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Dyno Differences - Measurement Technique

Entrenched In Science, Dynos Have Personalities All Their Own

Evan Griffey
Oct 5, 2010
Photographers: Charles Trieu, Sean

The chassis dyno has revolutionized engine tuning. These sophisticated machines accurately plot an engine's power curve as measured at the drive wheels by unleashing the genius of Sir Isaac Newton in a manner accessible to everyday enthusiasts. How does the magic happen?

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If we were to infiltrate a dyno's microchip we would see a data stream of physics starting with Sir Isaac's second law of motion, namely Force=Mass x Acceleration with Mass being determined by the dyno and the acceleration provided by the car measured in time. Next the machine's cerebral cortex would figure Work by using the Force in the calculation Work=Force x Distance with the Distance being determined via the circumference of the rollers. Power is ascertained by Power = Work divided by Time with Time being a variable revealed in the first calculation. Power in this case refers to torque so horsepower is calculated via Torque x RPM/5,252. There it is wheels-to-the-road horsepower measured in milliseconds.

This is leading edge stuff and it is no surprise that tech-savvy import enthusiasts have been leading the charge. In the early '90s, Dynojet put a unit on the back of a big rig and one the first stops was the Battle of the Imports at Los Angeles County Raceway in Palmdale, California. Soon shops like Dynamic Autosports, Paisley Automotive and Road/Race Engineering had in-house dynos.

There is more than one way to skin a cat and more than one way to measure horsepower at the wheels. While the basis of the calculation remains true in theory, the manner in which dyno machines go about getting that info differs.

Tuning Technique-Using The Tool
Bisi Ezerioha of Bisimoto Engineering is a big fan of the Dynapack. "It is extremely important to have partial throttle tuning ability when tuning a road race car or street car," says Bisi. "We spend much more time in that partial throttle environment than we do at wide open throttle. With the Dynapack I can speed set tune the engine by running it to 2,000, 2,500 or 5,500 rpm and leave it there then work on timing and fuel. I have the capability to tune every single cell or map. This kind of control manifests itself in the drivability of the car. It will have an OE feel in normal driving situations but hard-hitting tuned performance when pushed."

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Bisi is also quick to point to the Dynapack's repeatability. He says tire deformation, tire growth, wheelspin and even alignment issues in front-wheel-drive cars can skew results in roller style dynos. Alignment with too much toe-in or toe-out will cause what Bisi calls 'power scrubbing' where the friction of the misalignment results in an unrepresentative reading. Since the Dynapack mounts at the hubs, the tires are taken out of the equation. He was also quick to point out that since gear ratio is an input item, this variable is also off the table. He also says the unit is quiet, allowing the operator to hear the engine better, the cabinets can be moved out of the way opening up more room in the shop and since there is no strapping down a Dynapack is cheaper to insure.

SP Engineering has both a two-wheel drive inertia Dynojet and an eddy current four-wheel drive Mustang. SP's owner Alex Shen says newer, modern high-tech cars can react adversely to the loading effects of dynos. Irregular loads can trigger the car's electronic traction control system, which can severely skew results, render the drivetrain unreadable and possibly cause damage. "The Porsche 911 997 and Audi R8 aren't worth the dyno fee, they are truly unruly", says Alex. "The GT-R, Evo X and Subie STI light up like a fireworks show but there is no impact on performance or danger to the driveline and we test them all day long."

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Mustangs are known to read low and Alex knows why. He says it's the factory roller weight settings. "I was floored when my Nissan GT-R baselined at 374awhp," quips Alex. "This is way out of line, I mean the car is rated at 480 and the transmission is quite efficient. The roller weight is an assignable item so I played with the weight numbers until I found what I think is the sweet spot. Stock GT-Rs make between 395 and 405 on the dyno now. That's a conservative number accounting for 17 percent drivetrain loss." He says at Mustang factory settings an Evo X puts down 228, a V12 Murcielago 430, despite a 640 rating. Yep, the egos are getting battered pretty good.

Alex has no problems with his inertia-style Dynojet because he tunes to boost. He says turbo cars are difficult to hold steady state so he tunes at full boost first, then addresses drivability or part throttle performance by map trace tuning at wastegate spring pressure, i.e. low boost and/or other low boost setting. This gets the job 95- to 97-percent done. The final step is a few laps on the street. Most of the street runs are at speed throttle tip-in runs, nothing high-speed although Alex does admit to a good working relationship with the local PD. Up last is what Alex calls 'what if' runs, consisting of bogging the engine by accelerating from 35 mph in sixth gear and other antics of a bad driver.

