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Discussion Starter · #1 ·
Partially mine partially pinched form a friend read on! "with permission"

There is a lot of talk around the industry about dyno’s and their accuracy. Just how accurate are dyno readings? Why does the same car get different results on different days? Why does the same car get different results on dyno’s from the same manufacturer and as well as from other different manufacturers? One needs only to peruse some of the "forums" on the Internet, to see that there can be significant differences in the results produced by the same vehicle!!!

But some people say that the accuracy of a dyno is not as important as its ability to reflect the outcome when modifications are made to a vehicle, and its repeatability. Others say a dyno is only useful as a tuning tool and does not need to be relied upon to be accurate. Is it any wonder people get confused? Read on to get an understanding of the reasons behind these variations in results.

Once upon a time, there were really only 3 key reasons why a vehicle could show a different power reading on the same Dyno at different periods in time, they are:

1. The Dyno was not correctly calibrated.
2. The Atmospheric (Weather) Conditions had changed.
3. The Vehicle had a problem that affected its Power Output.

Early chassis dyno’s were only able to apply load, and subsequent models were extended to measure some basic outputs like torque, power and speed. The dyno operator had little influence over the dyno and had to accept the results that the dyno produced. Now, would it not follow that if the vehicle was unchanged and weather conditions were constant, that one vehicle could be expected to produce at least similar results on several of the same brand of dyno? Or all dyno’s for that matter?

Is incorrect calibration likely to be a factor? Perhaps, but with modern dyno hardware, a dyno is not likely to lose calibration through normal use. Dyno’s that are available these days have mostly evolved into state-of-the-art diagnostic tools and keep pace with the high-tech advances in motor vehicle technology. The better quality dyno’s can now generate vehicle specific information and a level of accuracy that was not conceivable even a few years ago. Faster computers and the flexibility of the Microsoft Windows
Operating system has given dyno programmers a far more powerful tool than they have ever had before.

So what other factors are involved? We would surely all agree that any dyno manufacturer worth their salt is interested in producing accurate and honest results from their dyno. An intrinsic part of producing accurate, honest and consistent results from a dyno requires that the dyno dictates the power produced by a vehicle without undue influence from the dyno operator. Let's look at how results can be influenced.


The evolved dyno operator of today has much greater influence over the results produced on a dyno than ever before, and the dyno operator has become another reason why a vehicle can show different power readings on the same dyno at different periods of time. Is it possible that at least some operators would mislead a customer about the extra power produced by an aftermarket add-on or a performance tune if they were able to do that? In a market place where more power seems to be better, would it be in a dyno operator's interest to satisfy a vehicle owner's desire for high power figures? Many dyno manufacturers have ensured that the "operator factor" is eliminated as far as possible, meaning that they lock the operator out of areas of the dyno controller where results can be "influenced" and follow a policy where all relevant dyno run factors are displayed on run files and printouts so that any attempt that has been made to manipulate the results can be easily identified.

But not all dyno manufacturers have taken that approach. At least one brand of dyno has built in an easily accessible "correction factor" that the dyno operator can use to increase or decrease the results produced and so for example, in the hands of an unscrupulous operator, the "correction factor" could be used to mislead a customer about extra performance when no performance benefit was actually achieved. It could also be used to convince a vehicle owner that a vehicle generates far more power than it is capable of.


Another example of why sometimes people get curious dyno results from different dyno brands is because one dyno manufacturer may attempt to factor in Vehicle Inertia while another may not. Vehicle inertia is best explained as this. If you are accelerating a car, then part of the torque produced is consumed to accelerate all of the rotating components of the car, i.e. Wheel/Tyres, Driveshafts, and Gears etc. This is not Driveline Loss, Driveline Loss is the friction generated from gears, bearings, universal joints and Tyre to Roller contact.

There is nothing wrong with trying to account for vehicle inertia, provided it is used correctly. The problem with applying vehicle inertia is that not every car is the same. For example, if you were to change from 17" Rims to 19" Rims on a car, you have effectively changed the vehicle inertia, because with inertia the further the mass is from the centre of a rotating object, the more inertia there is.

