Complete explanation to Dynoing

Hi, I know that on a Dynometer, you get the Hp and torque values of your cars. I'd like a little bit of a more in-depth explantion of what this really means. What do terms like "power curve" mean or "launching under the power curve" means. ANd what kind of dyno of graphs are considered ideal, superior, or inferior?

as far as "power under the curve" they are refering to, to put it simply, the average hp through the RPM. IE, peak power isn't as important as having the most broad power possible.

It really depends what application is being applied. Some forms of performance, peak power is more worthwhile than broad power. What tranny you use, for example, can change what is more desirable also.

Dyno's in general, is measuring HP (and torque if setup for it) at the tires, most commonly, adjusted for all possible variables such as altitude, temp, humidity, etc. Thats the theory anyway. There is a lot of variables, one of the biggest being the manufacturer of the dyno. Dyno readings can vary a lot between them.

The main point of dyno's is to be a tuning tool. That being, being familiar with a particular dyno, understanding its possible limitations, and doing before/after readings to verify peak performance, and/or tuning a particular setup using the results of the dyno runs.

The other side of dyno's is the bragging rights. Which is, the biggest number wins, regardless to its truthfulness. This is a fuzzy area, and I think is the bigger issue with dyno's. The shop that has a "optimistic" dyno gets more business for this reason. People prefer bragging rights over real results.

I always thought a dyno measured torque and from that the horsepower was calculated. I migh be wrong but I don't think HP can be measured and that it was just a marketing ploy when Watt came out with the steam engine.

common misconception.

It measure rear wheel torque, which is then calculated into HP, and then with an RPM reference, is calculated into torque.

Its not a simple "this happens first" though, since HP and torque are forever intertwined. The bigger issue is calculating out the gearing.....its much simpler to do that by caluclating the HP first.

For example, if you have no tach lead, you can't calculate a torque reading for the engine. But you can print a HP readout. That should be pretty telling on what happens when

Dynos, in general fall into two classes; engine dynos and chassis dynos.

A chassis dyno uses large rollers driven by the vehicle tires to measure the torque generated. From the measured torque and roller speed, backward calculations are made to include tire rolling radius, rear end and transmission gears, to obtain the torque produced at the flywhee. This torque value multiplied by engine rpm and divided by 5252 results in a calculated horsepower value. It is easy to see that chassis dyno readings are very inaccurate since they must estimate actual tire rolling radius, rear end and transmission friction losses and true rpm at any point in the chain. A chassis dyno can however, be very useful when used to compare one setup vs another setup but not very useful to determine absolute values.

Engine dynos are used to determine absolute values. The most common types of engine dynos are water brake and eddy current dynos. A water brake dyno uses a turbine filled with water to provide a rotational resistance to the engine rotation. It will apply a force to a lever arm and that force multiplied by the lever arm length will determine torque (units: ft lbs). Any dyno can be very accurately calibrated by simply hanging a know weight on the lever arm. The problems arise when we try to accurately measure engine rpm and to correct for various ambient climate conditions.

Eddy current dynos are the most accurate and can also be used to motor an engine to determine friction values. Iin principle, the eddy current dyno drives a generator which produces a measured amount of electrical energy. That energy corresponds to a given amount of force. Applying the same rpm/5252 values will produce horsepower.

Dynos do not measure horsepower. They are only capable of measuring torque values and the horsepower is calculated from measured torque. When tuning an engine on a dyno the horsepower should be ignored and attention should only be paid to the torque curve. In addition to the amplitude of the torque curve, we should pay close attention to the shape of the torque curve. The most power efficient engine speed occurs at the engine torque peak. That is where the combustion pressure produced by burning fuel is at its maximum. On either side of the torque peak the engine is less effective and less efficient.

A dyno is an excellent tool in the hands of a master tuner and will allow the correct fuel mixture and ignition timing to be determined under controlled conditions. In the hands of a bozo, any power value can be created simply by changing the correction factors that are used. No performance engine should ever be built without spenind some time on a dyno, either a chassis or engine dyno.

Dynos do not measure horsepower.

As I already stated.....you can't get a *engine* torque reading without a tach lead

But you can get a HP curve on any dyno, with or without a tach lead.

As I already stated.....you can't get a *engine* torque reading without a tach lead

But you can get a HP curve on any dyno, with or without a tach lead.

You can absolutely get an engine torque "reading" without a tach lead (maybe on a particular dyno you can't see it, but i think we're talking about how one works). You cannot "measure" horsepower, be it at the engine or the rear wheels, you can only calculate it. Likewise, you can't get an actual HP curve without a tach lead: you are probably getting the HP curve from the dyno measuring rpm itself (and torque :wink:). You measure torque and rpm to get a torque and horsepower curve.

