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Got that? OK. Things get a bit more complex. Let's compare three different cars, using the previously mentioned LS6 El Camino as an example. The first is an automatic transmission Stocker, the second is a stick shift Stocker and the third is an automatic-equipped Super Stocker. The Stockers are set up to use a 30-inch tall slick while the Super Stock car includes a 14 X 33-inch slick. We'll take a preliminary guess (guess definitely is the operative word) at picking a set of gears for the Super Stock and both Stockers. The following are the basic tire and gear ratio specifications:

Stock Automatic Stock Stick Super Stock
Axle Ratio 4.56 4.88 5.43
Tire Diameter 30.00" 30.00" 33.00"

Next, we'll compute the vehicle speeds at a specific rpm number. The formulas are as follows:

Loaded Radius Of Tire: 9X30" slick 14X33" slick
Loaded radius = Tire Diameter ÷ 2 30.00 ÷ 2 33.33 ÷ 2
  = 15 = 16.5

Next, we have to determine the tire revolutions per mile for each car. The following formula is used for the calculations:

Tire Revolutions Per Mile: 9X30" slick 14X33" slick
Tire Revolutions =
10084 ÷ loaded radius of tire
10084÷ 15 10084÷ 16.5
  = 672.2666 = 611.1515

The above figures are obviously meaningless, unless they are plugged into the next formula -- maximum speed. For this formula, we have to use the previously mentioned peak horsepower rpm numbers I arrived at earlier. There are two schools of thought when it comes to gearing contemporary drag racecars. One is to set the car up so that it crosses the finish stripe at the rpm at which peak power is made while the other conviction is to have the engine turning approximately 10 percent over the power peak at the finish line. We'll look at the math for the first concept (cross the finish line at peak power rpm) first:

Maximum Speed:
Maximum Speed = (rpm X 60) ÷ (Gear Ratio X Tire Revolutions Per Mile)
9X30" slick (auto) 9X30" slick (stick) 14X33" slick
(6,250 X 60) ÷
(4.56 X 672.266)
(6,250 X 60) ÷
(4.88 X 672.266)
(6.700 X 60) ÷
(5.43 X 611.151)
= (375000) ÷ (3065.5356) = (375000) ÷ (3280.661) = (402000) ÷ (3318.5526)
= 122.32 mph = 114.30 mph = 121.13 mph

If the respective combinations were set up to run for the 10 percent over peak rpm theory, then the results would be as follows (we won't re-do the math -- I've simply increased the rpm by 10 percent in each case):

Maximum Speed (10% over Maximum rpm):
9X30" slick (auto) 9X30" slick (stick) 14X33" slick
= 134.56 mph = 134.56 mph = 134.56 mph

There's quite a difference in speeds between the three car combinations at a given rpm level. The next thing to do is to consider one of the very first notions pointed out in this article. Automatics tend to slip as much as seven percent in a Stock/Super Stock application while tires tend to grow three percent for every 100 mph. Let's assume there is no clutch slippage for the stick shift car (although there could be some). This means the automatic-equipped stocker could lose as much as 9.4 mph while the Super Stocker could lose 9.3 mph. Meanwhile, each of the combinations (Stock and Super Stock) could gain roughly 3.75-4 mph with tire growth, given the three percent per 100 mph concept (keep in mind that we're not taking radial slicks into account for this exercise). In the end, the final speed for the three combinations works out as follows:

Maximum Speed (Revised):
9X30" slick (auto) 9X30" slick (stick) 14X33" slick
= 128.89 mph = 129.48 mph = 127.67 mph

When I'm crunching these numbers, the next thing I do is cross check the final (ultimate) speeds with NHRA national records to see if I'm on the right track. For example, both of the Stock Eliminator El Caminos fit into "A" (the last time I checked, the Classification Guide factored the Stock combination as 460 HP with a Factor of 8.07) while the Super Stocker fit into SS/EA (465 HP with a Factor of 7.98). At the time of this writing, the A/SA record is 131.81 mph. The A/S record is 135.72. The SS/EA record is 140.40 mph. As you can see, we're reasonably close with the gear ratio selection for the pair of Stockers, but light years away with the SS combination. What happened? I guessed wrong. I did the math again, this time with a 5.00:1 final gear for the SS combination. This time, the car "ran" 144.71 mph using the 10 percent over maximum engine speed rule. Taking converter slippage into consideration, then adding speed for tire growth, the final speed across the finish line worked out to 138.33 mph. As you can see, this is very close to the speed record. It's probably the right gear for the car (although in certain cases, a 4.86:1 or a 5.14 might work too). Just remember that aerodynamics comes into play with final speeds. The El Camino is basically a barn door, and it might not quite run as fast as something like a Corvette.

When all is said and done, the idea is to select the axle ratio for your car only after you've considered all of the variables -- and that includes the tire diameter, converter slippage, tire growth and of course, engine power peak characteristics. With the right axle ratio combo (one that's matched to the torque characteristics of your engine), your car will flat fly. If you don't have the right combination, it won't fall out of a tree. Use your calculator. It works.

Previous Stories
Import and Sport Compact Tech — 7/24/04
Del Sol H22A4 Engine Swap
Back-2-Basics — 7/22/04
Sometimes you just have to change your attitude.
Is the Future of Pro Stock in NASCAR Country?


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