Dyno Testing: An Explanation of Horsepower and Torque

By Mike Petralia

Even though the main goal of modifying engines is to make our cars go faster, there’s a certain pleasure to be derived from just testing engines on the dyno. I’ve learned to appreciate dyno testing much more than I do road or track testing. That’s because in the dyno cell I can control and monitor things that would be out of my grasp in the outside world. Likewise, things like traction and road conditions can play a big part in how fast your car can go, but have no bearing in the outcome of a dyno test. So it’s just easier to try to find the most power possible on the dyno.

This tech tip is from the full book, DAVID VIZARD'S HOW TO BUILD HORSEPOWER.

For a comprehensive guide on this entire subject you can visit this link: LEARN MORE ABOUT THIS BOOK HERE

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The dyno can tell you many things besides just HP and TQ. It’s also a great, although expensive, way to check and see if your engine leaks before installing it your car.

Dynos are practically a requirement in the aftermarket today. Successful companies like Edelbrock have installed a few dynos in their R&D facilities to help design and optimize products in a test environment they can best control.

Machine shops and engine builders also have dynos. Shown without an engine attached to it is Burbank Speed & Machine’s Stuska dyno in Burbank, California. The engine gets bolted to the rolling cart on the right and connects to the bell housing with a short driveshaft going into the impeller housing where torque is measured.

On the right is the throttle where the operator begins his “dyno dance” by going Wide Open Throttle and then sweeping the “Brake Control” lever, (center). He modulates the water flow through the impeller housing with the “Load Valve” on the left. This controls the speed at which the engine can accelerate.

Burbank Speed & Machine relies on a DEPAC Data Acquisition system to record the results of the dyno test. The DEPAC box transmits all of its data to a PC for storage and read-outs.

The word “dyno” is actually slang commonly used to describe an engine dynamometer, which is a rather uncomplicated apparatus used to measure an engine’s torque output at the flywheel, among many other things. There are also wheel dynos (a.k.a. tire dynamometers or chassis dynos) that measure actual power output at the tires on a car and then estimate the power output at the crankshaft based on any number of factors. There are even “tow-dynos” that get hitched to a vehicle and are towed behind them while measuring power output under actual road conditions. I prefer testing on the engine dyno, as its results are hardly contingent upon things like road conditions, traction, torque converter and/or clutch slippage, etc. However,a chassis dyno is invariably much easier to get onto and in the long run, probably less expensive to test on as well.

Since it’s difficult to watch the small displays on the data acquisition system, most dynos are equipped with some sort of “power scale”, either mechanical or digital, that the dyno operator watches to quickly see how the power curve is doing and when it begins to peak and nose over signaling the end of the test.

Funny Thing About Power

Did you know that an engine really does not make horsepower and a dyno doesn’t really measure horsepower either? The twisting force an engine makes is torque and that’s what the dyno measures. The data acquisition computer attached to the dyno then calculates horsepower from the observed torque. That’s because torque is the force that actually does all the work—turning what ever is behind the crankshaft in this case—and horsepower is the amount of work the torque provides over a period of time (measured in Revolutions Per Minute, a.k.a. RPM, in our case).

The formula for calculating HP from TQ is: (TQ x RPM) / 5,252. It works for all engines, regardless of make or size. Interestingly, using this formula always gives you equal TQ and HP figures at exactly 5,252 rpm on every engine you test. That’s the way the math works out. In most dyno tests where power and torque are listed at 5,200 and 5,300 rpm, they’ll be very close, if not equal. That’s also the point where, in a dyno graph, you see horsepower cross over and begin to surpass torque. Also, it’s impossible for a test engine to make more TQ than HP above 5,252 rpm when using this formula. The math makes it so. Conversely, if you know horsepower output, you can calculate TQ from HP using the inverse of this formula: (HP x 5252) / RPM. But, there’s hardly ever a case where you’ll know HP without first knowing TQ, unless you’re using one of those computer simulation programs that estimate HP based on the speed or E.T. of you car. Thankfully, any dyno worth its water will perform all these calculations for you, so there’s little math involved on your behalf.

Here’s an example of a strong 355cid Chevy small-block’s power curve. Look at the area around 5,200 rpm where you’ll see HP and TQ cross over. It’s that way in every dyno test because the math used to calculate HP from TQ makes it so.

Measuring Torque

Since torque is basically a twisting force, the guys who invented the first dyno had to figure out a way to measure the amount of twist an engine can provide. To do that, they turned to the most common substance on our planet: water. In a nutshell, when it’s bolted to the dyno, an engine works sort of like a big water pump designed to move lots of water, under little-to-no pressure. The crankshaft gets connected to an impeller that’s sealed inside a housing, which moves water in and out of a valved orifice. That housing is connected to a device that measures how much twisting force is being produced outside the impeller housing. Typically, the device of choice is what’s known as a “Strain Gauge” or “Force Transducer” or even sometimes called the “Load Cell.” These strain gauges are very simple little devices, and they do exactly what their name implies—they measure strain. Or, more precisely, the amount of strain the pump can induce upon a fixed device. The more pressure inside, the more the impeller’s housing will try to twist away from that force, causing the strain gauge to move, usually in the order of only a few thousandths of an inch. That movement is interpolated by the data acquisition computer as a TQ figure. The dyno must be precisely calibrated, and kept in top shape in order to work correctly and consistently.

Here’s the thing that measures the torque. It’s commonly called a “strain gauge,” a.k.a. “Force Transducer.” It basically gets pushed together and/or pulled apart; depending upon which direction the engine is rotating. The amount it moves is typically invisible to the naked eye, but it’s that small amount of movement that gets translated into torque by the data acquisition system.