So on a dyno that uses vehicle inertia correction, an operator may specify an inertia value for a vehicle, say for example a Commodore. But the available Tyre/Rim Combinations from early to late model Commodores can vary from 14" to 19" Rims. One model might also have steel rims while another has nice light alloy rims. It is easy to dismiss this as unimportant, however do not underestimate the difference in tyre/wheel combinations as the Tyre/Wheel is responsible for around 80% of the Vehicle's Drive Train Inertia because these items have a much larger radius than any other component in the drive train. A difference of 8KW's can be seen on a Subaru WRX by just changing from one tyre/wheel combination to another. The chance of over or under applying Vehicle Inertia is fraught with inaccuracy and can provide misleading results, and some dyno manufacturers choose to steer well clear of it. They take the view that at the end of the day, a dyno is measuring how much power is actually getting to road surface, and the power that gets to the road surface (Motive Force) will govern just how quick a car really is.


Modern dyno’s also have the ability to correct power on the basis of changes to weather conditions. A short explanation is needed here to fully understand atmospheric correction. Atmospheric correction is applied to compensate for changes to the combustive properties of the ambient air (the quantity of oxygen per unit volume) in an attempt to provide a level playing field between dyno runs. Atmospheric Correction Standards are defined by organisations such as SAE, ISO, DIN, ECE etc and each uses a slightly different way of measuring change. Using the widely accepted SAE J607 standard, on an ideal day when the temperature is 15 degrees Celsius, and there is 0% humidity, and the Barometric Pressure is 1015mbar, zero power correction is applied. Variations in any of these three atmospheric factors will either cause positive or negative power correction to be applied. If the temperature changed to say 19 degrees Celsius, the humidity to 34% and the Barometric Pressure to 989mbar, the conditions are not as ideal and the vehicle will not make as much power. By applying the SAE J607 atmospheric correction (in this case +3.89%), the power readings are corrected to what the vehicle could be expected to make on an "ideal" day.

The amount of power correction applied to a vehicle always needs to be an accurate reflection of how much power is actually lost by or gained by the vehicle as a result of unfavorable weather conditions. The SAE J607 standard specifies a maximum ceiling of 10% power correction, on the basis that any power correction in excess of 10% will produce a power figure that cannot necessarily be reproduced by the vehicle under optimum conditions.

So, a dyno that can accurately correct vehicle power according to changed weather conditions should produce more consistent and accurate results from run to run. A dyno that cannot correct vehicle power according to weather conditions or where weather conditions have not been regularly updated can produce results that vary considerably from run to run. A 5ºC inaccuracy in air temperature can lead to a 0.9% change in power figures. A 3 mBar (normal range 900 to 1050 mBar) inaccuracy in barometric pressure give a 1% change in power.


Follow these golden rules to limit the variations in results between dyno runs and give you a better idea as to whether the results of a dyno run can be relied upon to be accurate:

• Dyno Printouts should always clearly show which atmospheric correction standard was used (e.g. SAE, ISO) AND how much atmospheric correction has actually been applied to arrive at the end result.
• Be wary of any dyno that uses uncapped correction, as it can produce figures that can never be reproduced even under ideal weather conditions. If the percentage correction that has been applied is not specified, there is a good chance that uncapped correction has been applied.
• If the dyno uses the air intake probe temperature to calculate atmospheric correction, make sure that the probe is not improperly placed during the dyno run, and check that the ambient temperature and the air intake temperature on the printout are not unreasonably different.
• Use a dyno that provides automatic correction from an inbuilt weather station, or make sure that the atmospheric conditions have been updated just prior to the dyno run if the dyno cannot provide automatic atmospheric correction.
• Make sure that the operator uses consistent test parameters (gear, start speed, end speed, ramp rate) for multiple runs so that any variations in results are from the vehicle and not the way it is tested.

