Dynamometer accuracy, whats it all about

[PGS 4WD]

With the discussion regarding Dynamometer accuracy of late I decided to do some research into it, particularly due to the fact that the dyno figures off our dyno are consistently lower than that of other makes of dyno, this includes dyno days where people have had printouts from other dyno’s that read consistently 10 to 30 percent higher than we achieved with the same vehicle (no tuning changes). Obviously a concern I endeavored to work out what was the cause of the discrepancy between ours and other makes of dyno , in particular as all dynamometer manufacturers call their dynamometers accurate. What I found out is that it’s all about the standard.
For those out there who would like to understand dyno power figures, after doing some research, I have found some interesting documentation on the accuracy of dynamometers and the standards they comply to.
This is taken from one of the mainstream dynamometer manufacturers web site.
ATMOSPHERIC CORRECTION
Modern dynos 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

I wasn’t able to find what standard all the various dynamometers in this country comply to but for most commonly used it is SAE J607

So what is this standard and what does it mean?

From Standards international Web page http://www.sae.org/servlets/works/docum ... dOcDeTaIlS
Document Number: J607_198808

Revision Number:
Title
Small Spark Ignition Engine Test Code (Cancelled Aug 1988, Superseded by J1349)
Scope
The purpose of this code is to prove a uniform means of comparing small gasoline engines of 0.82 L (50 in3) or less in displacement and 15.1 kW (20 hp) or less in power output. It is not intended as a laboratory manual. Governor characteristics are considered part of the end item specification. Replaced with J1349

This would suggest that if you have a 22 year old engine of less than 0.82L capacity and 20 hp output then the average Dynamometer will be fine, otherwise I would suggest get on to one that complies with SAE- J1349, JIS-D1001, ISO- 1585, DIN-70020 and EEC-80/1269 such as that of Dynamic Test Systems. It would appear GM agree as they have commenced using the current standard, as documented here. http://www.sae.org/certifiedpower/details.htm
The Dynamic Test Systems Dynamometer, which is the one we use due to its accuracy, complies with these current standards. The same standards as vehicle manufacturers around the world use.
Any truly accurate Dyno must comply with the current standards for accuracy. SAE- J1349, JIS-D1001, ISO- 1585, DIN-70020 and EEC-80/1269. Anyone that wants a read the info is here.
http://www.land-and-sea.com/dyno-tech-t ... epower.htm
http://auto.ihs.com/news/newsletters/20 ... andard.htm
If you search Google on this standard it becomes quite clear that every major vehicle manufacturer is providing engine power figures based on this standard. Unfortunately most dynamometer manufacturers are content to let you believe the figures you are seeing are accurate and current even though they are using an obsolete 20 year old standard for small low power engines.
My thoughts were to have a dyno day for anyone who has doubts about the figures they have been told.
Happy reading

Joel

[jsttry]

a dyno is a tuning tool, just worry about the before and after figures rather than trying to compare different dynos and operators.

With the atmospheric corrections its easy to fudge a figure to make it look higher with some dyno models.

[Matt_85Lux]

a dyno is a tuning tool, just worry about the before and after figures rather than trying to compare different dynos and operators.

x2

I have had my car on 4 different dynos (with the same mods done each time) and have recieved 4 different readouts on power over a 20hp spread.

This is a good read as posted on the LS1 forums by one of the guys from HPF

Originally Posted by MattJ
I have touched on my despair of dyno figures and the importance most people put on the numbers at the top of the dyno sheet in a few posts over the years. I just thought I’d start a dyno Mythbusters post and pass on some of my findings over the years. This is something I have wanted to write for years as in my position of performance sales manager I have to deal with this sort of thing countless times a day. Not everyone will be happy reading this as a lot of people here and on other forums hold the dyno sheet close to the ego.

Also, I can see how some shops feel that they have to provide big numbers as the customer demands the same figure as his mate or some guy on the forum with similar mods. But in the end, it starts and finishes with the performance dyno shops.

