Do Dynos Lie?

How Dynos Work and What You Need To Do To Get the Best Power Numbers Possible

Jason Sands
May 1, 2012
Photographers: Jason Sands
One of the most common ways of measuring a diesel truck’s horsepower and torque is running it on a chassis dynamometer, known as a dyno, for short. Perhaps the simplest type of dyno is an inertia dyno, which is a device that measures the time it takes a vehicle to spin a large set of steel rollers. Using a mathematic formula [Force = Mass x Acceleration (F=MA)], the dyno’s computer spits out a horsepower number. Most dynos will also provide a torque number if the engine speed (rpm) is measured. Dynojet 248s are the most commonly used inertia dyno.
Photo 2/7   |   A few years ago, Richard Brown found that adding a Gear Vendors overdrive unit was worth almost 50 hp on an inertia dyno. It turned out that dynoing a truck in double overdrive loaded the engine so much it made more power. If you don’t put a modified engine under full load, it won’t make its peak power. With a best of 1,218 rwhp, we’d say Brown knows what he’s doing.
The second type of commonly used dyno is called a load-cell dyno, which uses a brake on the rollers to apply a known amount of resistance. These dynos can apply enough force so that, in many cases, the vehicle can be run at a specific engine speed (and held there) while at maximum power. Mustang and SuperFlow both make load-based dynamometers that are commonly used by the diesel performance industry.
So which kind of dyno should you use to test your truck? And more importantly: How accurate are they? Well, that’s what we’re here to discuss!
Running On an Inertia Dyno
A lot of people approach an inertia dyno with a What is there to know? attitude, but there are still some tricks one can use to get the best number out of a vehicle. Doing the run in the tallest gear available will put the truck’s engine under the most load, so doing the run in an overdrive gear will probably give the best power output number. Be careful, however, as you do not want to overspeed the tires or driveshaft. If you’re unsure about the condition of your driveline parts, try to limit dyno speed (truck’s wheel speed as read on the speedometer) to 100 mph.
Photo 3/7   |   Sometimes the numbers don’t add up, as was the case with this 6.0L-powered Ford. Though the truck’s dragstrip trap speed would indicate about 550 rwhp, it never dynoed more than 441 rwhp. We’re still scratching our head as to what the real power number would be.
Another technique to get the power number to be larger is known as brake boosting. This is a strategy in which the driver applies the truck’s brakes while rolling into the throttle to get the turbo to spool sooner than normal. Once the turbo is spooled, the brakes are released, and the dyno test is started. Brake boosting will usually give better torque numbers and often also better power numbers if the engine continues to make boost throughout its rpm range.
Running on a Load-Cell Dyno
While load-cell dynos are usually more costly and complex than inertia dynos, they are often better for diesels because the trucks can be loaded down to the point of achieving full boost. Basically, load-cell dynos do a better job of simulating the mass (in this case, a heavy diesel truck) the vehicle has to move. On a load-cell dyno, the engine is typically operating in a more real-world scenario. This is especially important for trucks with large turbochargers, as inertia dynos often won’t be able to load the truck enough to achieve full boost. Load-cell dynos can often also be run in inertia mode, timed mode, or load-percentage mode, giving a wide range of tuning abilities. When running on a load-cell dyno, also try to dyno in the highest gear possible to avoid tire slippage.
Photo 4/7   |   When Nick D’Amico’s turbo blew at Diesel Power Challenge 2008, everyone was sent running for cover as hot turbo parts blew out the stack and into the crowd. Standing right next to a truck at a dyno event is rarely a good idea, even if it is fun.
Do Dynos Lie?
Well, yes and no. Dynos depend on many different factors, including the dyno operator, type of dyno, and truck involved. We’ve seen one hopped-up Duramax make 490 rwhp on a Mustang dyno, and then make 530 hp on a SuperFlow on the same day. Which one is correct?
Photo 5/7   |   Four-roller Mustang dynos are known for wheelspin when trying to dyno diesel trucks. For the best traction, run trucks in Overdrive and use a light load setting.
If you talk to the dyno manufacturers, they will tell you they are both right. Changing dynos can be like changing torque wrenches halfway through a job. Despite the industry’s best efforts—there’s always some variation. The real power output of the Duramax truck in our example is likely about 500 rwhp, because comparing dyno to dyno is like comparing apples and oranges. Dynos should be used as tuning tools, and if you’re upgrading parts, try and stick with the same dyno to have a valid comparison.
How Can I Compare Two Different Dynos?
Correction factors can be used but should be taken with a grain of salt. At high elevation, horsepower losses can be seen (due to the fact that there is less oxygen in the atmosphere at higher elevation), but they’re usually only about half of what the actual correction calls for. Most of the numbers fudging we’ve seen comes from people using highly corrected numbers at high elevations. At elevations near sea level, any type of dyno will give you a fairly accurate power measurement, as long as the truck can produce boost on the dyno and doesn’t spin the tires. And remember, just because you made 450 rwhp on one dyno and your buddy’s truck made 460 rwhp on a different dyno, doesn’t mean he’s got bragging rights. Horsepower varies from dyno to dyno, and you’re not going to feel 10 hp in an 8,000-pound truck anyhow.
Photo 6/7   |   During recent dyno testing with our ’95 Dodge Ram, Triple Threat, we found its power increased as the load on the engine was increased. While it was only about a 20hp difference, it was a difference nonetheless.
If you don’t have a dyno in the area or you don’t feel like spending $75 to $100 for three runs just to get a number, there are alternatives. The most popular of which involves running a truck down a dragstrip and using trap speed to calculate horsepower. Based on your vehicle’s weight and rate of acceleration, rear-wheel horsepower can be calculated to a high degree of accuracy. Even if traction is an issue or the vehicle doesn’t shift correctly, trap speed should still remain fairly consistent. A lot of websites, such as, have horsepower calculators based on trap speed that are usually within 10 to 30 hp of actual dyno numbers.
Photo 7/7   |   Perhaps one of the purest expressions of horsepower is the dragstrip. As one truck enthusiast commented, “It’s hard to fake trap speed.”
In The End, It’s Just a Number On a Piece of Paper
The lesson here is to not get hung up on the fact that your truck makes a few less (or a few more) horsepower than your buddy’s when running on a dyno. Even if you’re on the same dyno, weather conditions can be different, the trucks may be driven differently on the rollers—even the temperature of the bearings for the rollers can change.
If and when you do put your truck on a dyno, try and use a shop that deals with diesels. Diesel dyno shops won’t get freaked out by the smoke, and they should know how to properly load your truck. In the end, if you don’t like the results, you can always improve your truck’s power—or Photoshop your dyno sheet.



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