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BACK ISSUES

I am told that a child inherits the gene for male pattern baldness from his mother. Evidently, my maternal forebears were blessed with full heads of hair, which may have helped them survive the frigid winters in Minnesota, my ancestral home. Though I'm not yet personally in need of hair replacement, I am deeply concerned about bald spots — not on my scalp but on starting lines.

Summer is the time of year when bald spots appear on dragstrips even faster than crab grass sprouts in manicured lawns. In drag racing parlance, a bald spot is a patch of starting line where rubber does not stick. While Rogaine offers hope for the hairless, the only treatment for bald concrete is a massive dose of traction compound, rosin, and rubber.

There is a misconception that a racing tire grips the track surface. In fact, the tire tread should ideally contact a thin layer of rubber that is bonded to the underlying concrete or asphalt. That is the impetus behind the burnout ritual — the objective is to deposit a layer of hot, sticky rubber on the racing line for maximum traction. Unfortunately, in the heat of a summer day, hundreds of starting-line launches can scour the starting line to a mirror surface. Sometimes, the aggregate (the rock "filler" used in concrete recipes) is exposed by wear, weather, or excessive grinding, creating a surface as shiny and slippery as a ballroom dance floor. A starting line needs a slightly textured surface with enough "teeth" to grip and hold the rubber.

Some tracks on the NHRA circuit have a known propensity to develop bald spots, and we plan our attack accordingly. For example, we know what to expect on Sunday afternoon at Indianapolis Raceway Park if it's hot and sunny during the U.S. Nationals. The situation we faced at Gateway Int'l Raceway during the Sears Craftsman Nationals was unusual, however. The right lane developed bald spots of different lengths underneath the right and left tires. Consequently, the cars running in that lane had a tendency to turn left because of the difference in grip between the two rear tires.

In four qualifying runs at Gateway, we never came to terms with the track. Nothing was fundamentally wrong with the surface; the problem was in our setup. By race day, I think we finally figured it out, but we had lost lane choice due to our less than stellar qualifying performance. Consigned to the right lane for the first round of eliminations, I knew what was coming. The car made a hard left turn as soon as I let out the clutch, but that's why they put steering wheels in race cars — it's the driver's job to guide the car back into the groove. I had a respectable light, Jeg Coughlin Jr. had an exceptional light, and even though I ran the quickest elapsed time of the round, we were done for the day.

It takes experience and a trained eye to learn how to "read" a starting line. Some racers have a thorough knowledge of every nut and bolt in their car but are oblivious to the different characteristics of starting lines. To be consistently successful, you must monitor the track conditions as carefully as you record changes in barometric pressure, relative humidity, and air temperature. Sometimes, it's enough to make you pull your hair out — but that particular cause of baldness has nothing to do with chromosomes or genes.

Now I'll take a hair-raising question from my National DRAGSTER mailbox:

I have heard that the optimum bore size for a gasoline-burning engine is around 2.6 inches, based on the burn rate of the fuel. Would this favor a smaller bore/longer stroke combination? How does piston-dome and combustion- chamber design affect this? Ken Casey Jr.
Denver, Colo.
While the burn rate of the fuel does affect engine performance, it would be a serious mistake to base the bore diameter on fuel characteristics. Bore size determines the available valve area, which has a much more significant impact on the performance of a racing engine. In most all-out racing applications, you want to maximize the valve area, which requires using the largest practical bore area (within the limits of the rules, the available hardware, and the ability of the rotating and valvetrain assemblies to operate reliably at engine speeds high enough to take advantage of the potential airflow).

For a given displacement, an "undersquare" engine (small bore/long stroke) typically has smaller valves that operate efficiently at low rpm but restrict breathing (and therefore performance) at higher rpm. An "oversquare" engine (big bore/short stroke) generally favors high-speed performance because the larger bore diameter can accommodate more valve area.

Certainly, the burn rate of the fuel has to be tailored to each particular combination. The cylinder bore diameter is part of that equation because it determines the valve and port sizes, which in turn dictate the engine's rpm range.

You must also consider the location of the spark plug in the combustion chamber. Obviously, a centrally located spark plug minimizes the distance that the flame front must travel across the cylinder. Most hemispherical and four-valve cylinder heads have spark plugs that are located virtually in the center of the chambers, and conventional wedge-type heads have spark plugs that are offset to the side. However, the development of purpose-built competition cylinder heads, like the GM DRCE big-block and SB2 small-block, show that it is possible to have spark plugs located relatively close to the center of the chambers even in two-valve, wedge-style cylinder heads.

The interaction between the combustion chamber and the piston affects both flame travel and cylinder scavenging during the overlap period when both the intake and exhaust valves are open. The taller you make a piston dome to increase compression, the more it can disrupt the flame front as it spreads across the cylinder. An intrusive piston dome also creates more nooks and crannies that can be difficult to purge of waste gases.

The shape of the piston dome has a significant effect on the volumetric efficiency of an engine. In a typical flow-bench test, you measure the airflow with the head mounted on an open hole that simulates the cylinder bore. In actual operation, that hole has a variable floor in it called a piston. As that piston travels up and down in its cylinder, it influences how efficiently that cylinder is filled and scavenged — especially at top dead center during the overlap period when both valves are open simultaneously.

You can appreciate the impact of the piston dome on airflow if you flow-test a cylinder head with a piston in the fixture. Locate the piston and valves in their relative positions during the overlap cycle, then flow the entire intake and exhaust tract with vacuum applied to the exhaust port. You may be surprised to see how different dome contours and heights can affect the airflow of the entire system. To truly understand an engine, you must examine every possible configuration in which it operates.

Those seeking the wisdom of "the Professor" of Pro Stock should send questions to Speed Reading, c/o National DRAGSTER, P.O. Box 5555, Glendora, CA 91740.