I bought a BF Typhoon about 2 months ago, the previous owner had the car tuned with injectors and a cat mod. He was given a dyno sheet stating 315rwkw. I did not buy the car because of the stated power nor am I for a second suggesting the previous owner pulled a shifty, but when I got the car back home, and without touching a thing, it made 279rwkw on the HP-F dyno. Where the missing 35rwkw fits into the list below, I don’t know. But this is the sort of thing that can make a customer think that one shop is (potty mouth)(potty mouth)(potty mouth)(potty mouth) and another the kings.

No dyno reads exactly the same! HP-F have two dyno’s, they read different. A dyno is not a ruler where 30cm is always 30cm wherever it’s measured. Every dyno company and just about every shop will say theirs reads more accurate or more repeatable which is great, but the simple fact is that every dyno reads different and comparing one to another is folly. The difference between one run and the next on the same dyno is what you need to keep in mind.

There is more than one way to dyno a car! HP-F have had the privilege to be able to build and tune some of the tuffest dyno cars in the country, the dyno HP-F use to tune the big bangers is set up to make 1700+hp turn the rollers with minimal slip and when Dyno Dynamics wants there latest technology tested they come to us. Utilising this latest technology gives us no choice but to follow the rules when strapping down a car. The Dyno Dynamics dyno is calibrated to have the rear rollers stay in constant contact with the wheels all the way through the run, if they come off, the power number will be inaccurate and read higher than it should. Strapping down of the car on the dyno has a huge effect on the final reading, a car with loose straps will ride up on the front roller for most of the run and then normally just after peak power the car will sit back in onto both rollers, you can see on the graph when this happens as the power line drops, looking something like (and often confused with) faulty valve springs.

The dyno is only as good as its operator! There is quite a few other ways for the dyno to read inaccurately, the incorrect placement of the intake air temp probe is a beauty and incorrectly typing in Barometer reading will have a real effect. All these things are easy to avoid for the alert operator and easy to tamper with for the dishonest. Calibration and regular service can also help maintain constant, reliable numbers.

Different tyres will read different power! We once had a car make a lot less power then what we thought it should for the given mods, we got the car up on the hoist, searched high and low, checked compression, double checked the tune, dropped the exhaust, changed intakes, you name it we tried it. Nothing doing. One of the guys took a backward step, had a look at the car and suggested we should try different tyres. The ones on the car were brand new but what did we have to loose? We pulled a couple of wheels of Rob’s ute and bung ‘em on. The car picked up the lost power, and some… 25rwkw from a tyre change!! Tell me, how do you compare one car to another?

Each engine is different! Two cars with the same mods rarely make the same power, even in standard form most cars make different power on the dyno. Or what about auto or manual? High stall torque converters? These thing all effect the power readings. Again, how do you compare one car to another?

A tune for the dyno and a tune for the road! Simple really, a lot of times the final run on the dyno needs to be trimmed up to stop the odd ping found on the road test, big problem with XR6-Turbo’s. The main issue with this is that you pick up your car with X-power figure on the dyno graph and a few days later you dyno the car somewhere else and it reads low or, your mate expects what your makes. What about a race cam in a road car, you want to replace valve springs every 20,000 kays or do you want to flycut the pistons to fit it in?? Nothing wrong with either of these two options if your prepared to go through it, but to base you figures for a ‘cam only’ upgrade on custom shop set up’s is folly.

And Finally…
Don’t let the facts get in the way of a good story! How the car was run up on the dyno is another thing. Race fuel? Shhssh don’t tell anyone. The problem with that is that one day your going to have to repeat it and one day the car might be out of your hands and the questions will start. Disclosure of the facts is important as anything else if you want to be taken seriously and not live a life where everyone now doubts your every move. So how can you tell? Simply, you probably cant, but like life, if it is seems too good to be true, it most probably is.

Ultimately, the car makes what the car makes, its fine to have goals but the basis of these goals has to be reality. Sorry for the long post...

[turbo gu]

The best dyno is a drag strip

Numbers don't lie there!!! No point having a ton of horsepower if you can't get it to the ground!!!!

[KiwiBacon]

The bit I love is getting the gear-ratios wrong.
Leads to guys with 1600cc petrols thinking they've got 400Nm and guys who've just fitted a K$N filter thinking they've doubled the factory torque figures.

[PGS 4WD]

a dyno is a tuning tool, just worry about the before and after figures rather than trying to compare different dynos and operators.

x2

I have had my car on 4 different dynos (with the same mods done each time) and have recieved 4 different readouts on power over a 20hp spread.

