2. The Horsepower Equation

Every gearhead and car guy should know this formula by heart. It’s simple, and there’s more than 100 years of internal-combustion effort and a Scotsman’s work on steam engines for this formula.

We won’t get into the entire history, but suffice to say, Scottish-inventor James Watt came up with this formula in the late-1700s to relate the power of his new steam engine to draft horses. The term horsepower was born. We’ll save you the details of how the 5,252 denominator was created. If you really want to know, Google can fill in the details.

Horsepower = (Torque x RPM) / 5,252

Here’s the inside information on making power. All internal engines make torque, defined as the twisting motion of the crankshaft. If you can make the same torque in less time (measured in revolutions per minute – RPM), then your engine will do more work and make more horsepower.

Let’s look at two examples:

Example A

A 454ci big-block Chevy makes 425 lb-ft of torque at 5,500 rpm. If we calculate the horsepower, it looks like this:

HP = (425 x 5,500) / 5,252

HP = 2,337,500 / 5,252

HP = 445

Example B

A much smaller 302ci small-block Chevy makes 333 lb-ft of torque but at a much higher 7,000 rpm.

HP = (333 x 7,000) / 5,252

HP = 2,331,000 / 5,252

HP = 443.8 rounded to 444

These two engines make close to the same horsepower, even though they are radically different. The key is engine speed. Of course, the big-block will make a ton-more torque than the little 302. But, you can see that if the engine is durable enough to live at a higher engine speed, it is a great way to make more power. This is no secret – engine builders have known this from the beginning of the internal combustion engine.

There are drawbacks to this high-RPM equation. Engines with long-duration camshafts don’t like to run at low engine speeds, while large displacement engines can rely on size to make monster torque and often offer much greater reliability.

All engine dynos use this basic equation to calculate horsepower. A dyno only measures torque and computes horsepower using this same equation. Automated dynos like this SuperFlow at Westech Performance do the math for you very quickly.

3. Estimate HP

This is our favorite formula, and the one we use most often. If you think it might be fun to be able to amaze your friends with the ability to predict – with surprising accuracy – how much power a normally-aspirated street engine makes, then you want to commit this simple formula to memory. But first, we need to lay the foundation on how this works.

The formula is based on two estimates: one for torque per cubic inch, and the other for the peak horsepower RPM point. The first estimate is relatively simple. For street engines on pump gas with a good cylinder head, intake, and exhaust systems, our buddy Steve Brule’ at Westech Performance likes to use 1.25 lb-ft of torque per cubic inch. So let’s say we have a 383ci small-block with good AFR heads, 10:1 compression, a decent cam, headers, and an Edelbrock Performer RPM dual plane intake.

Displacement x 1.25 = Peak TQ

So let’s use this first part of the formula on our theoretical 383ci:

383 x 1.25 = 478.7 lb-ft of torque

Over years of looking at engine power curves, Brule’ has noticed that street engines generally lose 10 percent of their torque at peak horsepower. This new number is the torque the engine will make at the peak power RPM point.

Peak TQ x 0.90 = torque at peak HP

So now let’s plug our torque number in:

478.8 x 0.90 = 430.8 lb-ft – let’s call this number Torque 2 or TQ2

Now, we employ the full horsepower equation just learned in the previous example. But, this is where the second estimate number comes into play. We must estimate the RPM point where the engine will make its peak horsepower number. Camshaft timing generally has the greatest effect on this number with a longer duration cam pushing this peak RPM point higher.

If we happen to know the exact peak torque RPM, then we can roughly add 1,500 to 1,800 RPM to the peak torque rpm to estimate the peak horsepower RPM. For example, if peak torque occurs at 4,000, then we can expect the peak horsepower RPM point at somewhere between 5,500 and 5,800 rpm.

For the purposes of this example, we’ll choose 5,700 rpm as our horsepower peak number. So now, we can just plug our TQ2 number into the horsepower equation:

HP = (Torque x RPM) / 5,252

HP = (430.8 x 5,700) / 5,252

HP = 467.5 – we’ll round that off to 467.