You can absolutely get an engine torque "reading" without a tach lead (maybe on a particular dyno you can't see it, but i think we're talking about how one works). You cannot "measure" horsepower, be it at the engine or the rear wheels, you can only calculate it. Likewise, you can't get an actual HP curve without a tach lead: you are probably getting the HP curve from the dyno measuring rpm itself (and torque :wink:). You measure torque and rpm to get a torque and horsepower curve.

you'll have to explain how the math works, and not use vague hand waving if you want someone to believe you

Frankly, you haven't spent much time around a dyno if you believe you can't get a HP reading without a tach lead

explain why this dyno chart is impossible.....no tach lead whatsoever
http://photos.imageevent.com/rz350/turboparts/websize/34262per3A8323232per7Ffp46per3Dotper3E232per3Aper3 D645per3Dper3C36per3DXROQDFper3E23235336per3A967pe r3Bot1lsi.jpg
(http://photos.imageevent.com/rz350/turboparts/websize/34262per3A8323232per7Ffp46per3Dotper3E232per3Aper3 D645per3Dper3C36per3DXROQDFper3E23235336per3A967pe r3Bot1lsi.jpg)

Lots of errors and misconceptions here. To keep it simple, lets stay with the Chassis dyno. (Actually Wheel Dyno).:

HP and/or Torque can be measured directly with or without RPM on most dynos.

In inertia mode, HP is computed directly, simply by measuring the acceleration of a known mass (usually a 3000 to 7000 lb drum that equates to 2000 to 2500 lbs of rolling mass). There are simple formulas in physics to determine how much HP (energy) it takes to move that mass from 40 mph to 41mph, and 42mph to 43 mph, etc. This calculation takes place every few millisecs and a simple graph of HP can be provided. (with no RPM per se' required)

If you do have RPM, then simply using the formula HP=Torque*RPM/5250 will back you into Torque. The dyno computer does this for you.

Now, a good dyno also has a "brake load" function also. In this case, it simply tries to hold the drive rolls from turning and it measures the amount of torque at the wheels. Now if I have engine RPM (or drive train ratios), the dyno can calculate HP with the above formula.

In the case of inertia mode (the upper example). The physics are pretty simple and ALL dynos SHOULD read the same (and most are pretty close). But, there are some calculation that most dyno controllers throw into the equation to try and get "true" wheel HP. There are some roller aerodynamic losses, frictional losses, etc. For example, many lower end dynos use two small pinch rollers. These require lots of correction calcs to compare them to a large roll dyno that more closely represents the road.

Then of course. industry standard correction factors are applied (SAE, STP, etc) to compensate for environmental conditions at the time of the run.

Inertia and Brake loaded modes both have their place in tuning, diagnosing and providing bragging :grinyes: rights. Both are required in a good shop.

In regards to engine dynos, most all of them are brake loaded (eddy current or hydraulic) and only a few inertia dynos are in use. (most on very small engines, or very expensive engine dynos)

BTW, I built my first dyno (engine) in about 1967 using a T400 transmission and a fishing scale (well sorta :grinyes: ) And.. it worked quite well. Today we have a couple hundred thousand invested in dynos (chassis and engine), and it does the same stuff, but makes nicer printouts. (Well, except for wide band a/f, Fuel Flow, Air Flow, Boost, OP, OT, EGT's and a few other things.)

BTW, Igor had his 350Z on our dyno many times.

Jim
SR Racing

explain why this dyno chart is impossible.....no tach lead whatsoever


Simple, It just calculates the amount of HP it takes to accellerate a known mass from any wheel speed to another. :wink:

And if you know the drive ratios you can even back into torque with that alone. But it's easier with the tach lead.

Jim
SR Racing

Simple, It just calculates the amount of HP it takes to accellerate a known mass from any wheel speed to another. :wink:

And if you know the drive ratios you can even back into torque with that alone. But it's easier with the tach lead.

Jim
SR Racing

Or knew the redline assuming the program could formulate the speed vs RPM

But my point was quite simple: without added info, you can't have engine torque, but you can have HP curve

Or knew the redline assuming the program could formulate the speed vs RPM

Not sure I understand you there...

But my point was quite simple: without added info, you can't have engine torque, but you can have HP curve

Well true on that cheap DynoJet, :wink: but better dynos also have water brake or eddy current loaders with strain gauges to measure torque directly. Then you use the formula to get back to HP. Either HP or Torque can be measured directly, then you back into the other.

But, I think you and I know this. This is all just rhetoric for the lurkers...:grinyes:

Jim
SR Racing

If you hit redline at 140mph, and redline is 14K RPM's, then its a rather simple formula to figure out torque based off the HP curve

Not sure how you can remove gearing and wheel diameter with no info, intertia or eddy. Without some reference to the RPM, torque is not measurable for the *engine*

You have an unknown car with unknown redline, unknown wheel diameter, and unknown final gear ratio. It makes 313hp at 120mph using the eddy. What is the engine torque at 120mph? Its impossible without more data

I don’t see why it matters which one is measured first since both torque and power are useful measures.