This is a good read as posted on the LS1 forums by one of the guys from HPF

Originally Posted by MattJ
I have touched on my despair of dyno figures and the importance most people put on the numbers at the top of the dyno sheet in a few posts over the years. I just thought I’d start a dyno Mythbusters post and pass on some of my findings over the years. This is something I have wanted to write for years as in my position of performance sales manager I have to deal with this sort of thing countless times a day. Not everyone will be happy reading this as a lot of people here and on other forums hold the dyno sheet close to the ego.

Also, I can see how some shops feel that they have to provide big numbers as the customer demands the same figure as his mate or some guy on the forum with similar mods. But in the end, it starts and finishes with the performance dyno shops.

I bought a BF Typhoon about 2 months ago, the previous owner had the car tuned with injectors and a cat mod. He was given a dyno sheet stating 315rwkw. I did not buy the car because of the stated power nor am I for a second suggesting the previous owner pulled a shifty, but when I got the car back home, and without touching a thing, it made 279rwkw on the HP-F dyno. Where the missing 35rwkw fits into the list below, I don’t know. But this is the sort of thing that can make a customer think that one shop is (potty mouth)(potty mouth)(potty mouth)(potty mouth) and another the kings.

No dyno reads exactly the same! HP-F have two dyno’s, they read different. A dyno is not a ruler where 30cm is always 30cm wherever it’s measured. Every dyno company and just about every shop will say theirs reads more accurate or more repeatable which is great, but the simple fact is that every dyno reads different and comparing one to another is folly. The difference between one run and the next on the same dyno is what you need to keep in mind.

There is more than one way to dyno a car! HP-F have had the privilege to be able to build and tune some of the tuffest dyno cars in the country, the dyno HP-F use to tune the big bangers is set up to make 1700+hp turn the rollers with minimal slip and when Dyno Dynamics wants there latest technology tested they come to us. Utilising this latest technology gives us no choice but to follow the rules when strapping down a car. The Dyno Dynamics dyno is calibrated to have the rear rollers stay in constant contact with the wheels all the way through the run, if they come off, the power number will be inaccurate and read higher than it should. Strapping down of the car on the dyno has a huge effect on the final reading, a car with loose straps will ride up on the front roller for most of the run and then normally just after peak power the car will sit back in onto both rollers, you can see on the graph when this happens as the power line drops, looking something like (and often confused with) faulty valve springs.

The dyno is only as good as its operator! There is quite a few other ways for the dyno to read inaccurately, the incorrect placement of the intake air temp probe is a beauty and incorrectly typing in Barometer reading will have a real effect. All these things are easy to avoid for the alert operator and easy to tamper with for the dishonest. Calibration and regular service can also help maintain constant, reliable numbers.

Different tyres will read different power! We once had a car make a lot less power then what we thought it should for the given mods, we got the car up on the hoist, searched high and low, checked compression, double checked the tune, dropped the exhaust, changed intakes, you name it we tried it. Nothing doing. One of the guys took a backward step, had a look at the car and suggested we should try different tyres. The ones on the car were brand new but what did we have to loose? We pulled a couple of wheels of Rob’s ute and bung ‘em on. The car picked up the lost power, and some… 25rwkw from a tyre change!! Tell me, how do you compare one car to another?

Each engine is different! Two cars with the same mods rarely make the same power, even in standard form most cars make different power on the dyno. Or what about auto or manual? High stall torque converters? These thing all effect the power readings. Again, how do you compare one car to another?

A tune for the dyno and a tune for the road! Simple really, a lot of times the final run on the dyno needs to be trimmed up to stop the odd ping found on the road test, big problem with XR6-Turbo’s. The main issue with this is that you pick up your car with X-power figure on the dyno graph and a few days later you dyno the car somewhere else and it reads low or, your mate expects what your makes. What about a race cam in a road car, you want to replace valve springs every 20,000 kays or do you want to flycut the pistons to fit it in?? Nothing wrong with either of these two options if your prepared to go through it, but to base you figures for a ‘cam only’ upgrade on custom shop set up’s is folly.