862 Posts
Discussion Starter · #2 · (Edited)
Here is the story behind Dynojet, and why they probally read a little higher than other dynos. If you dont want to read the whole story scroll to the portion I made bold.
The Story Behind the Dynojet Chassis Dyno – The Truth Meter

It’s a story as old as hot rodding itself. It starts with the sales pitch-”Buy my widget and your engine will gain 50 hp”-and ends with a disappointed customer with a car that sure doesn’t feel like it picked up 50 hp. A dragstrip doesn’t offer much proof one way or the other on incremental changes because there are too many variables involved, so the seat of the pants was for a long time the only way to tell if a modification or part really helped. That all changed in the ’90s with the introduction of the Dynojet, a portable chassis dyno that was in the financial grasp of most every mom-and-pop performance shop. Finally, power claims were proved or disproved as soon as the stuff was installed. And with the emergence of custom computer tuning, the Dynojet has proven invaluable to these shops; they can now tune the car without ever having to blast up and down a city street. They can thank Mark Dobeck, the machine’s creator.

Dobeck got his start tuning English sports cars in a Portland, Oregon, garage in the late ’70s. He had hot-rodded the shop’s Sun infrared exhaust analyzer to improve response time and became a wiz at using exhaust-gas carbon monoxide to optimize power on the go. The trouble came later when he moved on in 1980 to open a motorcycle shop in Wisconsin. Cars were one thing, but there was no way to haul a gas analyzer as big as a TV set on a motorcycle. So Dobeck talked his inventor/fabricator father into building a stationary rolling-road that could support the rear wheel of a motorcycle on a moving drum so he could continue tuning while “driving” with the big infrared analyzer.

The rolling road was designed with a hydraulic system that could be adjusted to work a bike engine harder at a given speed, something like the resistance controls on a Stairmaster machine. But because Dobeck and his dad were mechanics rather than mathematicians, they made the rolling drum heavy, and the homebuilt dyno had a surprising amount of inertia. It was accidentally pretty good at simulating a motorcycle’s ability to accelerate.

Dobeck’s new bike shop opened just in time for the arrival of Japanese superbikes equipped with constant velocity (CV) carbs, which were new to motorcycling. CV carbs provided good performance, economy, and emissions, but they could not be tuned and jetted using traditional methods. Many people recommended replacing them, a $600 solution. But Dobeck understood CV carbs from the days of wrenching on English cars and modified them to allow the new motorcycles to run with performance pipes and air cleaners. Before long, bikers were traveling from all over the upper Midwest for Dobeck’s dyno-jetting service. Meanwhile, in the evenings, Dobeck read magazine stories of hot rod bikes running exhaust-system shootouts on the torque-cell dynos of famous California super tuners.

Performance magazines loved dynamometers because they brought science to hot rodding. But torque-cell dynos, which load an engine by forcing it to pump water or generate electricity, are expensive, and using them has often required removing the engine from the vehicle.

“I started to realize I was doing something that no one else was doing,” says Dobeck, who was using his homebuilt inertial dyno to tune bikes with the goal of improving acceleration and responsiveness. “Eventually I built a few jet kits to see what we could do with them.”

Dobeck named his company Dynojet. His first big customer was K&N Filters, and it wasn’t long before he was selling lots of jet kits. His company grew at a rapid pace, and sure enough, a competitor sprang up with a similar product. “Their advertising was working,” Dobeck says. “They were taking away sales. But the product didn’t work. Not at all.” To prove it, he called several of the top engine-dyno suppliers to see if they would help him develop an affordable version of his homebuilt inertial chassis dyno that could live in the shops of Dynojet dealers to show the world what worked and exactly how well. “Every one of them laughed at me,” Dobeck remembers.

One of the biggest headaches of Dynojet’s go-it-alone chassis-dyno project was figuring out how to assign meaningful power numbers in the face of unknown inertia from the moving parts of the hundreds or thousands of engine, drivetrain, and tire combinations. Wrestling to fully understand inertia and powertrain losses, Dobeck and his team quickly realized that the standard physics formula of weight, time, and distance for converting acceleration into horsepower simply didn’t work-the derived number was always lower than accepted numbers. They poured on resources and burned up time and money investigating it, but no matter what they did, the math never added up.

