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).
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.
Mark Williams Enterprises sells ring and pinion kits for just about every model rear end. Per our application, we went with an 8260 alloy specifically designs for our high-shock application. Simply, high-shock is referring to the process of dumping the clutch, and accelerating the car to the 60-ft mark in less than 1.4 seconds. Also, Mark Williams' aluminum spool is hard-coat anodized for durability and it is half of the weight of its steel counterpart.
It's always good to have a stack of rear end shims laying around when performing this installation. We actually did this laying down on the floor of Robert's garage with a set of jackstands. If you have access to a lift, it makes life that much easier and will ultimately lower your chiropractor bills.
We set the entire spool and ring gear in place using the same shims that came out. This is a great starting point as often, you will use the same shims to make the switch. Getting the assembly in place is a little tricky, as it has to go in completely straight. If it is slightly off kilter, pull it back out and start over again. The last thing you want to do it damage the aluminum spool.
We then torqued the main caps into place and broke out the gauges once again to check the lash between the ring-and-pinion. Optimally, we are looking for approximately 7 to 8 thousandths of an inch. It actually took us three tries to get the correct shims in. We had just about every shim size from .001 to .040 and it saved us a great deal of time and effort.
At this point, we can reinstall the driveshaft with the two U-bolts and also install the rearend cover. Use both a gasket and a bead of silicon to ensure that you don't have a leakage problem down the road. Also, tighten the rearend cover bolts in a pattern that never has you tightening two bolts right next to each other. Bounce around and make sure that all the bolts are tightened fully.
We filled the rear with two quarts of Joe Gibbs Driven qualifying rear fluid. Designed specifically for race cars, Joe Gibbs Driven goes to great lengths to ensure its product is both safe and horsepower-friendly. Using a lighter weight rear fluid can improve performance, but the wrong weight can cause the ring-and-pinion to seize. Make sure you talk to a company representative before choosing.
At this point, we can slap the brakes back on and head to the track. As always, take your time when installing a ring-and-pinion set and make sure to take accurate measurements along the way. Not doing so can creep up to hurt you later when you are in a final round match up. Plus, no one wants to be stuck on the starting line with a broken rear. People tend to laugh.