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Real benefit of eddy current dyno?

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Hello guys! I understand that on eddy current dyno you can set certain rpm and load to tune ECU, but is it only time saving option? On inertia dyno you can also apply every tps (now i am talking about ITB motorcycle tuning) logging AFR and then just correct fuel. It won't be as convenient as on load dyno, of course, but it's possible, right?

There are many types of load-controlled dyno mechanisms, and the eddy current brake is only one of them. For example, the Dynapack hub dyno uses hydraulic pump control.

There is no comparison between the convenience and speed with which you can tune using a load-controlled dyno and an inertia dyno. With a load-controlled dyno you can do everything from hold a specific RPM / speed, to control the rate at which the engine is allowed to accelerate. On an inertia dyno, this is completely controlled by the inertia of the rollers and the torque output of the engine.

If using an inertia dyno to do part-throttle tuning of ITB's, you really need to have an adjustable / repeatable throttle stop, as you will be taking many, many runs and you want to the throttle position to stay the same. Being off by a percent or two changes everything.

I have 2 dynos (hub and engine), both of which are load bearing. As David mentioned my Dynapack's are hydraulic with the option (sometimes requirement) of water cooling. I can safely tune most of the lower power race setups I build on them without active cooling through a recirculating 200 gallon tank, however, on high(er) boosted/torquey engines I can maybe do 2-3 pulls without tripping the overheat limiters/alarms/etc so in comes the need for cooling.

My other dyno is an older Superflow water brake/shear setup using the same 200 gallon tank and a giant heat exchanger that I can again run a smaller ~175 ft lbs setup on for hours, whereas I can only run a 500 ft/lbs setup for about an hour before outrunning my cooling setup and needing to either dump ice/exhange water/wait for a day before getting consistent runs again.

An eddy current dyno worth anything shouldn't have issues with overheating/require nearly as much active cooling like mine do, however the results should be be the same: far more usable for real world and transient tuning than a roller/inertia dyno.

Thank you for the answers! For example dynojet i250 has eddy current retarder which requires 30A at max load.( 220 V) Am I understand right that max power it can hold is 220*30=6600 Watts? About 9 up... Not much

According to the specs on this page, while the dyno can require up to 30A input power, it is capable of handling 750 hp (about 560 kw), 750 ft-lbs torque (about 1016 Nm).

The operating power requirement, and the power/torque it can handle don't have a direct relationship with each other.

the “ professionals “ will laugh at this , but some of us have to adapt , i have an inertia dyno , but needed to be able to hold / have a load ( I have a Hayabusa powered land speed bike —- with M84 ecu ) I added 2 large discs to the dyno and a master cyclinder that i manually wind a threaded system up/ down , it works quite well , ( obviously the heat generated has an effect on a “ constant “ load )

I had thought of having the discs in an oil bath , but haven’t done that . the photo is my latest progress , the combination of the long arm to the the master cylinder, cable travel reduction and a very slow linear actuator ( remotely controlled). means i have very fine control over the applied load . I have only just finished this modification, and have done minimal testing

the dyno was using Dyno46. software, but that past its use by date , i am looking at YourDyno or the Australian DTec system . anyone use either of these ?

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Absolutely nothing to laugh at, it's very close to what was used in the very early "dynamometers" that were used to rate steam engines - it's why the old term was "Shaft Horse Power".

A dyno' doesn't measure power directly, but torque and the rpm at which it was produced. The earliest form used a simple loaded beam to measure the torque and water cooled rope wound around the propellor shaft that could be tightened to increase the friction and hence load. The rpm being generally low enough to make a count relatively easy, although other timing methods were also used. One of my textbooks at the time had a picture of this very test on a warship.

Many, many years ago my high school had something very similar to what you are using to illustrate the principles of torque, rpm and power. A vented (quite exotic at the time) disc brake was fitted to a short shaft driven off the flywheel and the caliper was mounted to a sring loaded free floating beam arrangement and had a screw adjuster on the master cylinder. As the pads gripped the disc the beam deflected and the amount indicated the torque, and a tachometer gave the rpm. This was on a small, post (pre?) war British 4 cylinder engine, so the heat buildup wasn't too much of a problem ;-)

Some people who race small engines still use exactly the same sort of tech for budget tuning, as any reasonably skilled fabricator can knock one up quite easily.