And Finally…
Don’t let the facts get in the way of a good story! How the car was run up on the dyno is another thing. Race fuel? Shhssh don’t tell anyone. The problem with that is that one day your going to have to repeat it and one day the car might be out of your hands and the questions will start. Disclosure of the facts is important as anything else if you want to be taken seriously and not live a life where everyone now doubts your every move. So how can you tell? Simply, you probably cant, but like life, if it is seems too good to be true, it most probably is.

Ultimately, the car makes what the car makes, its fine to have goals but the basis of these goals has to be reality. Sorry for the long post...

I used to work with Matt while he was at G&D Performance. And I do certainly agree with what he is saying. Interestingly HPF use a Dyno with the old standard, a recent vehicle from their dyno with a printout of 334 rwkW made 292 rwkW on our dyno which complies to the current standard. Trying to explian to the general public where their 40 kW went isn't always that easy. For obvious reasons we always do a before and after run if we are making any changes.

[Chook350]

ramp rates also have an effect on the output, also strapin the car down. i dont fully understand the setup for our dyno as our boss is to tight to get us trained. howeva wen it was installed we were told ramp rates, and to make sure the car is strapd down to keep pressure on the front roller.

[chimpboy]

I assume they need regular calibration like any other precision measuring equipment as well... but I bet most places don't do it.

Are you saying the big differences are just down to atmospheric pressure compensation though? I would think that would only explain a few percent either way.

[ISUZUROVER]

The bit I love is getting the gear-ratios wrong.
Leads to guys with 1600cc petrols thinking they've got 400Nm and guys who've just fitted a K$N filter thinking they've doubled the factory torque figures.

Some results seem to report rear wheel HP and Torque (maybe because they haven't put any ratios in at all?)

My series landie has 7000Nm of torque!!!

[KiwiBacon]

I assume they need regular calibration like any other precision measuring equipment as well... but I bet most places don't do it.

Are you saying the big differences are just down to atmospheric pressure compensation though? I would think that would only explain a few percent either way.

Depends what the limits of the altitude adjustment are. If you could crank it to think it was around 2000m up, it'd be about 20% out.

[coxy321]

a dyno is a tuning tool, just worry about the before and after figures rather than trying to compare different dynos and operators.

I'm sure that we can all see where the operators are coming from, just as JOel has said in regards to a customer getting x-rwkw at one shop, and then y-rwkw at another shop.

I know from conversations with my mates in regards to dyno figures, all they care about is the end number on the printout. I don't think i've ever heard one of them say "Jeez, those new injectors and lift pump gave me a nice 10rwkw gain". Its all about "my car makes this much power, or that much torque.

[PGS 4WD]

ramp rates also have an effect on the output, also strapin the car down. i dont fully understand the setup for our dyno as our boss is to tight to get us trained. howeva wen it was installed we were told ramp rates, and to make sure the car is strapd down to keep pressure on the front roller.

Ramp rates do effect read outs as does the wheel speed due to windage, it gets complex as the two impact each other.

In regards to the rollers it depends on the dyno, if using a dyno where the front and rear rollers are connected via a belt or chain such as the Dynamic Test System it dosen't matter as long as wheel slippage dosen't occur. On the older Dyno dynamics dynamometers it was an old trick to strap the vehicle forward onto the front roller so it would come off the rear roller under power hence showing a higher power figure as the vehicles power is not being consumed by turning the rear roller. A higher measured output will occur.

Joel

Joel

[nastytroll]

I'm not sure how the US does their calibration as all the power figures from the US seem much higher then ours. I have conversed with several people in the US about the set up I'm running on my bike, all seem to get the same HP. They all say they have 160rwhp and mine makes 130rwhp, all are stock engines with the identical aftermarket carbs. After talking with the tuner here, he said all the specs they get from the states are much higher then ours and he cannot see how they can make such huge gains from such small mods, and their fuel has a lower octane then ours.

One of the aftermarket air cleaner manufacturers claim a 25% power increase from just a air cleaner change. I think they are optimistic.

As far as a tuning tool, my bike went from 105hp running real fat to 130hp with a good AF ratio. Big numbers mean squat compared to drive-ability.