Just for fun, we dug up the dyno specs on a mild-383, pump-gas small-block from a previous story and charted the engine’s power curve. Our estimate of torque is almost perfect, but we’ll admit that we worked backward from this test to choose the peak RPM point. Given that, you can see the horsepower estimate is very close with 467, while the engine actually produced 453. Our assessment was high by 14 horsepower, which is only a 3-percent error factor.

This formula will work with any normally aspirated internal-combustion engine, but as you can see, it requires accurate estimates for both peak torque and the peak horsepower RPM point. Race engines with higher compression will make upwards of 1.55 to 1.6 lb-ft of torque per cubic inch. An NHRA Pro Stock engine will be even higher.

This formula can be used to estimate a race engine’s potential, regardless of the engine speed. Again, the key is carefully estimating torque per cubic-inch and peak engine speed.

As an example, with 427 cubic inches and 1.50 lb-ft per cubic inch, that equals 640 lb-ft of torque. According to Ben Strader at EFI University, race engines tend to lose 12-percent torque at peak horsepower, as opposed to 10 percent, so we’ll use his factor. Calculating 640 x .88 = 563 lb-ft, and if the peak horsepower occurs at 11,000 rpm, it equals to 1,179 hp. That’s pretty stout.

Now, you can use this formula to amaze your friends with your engine acumen and horsepower expertise!
383 Small-Block Chevy Power Curve

RPM

TQ1

HP1

3100

449

265

3200

456

278

3300

460

289

3400

464

300

3500

466

310

3600

468

321

3700

471

332

3800

473

343

3900

475

353

4000

475

362

4100

474

370

4200

469

375

4300

465

381

4400

463

388

4500

462

396

4600

461

404

4700

460

412

4800

459

419

4900

457

426

5000

455

433

5100

452

439

5200

448

443

5300

443

447

5400

437

449

5500

431

451

5600

424

452

5700

417

453

5800

409

451

5900

399

448

6000

389

445
We use this formula to estimate power for street engines, and it is often very close but, there are some exceptions. We built this pump-gas, 10:1-compression 6.0L (364ci) that was finalized with a set of TFS heads and a COMP cam, and it made 500 lb-ft of torque. That equates to 1.37 lb-ft per cubic inch. The engine produced 557 hp at 6,700 RPM, which was shy of the estimated 572 hp.

4. How to Convert 1/8-mile E.T. and Speed to ¼-Mile Numbers

There’s much more emphasis on 1/8-mile drag racing now that cars are running so quickly, and it is often difficult to relate 1/8-mile (660 feet) times to 1,320 numbers. Many moons ago, a good friend Dr. Dean Hill along with his friend Dr. D. Craig Hane, published a reference book called the Pocket Dyno. This was published long before the days of home computers, and the book is full of conversion tables to convert 1/8-mile to 1/4-mile elapsed times (e.t.) and other useful tables and charts.

We used Dr. Hill’s information and converted his numbers to a simple conversion equation.

1/4-mile E.T.= 1/8-mile E.T. x 1.54

As an example, our Chevelle recently ran a 7.051 in the 1/8-mile.

7.051 x 1.54 = 10.85 e.t. in 1/4 mile

We also have a simple conversion for M PH. This is a bit more generalized but seems to hold up for MPH estimations.

¼-mile MPH = 1/8th mile MPH x 1.25

In our case, we ran a 98.98 mph trap speed in the 1/8-mile.

1/4-mile MPH = 98.98 MPH x 1.25

1/4-mile MPH = 123.72 MPH.

There are numerous reasons why MPH may not always be accurate. Several variables come into play starting with examples where, at any speed above 90-100 MPH, the aerodynamic effects of older ’60s cars will lower the quarter-mile speeds compared to a slippery third-generation Camaro, for instance. But this formula still works as an estimate for quarter-mile times and speeds.
This is our Chevelle at Irwindale running low 7’s at 98 MPH. Those times convert to high-10’s at 123 mph for the quarter-mile.

This is the book written by Dean Hill and Craig Hane. It was published in 1974 and is now-long out of print. The information goes beyond just converting 1/8-mile times to 1/4-mile. It also contains dozens of charts to estimate flywheel horsepower based on weight and trap speed.