Steven, how did you recalibrate inertia in dyno software after you added brake discs?


AT this stage i only use the disc system to allow me to set load / rpm points in a map , the inertia system is its own system .

I am investigating how or if i can use the discs as a software controlled load system . The YourDyno advice is i may be able. to in corporate a load cell , i haven’t got my head around how that will work

the discs add a few kilos to the 200kg mass of the inertia roller . I am leaning to buying the DTec software , that will replace the old Dyno46 software, and is much more up to date . The disc s will still let me have a load capacity for engine “ problems “ or to still do mapping

the 2 discs are 300 mm ventilated large car discs , and the system easily controls / loads my 200 hp Hayabusa power

the discs get seriously hot —- as you would expect . it is certainly a simple and inexpensive load system . as i said previously, i had thought of having the discs in an oil bath ( more constant , controlled load ) but i am very happy with the outcome as it is


Thank you for sharing your experience! I've never heard about dyno'46. I am planning to use sport dyno software. Have you heard about that? What's diameter of your rotor? You mean brake discs affect inertia just a bit, so you don't have to consider that factor?

Dyno46 is a Malaysian package that was about in 2004/5. it wasn’t produced for long , i have struggled to keep it working ,and find any

information on it . the inertia roller i have is 700 mm x 320mm in diameter, it weights 200kg s . Funny story , i had my hayabusa in 3 gear doing maybe 134 / 140 ks and the balancing weights on the ends of the roller came un welded , me , the bike and the dyno went bouncing around my workshop ——— great fun

Wow, its dangerous. Do you have FB or instagram?

My wife thought there had been an earthquake.

im not on facebook , i look at intstagram ( but don’t post on it )

getting the roller re — balanced ( to do about 3500rpm ) was done by a business that balances large electric motor armatures for the mining industry

Haha,Steven,that's fun. I am interested if its possible to make close loop system with car brake to hold certain rpm.. Have you though about that?

By the way, how long brake pads last in this car brake dyno?

With eddy current dyno's, something that's been buzzing away in the back of my mind for years would be if it's possible, and practical, to feed it back into the main grid to offset the power bill, or even for a bit of a cash return?

Depending on how much use they get, the power levels of the vehicles, and the power charge rate, it could be several hundred dollars a week - a few thousand a year, which would help offset the TCO for the machine.

Hi Gordon,

I spoke to one of the electrical engineers here about the use of the eddy current dyno to generate energy to feed back into the grid, and his answer was that it wouldn't work. This is due to the coils needing to be energised to generate the magnetism in the disk needed for braking, and if they are not energised, then the magnetic fields that are needed for the generation of power do not exist.

Uh, might be a mis-communication there - probably on my part, it seems.

While there is a current required for running the dyno', there is also a LOT of waste energy that has to be dumped that is produced by the brake that has to be dumped - it's basically a variable load electrical generator - and most systems seem to use it to heat water (possible heat exchanger for HVAC heating of the building?) or otherwise fail to make use of it.

It would seem what I was thinking of was an "AC Dynamometer", which seems to be intrinsically suited for feed the grid, but the models I have found seem to be rpm limited, power limited, or both. They are also expensive and the ability to power the engine, which would be very useful for establishing frictional and pumping losses, for example, may fall short of justifying the expense for most.

Gord, the only thing an eddy current retarder generates from the absorbed torque/power is heat. The vast majority of chassis dyno eddy brakes are air cooled so the heat is generally just rejected to the atmosphere and is not easy to harness for anything useful. If you had a water-cooled one in theory you could possibly recover the heat from the water and convert that into some more useful form - but the path to generating electricity from warm water is a long one...

As you suggest, AC dynamometers can be and often are connected to the grid. If you dont connect to the grid they need a massive load bank of resistors to dump the energy which I imagine is not cheap either. Typically the AC dynos are only used by big OEM's though due to the cost - both the absorber and all the control gear is big $.

I have read an article a while ago about one of the OEM's that was using the dynos to directly power all the lighting in one of their production line buildings. I cant remember who that was though.

On a more down to earth level there are also other variations of the theme such as the original rototest hydraulic hub dynos that used the hydraulic fluid power from the dynos to drive the cooling fans and dyno room ventilation. I think their newer electric hub dynos are re-generative.

Yup, I had been conflating two different variations.