Small gear changes contribute in a big way when it comes to the maximization of a race car's quarter-mile capabilities, and the proper choice determines the quickest elapsed times. The test mule for our gear change swap is an NHRA Stock Eliminator '70 Nova powered by a steel-headed 402ci big-block. Last year, the car made the switch from a 350 ci small-block pumping out approximately 400 hp that could get down the 1,320 in approximately 10.8 seconds to the big block, which should be good enough for 9-second passes at its lightest weight.
Once owner Robert Twynam worked out all the bugs, he got the car down the track in B/Stock trim with some 10.20-second passes, good enough to place him near the top of the qualifying sheet. The problem that had become more and more evident was the over-revving of the big block with stamped steel rocker arms to 8,000-plus rpm, which led to audible valve float for the last hundred feet.
To make it in Stock Eliminator, the cars must be wrung out to the absolute max and this means pushing the factory parts to the limit and (sometimes) beyond. Rules stipulate the use of stamped steel rocker arms, a GM 12-bolt rear end, and a four-speed transmission (most competitors use either a Jerico or G-Force manual trans).
Just about everything out...
Just about everything out back on this hot rod is from Mark Williams Enterprises. Its drag brakes are some of the lightest in the business, as are its super-strong gun-drilled axles.
Making a rear gear change affects performance for the entire length of the quarter-mile and it doesn't matter if you are talking about a full-bore racer or a Saturday night cruiser. The trick to being successful is melding the first 60 feet with the 330 feet, followed by the eighth mile, and ultimately, the quarter.
Using 30-inch diameter radial drag slicks and trapping 126 mph, a change from 5.38 gears to 5.13s should drop trap rpm approximately 250, eliminating the valve float, and allowing the engine to work at maximum capacity.
We contacted Mark Williams Enterprises, which spec'ed out a 5.13 ring and pinion. It is lightened and polished for the strength and speed. Removing the spinning weight of the drivetrain is especially effective as you are not only lightening the automobile, but you are adding horsepower at the rear tires via a lower coefficient of drag from the flywheel to the wheels. With the 5.13s installed, Bob saw a three full mph improvement in trap speed (to 130.97), eliminated valve float and dropped 0.08 from his best e.t.
 First and foremost, we disassembled...  First and foremost, we disassembled the brakes and brake brackets and gently pulled the axles out. We then could unbolt the main caps, and gently pry the spool assembly out of the 12-bolt housing. Next, we can unbolt the pinion and pull it out. Before taking another step, we made sure that both pinion bearings were in good shape. We checked the gear oil that came out for any shards or metal and once we were satisfied it was OK, we continued the installation. |  The first step is to install...  The first step is to install the pinion bearing and spacer to achieve the desired depth. Using a cylindrical, we gently hammered the bearing into place on the pinion. It is important to keep your ring and pinions together as you never want to mix and match these. Once used, they develop unique wear patterns and should constantly be checked for any cracks or distortions. This little Nova leaves the starting line at approximately 6,000 rpm and really puts a hurting on the ring and pinion. |  We installed the pinion with...  We installed the pinion with some grease for lubrication and set our gauge to find the pinion depth. We were dead-on at 2.74 inches. This is the time to check pinion depth because you can easily make changes by pulling the pinion, removing the bearing and installing/deleting spacers. |