Correcting the Issues

To keep things equal around the world when we dyno test an engine, all dynos usually employ some sort of correction factors. These factors are mathematical calculations that “equalize” the test data for comparison on any day, and in any part of the world. Since engines breathe air and make power from it, an engine tested a sea level in Los Angeles will always make more power than if the same engine were tested at high altitude somewhere like in Denver. The higher altitude has less oxygen and a typically lower barometric pressure; consequently, in Denver the engine gets less air to make power with. Things like altitude, temperature, humidity, and barometric pressure, (a.k.a. “baro”) all have an effect on how much power your engine can make. But it’s important for someone in New York to know that his test results can be compared to someone in California.

That’s why the SAE (Society of Automotive Engineers) came up with standard factors that “correct” any dyno results to data collected elsewhere. The correction factors also help keep results comparable from morning to noon to night during a long thrash in the dyno cell. In other words, if I tested an engine in Louisiana on a hot, humid summer day with a storm rolling in off the Gulf coast, the barometer might drop as temperature and humidity rise during the day. Any of these items alone would rob power from an engine, but by “correcting” the data to standard figures for temp, baro, and humidity; I can keep all my results within comparison. So when you see dyno figures listed somewhere, it’s important to note if they’re “Corrected” or “Observed,” which would be the actual figures as measured on the dyno.

This is also something that makes dyno testing confusing, because the correction factors are very easy to manipulate. And any savvy dyno operator can produce a higher power output from an engine simply by tweaking the correction factors. While I don’t personally know of any shops that do this, it’s still a good idea for you to understand all this and ask to see the correction factors being used on the day you test. It’s also interesting to look at and compare uncorrected data to tell if something is awry. Sometimes the corrected figures can be lower than the observed figures, but that’s rare.

What Can the Dyno Tell You

Besides the obvious TQ and HP, the dyno is a very useful tool in recognizing performance trends for a particular part or even a whole engine, as well as a simple means to break-in a new engine before dropping it into a car.

Something all dyno operators learn over time are the “trends” that certain parts share. If a guy brings his engine to a shop for dyno testing and has a monster 1,200 cfm race-prepped carb sitting on top of his 9:1 347-inch small-block, a sharp dyno operator typically lets him run the engine first with his carb, then offer to loan him one of the shop’s better-suited carbs, say a 750 cfm carb, and show how much better the engine might work with it.

A bolt-on is: “Any part or product you can install on the engine with it still in the car.” At least in my opinion. The Holley HP carb counts as one.

Now, if his customer is smart, and doesn’t have an ego bigger than that carb he brought with him, the dyno operator will be his hero and he’ll have a very happy customer because he knows the “trends” that some parts will follow. While, it’s for sure that you can make a carb that big work on a 347 ci engine, it takes a lot of pre-thought and definitely the right combination to do it. That’s why the dyno operator let’s him try it first—because he doesn’t know if this guy’s done his homework, or has just seen too many Dominator carbs on small-blocks at the car shows.

Also, another way the dyno operator can take advantage of a trend and help his customer out is by suggesting that he try another combination inside the engine. Maybe a new cam or different ratio rocker arms will work better, and some dyno shops have inventory that you can borrow for your test, or at least rent for a few extra bucks. Then, if you really want to be good to them, if the part works, you can offer to buy it directly from the shop giving them the chance to make some cash on it. Believe me, this will make friends fast.

Since fuel is something you pour into the engine, essentially you “install” it; I consider fuels a bolt on for performance too.

Throwing Parts at It

My experience on the dyno has taught me that if you just throw parts at an engine without doing any research first, your engine will probably suck. One of my earliest engine tests involved a big-block that I bet could make 600 hp with two plug wires removed. It didn’t. In fact, it could barely squeeze out 400 hp. After a tiresome day of head scratching, one of the regulars who often hung out at the dyno shop asked me about my engine’s displacement. Even though this was a big-block, it was a very small displacement big-block at only 402 ci. At the time, the dyno shop only had big-block headers with 2-1/8-inch primaries and 4.0-inch collectors, and I didn’t bring my own. My engine also ran a worked-over single plane intake manifold and it was suggested that we first swap on a set of 1-3/4-inch headers and try a good dual-plane intake manifold instead.

I felt strongly that the single plane intake manifold was correct for this engine, but agreed to swap the headers, although I wanted to try a 2-inch primary tube instead. He reassured me that my pipes were way too big and even called the folks at a local header company to get a set of their 1-3/4-inch tubes with 3-inch collectors. I resentfully bolted them on and gained over 30 lb-ft of torque! And, as I explained earlier, torque = horsepower, so the gains in power across the board were staggering. Right away I swallowed any last ounce of pride and bolted on the dual plane intake manifold as quickly as I could. Whammo, another huge increase (over 25 extra peak hp and as much as 20 ft-lbs more TQ)!

After another day spent re-tuning the engine for its new components, I finished the session making almost 480 hp, and I started out making under 400 hp. Still a far cry away from the 600 hp I’d been expecting, but those three days on the dyno taught me a lesson that I needed to learn again and again. I needed to pay closer attention to the experts that had already learned such lessons, and to what I already know. I have gone through many years and literally thousands of dyno pulls, and I still learn something every time I test. Now, I have a tendency to be too conservative in many of my parts selections, only to find more power when I bolt on slightly bigger parts. It’s come full circle and I’m moving towards the bigger and better parts as I build 400-ci small-blocks that produce over 600 hp now!

This tech tip is from the full book, DAVID VIZARD'S HOW TO BUILD HORSEPOWER.

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