[JrZook]

ramp rates also have an effect on the output, also strapin the car down. i dont fully understand the setup for our dyno as our boss is to tight to get us trained. howeva wen it was installed we were told ramp rates, and to make sure the car is strapd down to keep pressure on the front roller.

Ramp rates do effect read outs as does the wheel speed due to windage, it gets complex as the two impact each other.

In regards to the rollers it depends on the dyno, if using a dyno where the front and rear rollers are connected via a belt or chain such as the Dynamic Test System it dosen't matter as long as wheel slippage dosen't occur. On the older Dyno dynamics dynamometers it was an old trick to strap the vehicle forward onto the front roller so it would come off the rear roller under power hence showing a higher power figure as the vehicles power is not being consumed by turning the rear roller. A higher measured output will occur.

Joel

Joel

What sort of dyno's are we talking about here? Inertia or absorption? Yes in inertia dynos the ramp rate highly dictates the overall calculated power output as they measure the acceleration of a given mass in the rollers a static load. The absorption dyno's measure both speed and torque by using a dynamic loading system typically an electromagnetic brake (eddy current). This brake is connected to a strain guage load cell which measures the force moment. So given the distance between the brake and the strain guage, the measured force on the strain guage and the speed of the rollers connected to the brake both torque and power can be measured.

Ramp rate will also play a part in the maximum power output but no where near as critical and dependant as the inertia style dynos. This rate is controlled by the eddy current brake and the programs built into the dyno software (modes). So with all this yes it comes down to how the dyno operator runs his machine and the control system software running the brake. If a no bs maximum power output wants to be obtained then the vehicle needs to be gradually ramped up to where the power measurement stabilises or maximum speed is reached. All descent engine builds should be able to sustain a few seconds running at maximum output.

Im no expert in all these dynos out there and the way they all function but have been involved in designing an eddy current brake absorption dyno from the mechanical hardware, to the control circuits for the brake as well as the control software to measure the output power.

Dan

[PJ.zook]

Im amazed that dyno's still get strapped down to the rollers and hope they grip. Why dont they instead support the car, remove the drive wheels and bolt the wheel studs to appropriate gantrys that have easily swappable flanges, and can move side to side for almost any width of vehicle. Sure it would take a little more effort, but more accurate figures would be produced.

[KiwiBacon]

Im amazed that dyno's still get strapped down to the rollers and hope they grip. Why dont they instead support the car, remove the drive wheels and bolt the wheel studs to appropriate gantrys that have easily swappable flanges, and can move side to side for almost any width of vehicle. Sure it would take a little more effort, but more accurate figures would be produced.

One big difference is that removes the tyres from the equation. Good for engine tuning I guess but not as good for knowing how much gets to the road.

The only time I've seen vehicles dyno'd they were using dyno packs bolted to the hubs.

[jsttry]

You can get hub based dyno's that remove the tyre from the equation, but still doesn't factor drivetrain loss. Reason they are not commonly used is the extra effort to set the car up.

have seen them used in both Harrop and on a car in one of the V8 support categories.

[PGS 4WD]

ramp rates also have an effect on the output, also strapin the car down. i dont fully understand the setup for our dyno as our boss is to tight to get us trained. howeva wen it was installed we were told ramp rates, and to make sure the car is strapd down to keep pressure on the front roller.

Ramp rates do effect read outs as does the wheel speed due to windage, it gets complex as the two impact each other.

In regards to the rollers it depends on the dyno, if using a dyno where the front and rear rollers are connected via a belt or chain such as the Dynamic Test System it dosen't matter as long as wheel slippage dosen't occur. On the older Dyno dynamics dynamometers it was an old trick to strap the vehicle forward onto the front roller so it would come off the rear roller under power hence showing a higher power figure as the vehicles power is not being consumed by turning the rear roller. A higher measured output will occur.

Joel

Joel

What sort of dyno's are we talking about here? Inertia or absorption? Yes in inertia dynos the ramp rate highly dictates the overall calculated power output as they measure the acceleration of a given mass in the rollers a static load. The absorption dyno's measure both speed and torque by using a dynamic loading system typically an electromagnetic brake (eddy current). This brake is connected to a strain guage load cell which measures the force moment. So given the distance between the brake and the strain guage, the measured force on the strain guage and the speed of the rollers connected to the brake both torque and power can be measured.

