18 Degrees of HP

Dyno Testing Trick Flow's New 18 Degree Chevy Heads

If there is one constant in this big, crazy, spinning-wildly-out-of-control world, it's this: horsepower wins races. Oh sure, racers will tell you stuff like suspension settings, driver skill, and plain old dumb luck are important. But deep down in the furthest corner of their souls, they know it's raw, screaming horsepower that takes the checkered flag.

The cylinder head crowd at Trick Flow Specialties understands this Primary Law of the Universe -- their famous Twisted Wedge and R-Series aluminum heads are ample proof of that. Now they've gone to an even greater degree in the search for more power. Actually, make that 18 degrees, as in their new 18 degree aluminum heads for small block Chevy. And do those heads make power -- we have the dyno tests to prove it.

Before we get to the dyno results, let's get into the theory behind 18 degree heads. The number refers to the angle of the intake and exhaust valves in relation to the head's deck surface. By contrast, conventional small block Chevy heads have a valve angle of 23 degrees.

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.

In 18 degree heads, the valves are right on the cylinder bore centerline. The valves are also relocated to position the intake valve closer to the bore center, and the exhaust valve closer to the cylinder wall. This unshrouds the intake valve at maximum lift and allows the use of bigger valves.

The Trick Flow 18 degree heads are machined for 2.150 in./1.625 in. valves, and can accept intake valves up to 2.180 in.

To fully take advantage of the high-flow characteristics of the shallower valve angle and larger valves, 18 degree heads have raised intake runners; the runners on the Trick Flow heads have floors raised 1.150 in. from the deck. And when you combine big, unshrouded valves with raised ports, you get gobs of power-building airflow at high rpm.

18 degree heads 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.

The Trick Flow 18 degree heads we dyno tested came with as-cast 250cc intake/100cc exhaust runners, 56cc combustion chambers, 2.150 in./1.625 in. stainless valves, and 1.550 in. dual valve springs. The flow bench data on these heads was impressive: maximum intake flow was a steady 328 cfm from .550 in. to .700 in. lift; max exhaust flow was 255 cfm at .650 in. lift. In the hands of a skilled head porter, the Trick Flow heads can be made to flow a bunch more.