Understanding Acceleration - Balancing Torque & HP

Years ago, I discovered that engines which are equal in horsepower and torque have a balanced feeling. "Balanced" engines have a usable power range from about 4,000 to 6,000 rpm's. "Horsepower" engines are those with more horsepower than torque. They're sluggish in the lower RPM range and need revving before they feel good.

"Torque" engines, on the other hand, feel strong on the bottom end but go flat at the top.

Horsepower to Torque Ratio
In Chart A, we show the balance of horsepower to torque graphically. Dividing torque into horsepower gives us the Horsepower to Torque Ratio

Study Chart A. You'll notice that automobiles listed in the "Balance Zone" (see brackets { ) offer excitement without wearing out the driver. Autos with a horsepower to torque ratio greater than 1.15 tend to feel high strung while those falling below .80 feel dull. Note: the peak RPM for torque and HP. A good rule of thumb is this: in building a performance engine for everyday driving, keep the ratio close to the balance zone, or the car's temperament will eventually wear on you.

Gearing
Before we look at how the Horsepower to Torque ratio affects gearing, let's establish on overall ideal rear gear ratio. Of all the rear gear ratios that BMW offers, I'd say that the 3.45 or 3.64 gears seem optimum for the most BMW models. Averaging these two gears together, reveals a single ideal gear of 3.55:1

In part 2 of this series, we talked about how an engine that develops high RPM horsepower needs to be re-geared to bring up rear axle torque. To balance out engine torque on a high RPM HP engine, we simply need to multiply the horsepower to torque ratio times 3.55 (our ideal rear gear).

Horsepower Torque x 3.55 = New Rear Gear Ratio

Now, normally you'll never see a rear gear numerically lower than say a 3.00:1. That's because gears lower than that are counter productive to torque (below .85 Torque Ratio) and torque is what moves cars! Use the chart simply as a general guide. Other things that effect gearing would be a big change in vehicle weight, tire size, gearing for a particular track, etc.

Captain Says
If I had to put my finger on what makes a car accelerate, it would be the amount of axle torque at the rear wheels versus vehicle weight - (Part 1, July '89). In Part 2 and 3 of this series, (August & September) HP and Torque were discussed. Now, in Part 4, we've considered how to create a balance between Horsepower and Torque through gearing. So, although you were given the "key" to acceleration in the Part 1; Parts 2, 3, & 4 tell you how to achieve it.

So, more power to you ... beam me up.