Ramp rate will also play a part in the maximum power output but no where near as critical and dependant as the inertia style dynos. This rate is controlled by the eddy current brake and the programs built into the dyno software (modes). So with all this yes it comes down to how the dyno operator runs his machine and the control system software running the brake. If a no bs maximum power output wants to be obtained then the vehicle needs to be gradually ramped up to where the power measurement stabilises or maximum speed is reached. All descent engine builds should be able to sustain a few seconds running at maximum output.

Im no expert in all these dynos out there and the way they all function but have been involved in designing an eddy current brake absorption dyno from the mechanical hardware, to the control circuits for the brake as well as the control software to measure the output power.

Dan

force=mass x acceleration. Absorbtion dyno's still have to accelerate a mass, the rate of acceleration is controlled by the brake true, this dosen't change the fact that to accelerate a mass at a faster rate requires more force, the mass is the same(roller, tyres drivetrain) and the equasion needs to remain balanced. Unfortunatly this force is a loss as we are only measuring whats left. A static power figure will always be higher than an accelerated one at any particular speed and rpm as the driving force is no longer being consumed in accelerating the mass, the losses are now friction and windage only. The dyno samples hundreds of times a second so the rate has no effect on the software. The problem arises with very slow ramp rates that other factors can cause power losses. Turbo cars will heat soak, efi systems will remove timing from high water an intake air temperatures and so on. A static power figure doesn't produce the shinny graph that everyone loves so much.
Joel

[PGS 4WD]

Im amazed that dyno's still get strapped down to the rollers and hope they grip. Why dont they instead support the car, remove the drive wheels and bolt the wheel studs to appropriate gantrys that have easily swappable flanges, and can move side to side for almost any width of vehicle. Sure it would take a little more effort, but more accurate figures would be produced.

Dynapak make such a dyno and I know of a couple of reputable shops that use them. For us we do ten cars that make 100 rwkW for every one that makes 300 rwkW and to tie down is much quicker. We have no issues with traction due the roller design although tyres are a source of inconsistancy in particular due to windage. Even the amount of tread makes a difference. The Dynapak would also reduce a significant amount of friction and windage and I would expect higher figures off a Dynapak dyno.

[nastytroll]

My brother inlaw uses a Land ans Sea water brake dyno for tuning, capable of 800BHP. It is an engine dyno though but seams very good for its purpose.

[JrZook]

A static power figure will always be higher than an accelerated one at any particular speed and rpm as the driving force is no longer being consumed in accelerating the mass, the losses are now friction and windage only.
Joel

What I was trying to say was that if the ramp rate on the brake is relaxed ie fast, the intertia of the roller through the quick acceleration will show a higher power output from the speed overshoot due to this inertia. Flywheel effect.

Dan

[MightyMouse]

Sorry Dan but a high ramp rate will produce lower INDICATED power as accelerating the rollers takes energy, thus leaving less for the brake unit to measure...

Note the engine is still making the same power, just that the inertia effect becomes significant at high ramp rates. If this was not true Inertia Dyno's wouldn't operate.

This is a bit of a catch 22 with some operators, they use high ramp rates to try and minimize engine stress but overlook the fact that they basically are using then brake dyno as an inertia dyno without the inertia dyno measuring system..... dumb.

Reputable dyno's actually limit the correction factors possible to avoid operator "abuse" to get inflated numbers and print the correction percentage very prominently

Dyno's are fantastic comparative tools...... for the professional accuracy actually isn't the first priority - consistency is. Who cares if its 10% high or low as long as it can be relied on to produce the same numbers and indicate changes consistently.

So what matters is consistent measurements of the baseline and modification. If you just want big numbers its far cheaper to save on modes and go find a dyni operator who is prepared to generate BS readouts for you.

[PGS 4WD]

Mighty Mouse is correct, I think JrZook is talking about something slightly different, when I started tuning 15 years ago it was on a water brake dyno and it couldn't load at low roller speeds, when you applied the brake and accelerated the engine it would overshoot the power figure as the brake became effective, this is using the inertia of the roller to produce a false high. Even in those days we would wait for a stable power reading, the old water brake we had couldn't do printouts it just had a big power and torque dial.

