I am truly astounded by the choices in competition cylinder heads that are available today. When I started my racing career 27 years ago, the only source for cylinder heads was a junkyard. Now racers can buy heads that are better than anything we raced in Pro Stock not too many years ago.
Unfortunately, too many choices can also lead to confusion. Back when the only alternatives were cast-iron and aluminum factory heads, the decision-making process was simple: you used what you could afford. Now, however, an engine builder has literally dozens of aftermarket cylinder heads to consider, with a staggering variety of port layouts, runner volumes, and combustion chamber designs.
I've seen the effects of this revolution in cylinder heads firsthand at our shop. We developed our line of "Super Series" big-block Chevrolet V8's to take advantage of the choices in Rat motor heads. By selecting the right castings from manufacturers such as Dart and
Brodix, we can match the heads to the engine displacement and rpm range almost perfectly without expensive porting and elaborate machining. As a result, the cost per horsepower has declined tremendously.
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The most common mistake racers make when selecting cylinder heads is to choose ports that are too big for the engine displacement. Racers have a tendency to believe "bigger is better," but that is usually not true when it comes to runner volumes. If you are debating between heads with different port sizes, my advice is to choose the smaller runners.
For years we have heard that oval-track engines want small ports because they need torque over a wide rpm range, while drag racing engines require big heads because high-rpm horsepower is important. Well, I'm not sure these rules of thumb still apply. On a long oval track like Daytona or Talladega, the engine speed in a NASCAR stock car varies only a few hundred rpm. A drag racing engine, on the other hand, has to operate over a much wider rpm band when the clutch engages (or the converter hits) at the starting line, and then on each successive gear change.
The best tool that we have available to simulate what happens during a quarter-mile run is a dynamometer acceleration test. Unfortunately, even an acceleration test on a conventional dyno can't accurately recreate what happens when a load is suddenly applied to an engine. During a typical gear
change, the |
