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gradually toward the short side radius and intake valve. Hand porting a set of cylinder heads takes skill and expertise to achieve consistency. Even then, it’s hard to match the results that can be achieved much more quickly and easily with CNC porting. With CNC porting, once you have developed the port configuration you want on a flow bench, you can digitally map the port and replicate it again and again with near perfect consistency in the rest of the ports in the cylinder head. And once you have a good port configuration mapped, you can also modify it with CAD/CAM software for different engine applications.
CNC Porting To port cylinder heads with a CNC machine, you need a machine with 5-axis capability. A 5-axis machine can move the head and tooling to reproduce all of the complex moves it takes to completely machine intake and exhaust ports. Once the CNC machine has been programmed, it is automatic, requires no manual labor other than loading the machine, and is repeatable from one head to the next. The motions of the tooling are carefully choreographed so that metal is removed in exactly the right places. Figuring out how to program such intricate movements requires a fair amount of CNC know-how as well as software that is up to the job. If you’re new to CNC porting, don’t expect to start porting cylinder heads in the first few weeks after you’ve purchased your equipment. It might be six months to a year before you can CNC port heads confidently. A port that has been CNC machined usually requires little or no additional finishing. It will often flow just as well as a port that has been smoothed out and finished with a die grinder. CNC machining unfinished castings is probably the fastest and easiest way to custom port heads. Most aftermarket heads are available with cast ports, CNC machined ports or as raw unfinished castings so you can do your own porting. The problem with trying to modify an existing port is that if the casting is not thick enough, you may grind through a thin spot in the port wall. Besides, most of these heads flow so well out of the box that there’s little additional power to be gained by additional porting. Of course, sometimes porting may be necessary if a set of heads is being used on an engine with a big stroker crank or a large displacement aftermarket block that needs more airflow. It’s often possible to take a set of aftermarket performance heads that flow 260 to 270 cfm as is out of the box and rework them so they’ll deliver 330 to 340 cfm. If you are porting a new raw casting, you also have to be concerned about head stability. A green casting that has not yet been thermally cycled on an engine can actually shift around quite a bit. As the metal heats and cools, the head may distort and change the location of guides, seats and decks, which in turn may affect valve sealing and valve life. A new casting can be stabilized prior to machining it by heating and slow cooling it in an oven, cycling it through a hot parts washer several times, by shaking it at a frequency of 100 Hz to stress relieve the metal, and/or by cryogenically freezing the head. Once the casting has been stabilized, you can machine it and be confident that it will not change. ■ Circle 20 for more information 20 August 2013 | EngineBuilder