[MightyMouse]

Ramp rate is very critical in analysing the dynamic performance of an engine. In real operation the engine is rarely operating at a steady state and therefore the inertial effect of accelerating the engine and drivetrain components must be taken into account otherwise your reading numbers that just aren't available during dynamic operation.

Inertia dyno's are great for this type of dynamic tuning BUT the inertial mass needs careful selection to ensure the engines ramp rate is as close as possible to that experienced during real operation. Once again this is understood by tuning professionals.

Really good inertia dyno's will have an array of masses available to cater for different engines - with selections in the controlling software. - crap ones don't even consider this issue.

I've seen people juggling with inertal constants to try and match the engines "nominal" power with what they are reading for one engine and then wonder why its wrong on others.......

I write Dyno control software as part of my job....... so i've worked with some very professional dyno operators who understand that a dyno's a tool that needs to be understood and others who may as well use their cash registers to generate numbers - the variations between idiots and experts is just that large.

[-Scott-]

Ramp rate is very critical in analysing the dynamic performance of an engine. In real operation the engine is rarely operating at a steady state and therefore the inertial effect of accelerating the engine and drivetrain components must be taken into account otherwise your reading numbers that just aren't available during dynamic operation.

Inertia dyno's are great for this type of dynamic tuning BUT the inertial mass needs careful selection to ensure the engines ramp rate is as close as possible to that experienced during real operation. Once again this is understood by tuning professionals.

Really good inertia dyno's will have an array of masses available to cater for different engines - with selections in the controlling software. - crap ones don't even consider this issue.

I've seen people juggling with inertal constants to try and match the engines "nominal" power with what they are reading for one engine and then wonder why its wrong on others.......

I write Dyno control software as part of my job....... so i've worked with some very professional dyno operators who understand that a dyno's a tool that needs to be understood and others who may as well use their cash registers to generate numbers - the variations between idiots and experts is just that large.

OK. So, if I understand this correctly, an inertia dyno uses vehicle mass & aerodynamic figures to "compensate" the final figures for ramp rate, to more accurately replicate "road" conditions? I think I can understand that.

But don't brake dynos compensate for roller inertia too? Surely computers these days can look at the force reading on the load cell and also calculate the force used to accelerate the roller? I would've thought that, within limits, software could compensate for different ramp rates on a brake dyno.

I did notice on the weekend that only the front roller was "ribbed", and the rear roller was smooth. Does that indicate that inertia of the rear roller is irrelevant to the calculations/measurement? I didn't notice if the rear roller continued to accelerate when vehicles rode up on the front roller.

[PGS 4WD]

I have had a number of stock vehicles on the dyno that has enables me to use the newest version of the drive train loss calculator.

I ran a 40 000 klm Navara, the new 2.5 rated at 130 engine kW, our dyno mearured 127.7 engine kW.
A series one 3.0L rated at 117 engine kW with 100 000 klms measured 114.5 kW.
Most recently I tuned an LS2 with less than 1500 klms on it, the factory rated 400 engine HP engine measued 399.4 engine HP when tuned to premium with a 3" exhaust.

We use this function when customers feel their standard vehicles are down on power to compare to factory engine power figures.

Cheers

Joel

[Dzltec]

Care to elaborate on how that works. What happens if you have larger tyres or mud or simex's fitted. Will it show how much power they consume?

A good read so far.

Andy

[chimpboy]

Some interesting info here:

http://yarchive.net/car/dyno_accuracy.html

[PGS 4WD]

Care to elaborate on how that works. What happens if you have larger tyres or mud or simex's fitted. Will it show how much power they consume?

A good read so far.

Andy

It's all about the math. The engine calculation involves two run downs from 95 kph. The first, in neutral, lets the rollers wind down on their own, the second run down also from 95 kph and in neutral the dyno applies a set amount of brake. During this the engine rpm must be accuratly fed into the dyno by the sensor or a calctulated value from the roller speed. This enables the software to gather enough data to fill the missing pieces of the equasion. As the loss is not static, it is relative to road speed to factor in windage for various tyres and frictional losses. I am no mathematician but I understand the principle and I know 2 mil was spent on software developement and independant testing by Rochwell Research.
I have not yet been able to fault it.

Cheers

Joel