Keys To Success
The key to power tuning nirvana is in the rollers for sure but it still takes skill. Beyond the challenges of electronic traction control personality control can be a real downer. "Dynoing is really a headache these days," says Alex. "The Mustang is a heartbreaker because it always reads lower. The Lambo guys get mad and all bent out of shape because they don't have big dyno sheets to brag about. They usually storm off telling us to 'fix' our dyno. Porsche guys whine about how our fans aren't good enough, can't duplicate vehicle speed on the street. They are all like little girls. I know that and the fans aren't trying to duplicate anything. The dyno is a tuning tool, the size of the number they print out is irrelevant and the difference between 'before' and 'after' is all that counts."

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The best advice is to pick a shop/dyno facility carefully and stick with that dyno for your entire build-up. This will ensure you get the most comparable results, best overall tuning experience and most responsive and powerful engine under your right foot. And power on the street, not the rollers, is what the tuning experience is really all about.

Reading Dyno Charts
The modern chassis dyno is a tuning tool not a bragging rights dispenser. Too often the number at the top of the graph is as deep as most enthusiasts look.

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The first order of business is understanding your engine. Where in its rev range does it make power, in your typical driving routine where in its rev range does it spend most of its time. This is where you need to direct your tuning attack. If your engine spends a lot of time between 4,000 and 6,700 rpm adding a set of cams that produce peak power at 8,000 rpm makes no sense. To intensify the driving experience you need to make improvements in the low- to mid-range of the powerband.

The chart is basically an x-y axis graph with the y-axis containing output numbers while the x-axis represents either engine speed or vehicle speed depending how the dyno is set-up. The chart illustrates both horsepower and torque in a different color. More than one run can be plotted on a single chart to compare tune changes or part additions. A cursor can be scrolled to see the gain difference at different points on the graph. In some scenarios boost pressure and air/fuel ratio can also be charted, giving the tuner a better view of what is happening within the engine.

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Here we see before and after runs starting with a baseline stock run. The cursor is positioned to show the biggest gain, 76 whp. This chart also includes air/fuel ratio and boost pressure data logging. Note how the AFR drops off the chart just past 6,000 rpm and how boost tends to taper off as the run progresses; both tell-tale habits of a 4G63 Evo.

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This staged testing chart shows runs for each component. 1) ECU tuning leans out the factory 'flood gate rich' tune, unleashing big power. It's good to have the power but also remember how often does the engine see 7,100 rpm to use it? 2) The red and green plots are for an intake and exhaust. Since these mods reduce factory restrictions they tend to make their best gains in the upper rpm range where flow becomes an issue. 3) Typically a boost up, from stock to 26 psi in this Evo IX, will bump the curve uniformly throughout the rev range. Upgrading the turbo would result in a decidedly different plot with different spool-up power on the low end and significantly more power on the top.

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Look past the peak. These two turbos are pretty much a dead heat on the top end (box) but the red line of the graph shows a much quicker spooling turbo that provides a 30 whp gain at 4,500 rpm and a 35+ whp gain at 5,500, (shaded area) right in the engine's sweet spot. The red line engine will generate more thrills in the real world.

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Dyno Differentials
Dynos read different, there is just no escaping this fact. The accompanying chart illustrates the readouts from a mildly boosted up Evo IX. All runs were made on the same day except for Dynojet-2, which was run earlier but included to show how two of the same make of dyno compare.

Mustang 292.7
Dyno Dynamics 318.6
Dynojet 312.5
Dynapack 324.4
Dynojet-2 315.9

Measurement Technique-Inertia
The early Dynojet units were called inertia style dynos, in that they measure power by using drums of a given circumference and weight and calculating the time it takes to accelerate the rollers/drums to a certain speed.

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Some of the shortcomings of these machines were their inability to perform steady state tuning where the engine is revved to a certain point held in check and tuned on. Inertia dynos are all or nothing, a run is a full-throttle blast to redline and nothing else. Dynojet's have evolved since the early inertia-only days... more on that later.

Measurement Technique-Eddy Current
Eddy current dynos rely on an intense magnetic field to create the 'Mass' or resistance needed to make proper calculations. The eddy current dyno uses iron discs in its inner workings. These discs are subjected to a magnetic field to generate resistance and the field can be controlled to provide more or less resistance. Since the eddy current dynos allow for sustainable resistance the engine can be held at a steady speed/load for closed-loop tuning. Popular eddy current dynos include Mustang, Superflow, Dyno Dynamics and Dyno-Mite.

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Measurement Technique-X-Factor
Today Dynojets can be ordered as a dual-purpose unit incorporating an eddy current set-up. Denoted by an LC for Load Control these dynos are outfitted with eddy current load absorption units to provide all the capabilities of an eddy current with the ability to be quickly run in inertia mode.

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Measurement Technique-hydraulic
The Dynapack is the leading example of hydraulic-style chassis dynos. These dynos use a hydraulic pump, control valve and fluid to generate resistance. The Dynapack consists of cabinets that contain the hydraulic brake system. Hub adapters that bolt directly to the test vehicle in place of its wheels are used to connect the car to the dyno.

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By Evan Griffey
272 Articles

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