How Much Should We Pay Attention to Dyno Numbers?

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Dyno

Numbers can be deceiving. We dive into the accuracy of dyno.

Mercedes, specifically their AMG in-house tuning arm, produces some of the most potent consumer cars on the planet. Not surprisingly, there’s a healthy community of fans dedicated to extracting more power from vehicles of all marques.

Google the name of your favorite car and ‘dyno sheet,’ and you’ll be rewarded with a plethora of forum threads, complete with PDFs on which a squiggly line indicates a car’s precise power output, as measured by a dynamometer, or ‘dyno.’ But how accurate is this number?

Dyno Bites

Conventional wisdom holds you should be able to compare one car’s graph to another’s and find out which one generates more power. The problem is, different dynos produce different power outputs, and manufacturers have made it a practice to underrate their cars since the industry standard shifted from gross to net SAE horsepower ratings.

To get the picture, you have to understand how dynos work. There are different types, and each one records horsepower differently. In fact, some people even use math formulas to compare measurements from the different types of dynos side by side, but it’s not an exact science.

You’ve Been Lied To

Horsepower can be measured two ways. You can run an engine outside of a car against a dyno to measure power called ‘at the crank,’ or you can measure power ‘at the wheels’ by mounting the car to a different kind of dyno. Power at the wheels will always be less than at the crank, because it takes some energy to move the car’s various drivetrain components.

Manufacturers typically market their cars using horsepower at the crank, but recently it’s become a popular practice to under-report how much power a car produces.

It is reasonable to assume about 15 percent power loss between what an engine generates at the crank vs. at the wheels in a rear-wheel-drive car. That means that with 503 crank horsepower, the AMG GT-S should produce about 430 horsepower to the wheels, but in fact, it produces approximately 480.

So, as you can see, there is no gold standard for horsepower ratings. Manufacturers sandbag these days, but they’ve done the opposite in the past, and no one, automaker or private shop, can completely standardize the way a dyno measures horsepower. The next time someone shows you a dyno slip, take it with a grain (or maybe a shaker) of salt.

Chime in with your thoughts on the forum!

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