Dynojet’s final number-fudge was arbitrarily based on a number from the most powerful road-going motorcycle of the time, the ’85 1,200cc Yamaha VMax. The VMax had 145 advertised factory horsepower, which was far above the raw 90hp number spit out by the formula. Meanwhile, existing aftermarket torque-cell engine dynamometers delivered numbers that clustered around 120. Always a pragmatist, Dobeck finally ordered his Chief Engineer to doctor the math so that the Dynojet 100 measured 120 hp for a stock VMax. And that was that: For once and forever, the power of everything else in the world would be relative to the ’85 Yamaha VMax and a fudged imaginary number. Dobeck’s engineering staff was dismayed by the decision, but the Dynojet 100 exclusively measured surplus power available to accelerate the vehicle’s mass-no more, no less-and that was true even if the modification was a low-inertia flywheel or lightweight wheels. As long as the inertial dyno’s numbers were repeatable, the critical question (did a particular modification make the engine accelerate faster or slower?) would be answered correctly.

Dobeck then turned his attention to providing the dyno to bike shops across the country. The first 20 early adopters of Dynojet kits were customers who had defeated the replace-your-CV-carbs drumbeat seven years earlier. “These guys believed in what we were doing,” says Dobeck. “I called, said I’ve got this dyno, and it costs $6,500. And they said, ‘Send it.’”

When a small network of the most important dealers had dynos, Dobeck took to the road with a mobile bike dyno mounted in a trailer. He would ask performance-shop owners, “Aren’t you sick of being the scapegoat for stuff that doesn’t work as advertised?” They were, and they started buying dynos. In subsequent years, Dobeck demonstrated his bike dyno everywhere from Montana to communist China. Then he took on the world of cars.

The pre-Dynojet world of hot rods circa 1993 had a lot of information, misinformation, and disinformation. You can’t feel a 5-10hp boost on a car, so many engine modifications were faith-based efforts made with a screwdriver and a prayer. Hot rodding had left more than a few hapless victims with fading dreams of glory and empty pockets. The onset of computerized engine controls in the ’80s made increasing horsepower even more complicated-escalating the opportunities for the unscrupulous or incompetent to fleece those with the need for speed: Install this electronic doohickey, double your power. Car guys needed a cost-effective, repeatable B.S. meter every bit as much as bikers. Dobeck hired his dad and put several engineers on the project to handle critical design issues and the team constructed the original Dynojet 248A using two 48-inch-diameter, 1,200-pound rollers, later increased to 1,600 pounds.

When it came time to market the new car dyno, Dobeck realized that although his company was big-time in the motorcycle universe, no one in the door-slammer crowd had ever heard of him. So he went on the road again. The import crowd embraced the new Dynojet first, since they were the victims of a lot of bogus power claims from unscrupulous manufacturers. Then Dobeck visited some of the bigger aftermarket companies. The Dynojet often brought bad news to hot rodders and manufacturers-now everyone on the street knew exactly how much power the parts were worth. But the good news was, in the right hands, the dyno could find “free power” through tuning 8 out of 10 times.

With the automotive aftermarket sold on his Dynojet, Dobeck wanted to relax. By 1996, he was running on fumes and on the road way too much working like a madman. “I had no normal life,” he says. An investor group was looking at buying the company, but it was on the fence, so he chased after a NASCAR licensing agreement. Back in the trenches he went, this time to offer his Dynojet to the NASCAR teams in North Carolina. They bit, and after a while, NASCAR agreed to his humble terms and made it “The Official Dynamometer of NASCAR” for three years. The NASCAR teams bought dynos, and Dynojet designed fabulous NASCAR chassis-dyno rooms that purportedly generated six times the revenues of the dyno itself. At that point, he sold the company for six million dollars in cash.

Over the course of his 27 years of work, Dobeck helped make hot rodding more honest. Performance consumers now expect to know dyno results for speed parts, and dyno tuning and development has become essential for serious racers and hot rodders. Chassis dynos from Mustang, Superflow, and others now provide an alternative to Dynojet, but Dobeck’s little bike dyno is the one that started it all.

What’s he doing now? Dynojet was recently sold again, terminating Dobeck’s non-compete clause, so he’s back at it with Dobeck Performance. He reassembled technical talent from the old Dynojet days and has created a handheld gas analyzer (The Sniffer) and a computer interceptor that allows fuel tuning in an EFI car or bike (The Fuel Nanny). He’s also looking at a new chassis dyno based on proprietary patented inertial and torque measurement technology. Meanwhile, he’s on the road, as always. “Again, I did the routine that works: I put myself right out there in the pits, at the track level, playing around.”
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