studs. Eighteen degree pistons with domes and valve reliefs matched to the combustion chamber shape and valve locations are needed. And then there's the big kicker—18 degree heads must use shaft mount, offset rocker arms and mechanical roller cams with offset lifters. These components are readily available, but often cost more than 23 degree stuff. Nobody said going fast was gonna be cheap.
