GEARING FOR GO

The Right Way To Choose A Gear Ratio Combination

By Wayne Scraba
8/9/04

Photo by Dale Bryant

pin the clock back a few decades. In the days before on-board computers and electronic fuel injection, it was pretty easy to select a gear set for your racecar. The selection process was dirt simple: You just used what the local hot dog ran (which invariably was a 4.56:1 or deeper ring and pinion -- if you had 5.13's or 5.38's out back, you were approached with some sort of reverence). Bigger numbers and consequently, lower gears were king. If you were sophisticated, then the only real concern was the rpm at which the car crossed the finish stripe on the drag strip.

Fast forward: Today things are much more evolved, and so is drag racing. Hardware that influences the gearing in your racecar is much more predictable. For the most part, you know how much a certain slick grows. You can find out how much a torque converter slips. Tachometers are ultimately reliable and in many cases, they can "recall" what the maximum engine speed was during a lap down the 1320. And in many cases, you don't even need to rely upon a tach or your faded memory to know what the engine rpm was when the car crossed the finish line: The on-board computer will heartlessly inform you of the peak rpm. In simple terms, no fudging is allowed.

It has probably been said dozens or even hundreds of times over the years but it still holds true: "Far too many racecars are over-geared." It was something I examined years ago. One part of the puzzle that has not been suitably addressed completely is the actual mechanics of selecting the appropriate rear axle ratio for your racecar.

The real key to gear ratio selection is to simply take all of the variables into consideration. Those variables can include the engine power band, the transmission gear ratios (and indirectly, the torque converter stall speed if the car has an automatic transmission), the rear axle or final drive ratio and the diameter of the drive tires. In the words that follow (and I’ll warn you, there are no photos), I'll take several hypothetical real car combinations and examine the variables. Additionally, we’ll look at how these variables intertwine and how you can use a calculator to work out the correct gear ratios for your personal racecar.

When you take a close look at the rear rubber used on the majority of today's drag cars, you'll find that it's usually 32.0-inches or larger. According to Marv Ripes at A-1 Automatic Transmissions, slicks typically grow three percent for every 100 mph. Ripes also notes that some Super Stock converter combinations can exhibit as much as seven percent slippage while other converter combos (particularly those used in SS/Modified and Comp Eliminator cars) can have as little as 4-4-1/2 percent slippage. It's easy to see that in some cases, tire growth can almost completely cancel out converter slippage. Keep in mind too that the engine must have enough grunt to push the vehicle to the top speed figure in the first place (as you can see, the formulas that follow do not take power or torque into consideration). The purpose of the mathematical examples is to show just how the numbers are crunched.

When it comes to picking the right axle ratio for your racecar, the real place to begin is the engine combination. For this comparison, I decided to include a trio of vintage big block combinations. The basic combination is an off-the-wall affair: It's a LS6 1970 El Camino. Although it's in the official NHRA Stock Car Classification Guide, I've never personally seen one in Stock or Super Stock trim (for that matter, neither have any number of tech gurus I've spoken with). Perhaps the reason they aren't popular is the fact that it could become difficult (if not impossible) to hook consistently, particularly with some real steam under the hood. None-the-less, two engine combinations were used: one for a Stocker and one for a Super Stocker. What we're searching for is the maximum horsepower at a given rpm level from the engine(s). What follows are some hypothetical numbers (and please remember, these are definitely hypothetical):

As you can see, we have two very different power peaks. Because of this, it shouldn't take a rocket scientist to figure out that each of these engines might "like" a different transmission and rear axle ratio combination. The bottom line is simple: You should select the drive line combination to fit the power and rpm characteristics of the power plant. Otherwise you (and your engine) won't be terribly happy.