The Quick and the Fast

“If it don’t go—chrome it!” There’s not a ton of chrome in CHP. That’s because we like to go fast—quickly. If you’re looking for the latest tricks on paint and polish, we’ll cover that some other time. This story is all about The Quick and The Fast.

Acceleration depends on two simple variables—weight and power. Given the same power and adequate traction, a light car will accelerate quicker than a heavier car. A very quick street or race car usually combines excellent power with less weight. Of course, you could emulate our buddy Nick Scavo who used to race a 4,200-pound Impala in NMCA Super Street against a bunch of lightweight Mustangs and Camaros. While his effort was admirable and downright impressive, it’s not the best approach if you really want to be among the quickest.

To really be quick, it’s important to pay attention to variables like torque, horsepower, gear ratios, transmission selection, traction, weight, and chassis science. All of these factors work in concert with each other to create a signature acceleration rate. Once the major components are in place, it’s up to the owner/driver to tune his combination to create optimum acceleration.

Power Curves

Everybody always talks about horsepower. Ask most any car freak and he’ll probably end a statement about his car with, “Yeah, but it could use more horsepower.” If you’ve been reading this magazine for awhile, you may have noticed that we place an inordinate amount of emphasis on torque. Torque is the twisting motion created by the engine. This is what the dyno measures. Horsepower is calculated from torque generated over time. Make the same torque in less time (higher rpm) and you make more power. We’ve placed the formula for horsepower in the “Formulas for Success” sidebar.

In a typical power curve, as the torque drops off, horsepower continues to climb, because the rpm number in the equation continues to get larger. This means that if you can make torque up high, you’ll make killer horsepower. But this also creates a very “peaky” engine with sluggish power at part-throttle engine speeds. The key to >> making power for a street engine is creating a broad power band since street engines operate over such a wide rpm band. This is why a pool-table–flat torque curve is more desirable than a curve that looks like Mount Everest. Also, the more torque you can make over a wider rpm range, the less gear ratio you need. This makes the car easier to drive without sacrificing acceleration.

Of course, this torque must be manageable. Top Fuel cars have to slip the clutch in the first part of the run, because they make more power than they can put to the ground. Mega-inch engines, nitrous-sprayers, or turbo/supercharged engines can make awesome power, but it’s only useful if the power can be put to the ground. This is one reason why nitrous and supercharged engines are so quick. These power-adders radically increase the torque curve. For example, a 100hp nitrous hit at 3,000 rpm will boost a small-block’s mild 380 lb-ft of torque up to roughly 550 lb-ft of torque. That’s what you feel when you hit the button, and that’s why the car accelerates so much quicker. A simple 150hp nitrous kit will bump a 14.30/98-mph street car to a 12.50/109-mph blaster (assuming adequate traction). It’s the massive torque increase generated by the nitrous combined with the 150hp gain that makes the car 1.8-seconds quicker in the quarter.

The Trans Mission

Now let’s jump into transmissions. Automatic transmissions are now the typical choice for most street performance cars, because they offer a smooth application of power compared to a manual transmission. The torque converter is an interesting and often misunderstood piece of equipment. It also is a torque multiplier for the initial movement of the car. At maximum stall, a stock converter will multiply engine torque by a factor of between 1.9 and 2.2:1. This means that just as you nail the throttle at, let’s say, 3,000 rpm with a small-block making 400 lb-ft of torque, the converter will push roughly 800 lb-ft of torque through the transmission input shaft for the first few feet the car moves. This torque multiplication decreases rapidly as the car >> gains speed so that by the 60-foot mark, torque multiplication has virtually disappeared.

This torque multiplication helps launch the car, but there is also inherent slippage in a torque converter since it is a fluid coupling. This slippage, depending upon converter design and stall speed, can be 5 to 7 percent. This means that in high gear (with a 1:1 ratio), an engine spinning 6,500 rpm will only spin the output shaft a little over 6,000 rpm. This slippage turns into heat, which is why torque converters get so hot. This is also why car manufacturers now build lockup converters to eliminate the slippage to improve fuel mileage. Even cruising down the road, a tight factory converter (without lockup) will still slip roughly 200 rpm. High-performance converters with a higher stall speed will slip even more. Obviously there is an advantage to using a transmission with a lockup connection between the flywheel and the transmission input shaft.

One limitation of an automatic are the first three forward speeds up to 1:1. Basically, the deeper the First gear ratio, the greater rpm drop there must be between First and Second and Second and Third gears. The TH700-R4 is the worst offender with its 3.06:1 First gear. There’s a huge 46 percent rpm drop between First and Second that really hurts acceleration at the gear change compared to a TH350 with its 2.52:1 First gear. For most performance cars, the TH350 will offer better overall acceleration despite its taller First gear ratio. This concept of rpm drop between gears holds true for manual transmissions as well, but the four-speed offers another gear, which reduces the rpm drop. A deep First gear ratio like 2.88 gets the car moving but also requires a greater rpm drop between gears. The Richmond five-speed takes this one step further with its 3.25 First gear. By reducing the rpm drop between gears, the motor stays in its power band and accelerates more quickly.

Gettin’ Grip

Power does little to improve acceleration if it all goes up in clouds of tire smoke. But the solution is a bit more complex than just stuffing wide tires under the rear wheelwells. Tire diameter has a big impact on effective gear ratio as well. Again, let’s hit the basics and see if we can make some of this stick.

Like the Pontiac commercial says—wider is better. What may be less obvious is that a taller tire will also increase the traction footprint. Increasing the diameter also puts more rubber on the track. Also, a taller, soft sidewall tire tends to absorb more initial “hit” than a tire with a shorter, stiffer sidewall. The Mickey Thompson E.T. Street and M&H DOT-legal tires work well, >> because they are basically drag slicks with a few grooves cut in them. The drag radials from BFGoodrich and Nitto are soft compound tires, but because they are radials, they utilize a stiffer, less compliant sidewall. These still work well, but not as well as a two-ply bias tire.

Changing tire diameter can also have a significant impact on the effective gear ratio. If you have a Camaro with a 26-inch tall rear and a 3.55 gear, a 28-inch tall tire will knock the effective gear ratio to a 3.29. Stuff a 30-inch tall tire in place of the 26-incher and the effective ratio drops to 3.07. This is one reason why drag cars need such deep ratios, because while a 30-inch tall tire puts more rubber on the ground, it also tends to hurt the gear ratio.

Chassis Science

While tires are critical, suspension modifications are equally necessary to help plant the tires. Improperly modified front and rear suspensions can actually contribute to increasing tire spin rather than improving traction. Leaf spring wrap-up, squatting rear suspensions, and too loose or too tight front suspensions all contribute to killing dragstrip e.t.’s. Also, as you work to increase power, you must also work to improve traction because the chassis setup will vary. Shock valving, chassis preload, front suspension tricks, alignment, and at least a dozen other variables all contribute to making the car work as well as the engine does. But when you get it all sorted out, it’s a heck of a ride.

As you can see, a quick pass down the quarter-mile is no easy venture. While all this information may sound like a ton of work (and it is), there’s nothing more rewarding than a quick blast down the track when the car works great and you’ve just run quicker than you’ve ever run in your entire life. That’s when you know it was worth every second of your effort, and you’re instantly on a mission to figure out how to go quicker yet.