Nevertheless since this something that bugs me…


Well true on that cheap DynoJet, :wink: but better dynos also have water brake or eddy current loaders with strain gauges to measure torque directly.

How is measuring strain first and then converting it to torque any different than measuring torque and then converting it into power?

Technically speaking neither torque or power are really measured directly. More to the point, is strain even measured directly or is it voltage that is measured?

If you hit redline at 140mph, and redline is 14K RPM's, then its a rather simple formula to figure out torque based off the HP curve

Yes if you know any RPM at any Speed then you have all you need in that particular gear.


Not sure how you can remove gearing and wheel diameter with no info, intertia or eddy. Without some reference to the RPM, torque is not measurable for the *engine*

Yes, you need drive ratios.

Jim

I don’t see why it matters which one is measured first since both torque and power are useful measures.

It doesn't really as long as you understand the concepts and physics involved.

How is measuring strain first and then converting it to torque any different than measuring torque and then converting it into power?

See above. It doesn't really matter.

Technically speaking neither torque or power are really measured directly. More to the point, is strain even measured directly or is it voltage that is measured?

Well NOTHING is measured directly. I need the concepts of time, (RPM or controller clock), even with my "fish scale" dyno, I needed to convert the spring resistance to a dial gauge. Yes, the strain gauge resistance is simply measured with a calibrated voltage.

The real differences in the is HOW it is measured (inertia or brake) and that does make a difference.

For example, using two engines producing the same brake HP may be wildly different when measured in inertia mode. All other things equal, if one engine/drivetrain has much greater rotating mass, it may measure X HP in the brake mode while the lighter engine measures the same thing. However in inertia mode, the lighter engine will put more HP to the rear wheels.

This is critical in brake dyno readings. To compare one engine to another you have to use a similar acceleration rate (300 rpm per second as an example. This is often used as a standard).

There are a lot of different procedures when brake dynoing to either evaluate the inertia losses, or to negate them, depending upon what the planned use of the engine is. A diesel for example does pretty poorly in inertia mode, but does pretty well in steady state brake mode. :grinno:

Jim

I don’t see why it matters which one is measured first since both torque and power are useful measures.


I think this is the "what is better, HP or torque" argument contaminating the dyno theory. Some believe that Torque is everything, and they allow their understanding of dyno's to be corrupted by this....fanatical belief

Torque is a static measurement that has little bearing on real world measurments. That is partly why you need more info to calculate engine torque.

No it doesn't really matter which comes first, if you have all the data, you know both, and having both is good. But some like to argue about which comes first as if it matters, and they don't like the reality. HP comes first

common misconception.

It measure rear wheel torque, which is then calculated into HP, and then with an RPM reference, is calculated into torque.

Its not a simple "this happens first" though, since HP and torque are forever intertwined. The bigger issue is calculating out the gearing.....its much simpler to do that by caluclating the HP first.

For example, if you have no tach lead, you can't calculate a torque reading for the engine. But you can print a HP readout. That should be pretty telling on what happens when

I disagree on that.
Most vehicles are dyno'd in a straight through gear (1:1) to minimise losses through the gearbox.
So the only things necessary to convert from wheel force to flywheel torque is the diff ratio and tyre size. Both are easy to find without needing a rpm feed from the engine.

The torque and HP curves at the wheels will give exactly the same shape as the torque and HP curves at the flywheel. They just need scaled, which depends on the gearing.

explain why this dyno chart is impossible.....no tach lead whatsoever
http://photos.imageevent.com/rz350/turboparts/websize/34262per3A8323232per7Ffp46per3Dotper3E232per3Aper3 D645per3Dper3C36per3DXROQDFper3E23235336per3A967pe r3Bot1lsi.jpg
(http://photos.imageevent.com/rz350/turboparts/websize/34262per3A8323232per7Ffp46per3Dotper3E232per3Aper3 D645per3Dper3C36per3DXROQDFper3E23235336per3A967pe r3Bot1lsi.jpg)

Easy.
Power = Force * Velocity.
can also be written as
Power = Torque * rotating speed

The torque at the rollers is measured. The speed they are rotating at is directly measured also.
From this power is calculated.

The real differences in the is HOW it is measured (inertia or brake) and that does make a difference.

For example, using two engines producing the same brake HP may be wildly different when measured in inertia mode. All other things equal, if one engine/drivetrain has much greater rotating mass, it may measure X HP in the brake mode while the lighter engine measures the same thing. However in inertia mode, the lighter engine will put more HP to the rear wheels.

This is critical in brake dyno readings. To compare one engine to another you have to use a similar acceleration rate (300 rpm per second as an example. This is often used as a standard).

There are a lot of different procedures when brake dynoing to either evaluate the inertia losses, or to negate them, depending upon what the planned use of the engine is. A diesel for example does pretty poorly in inertia mode, but does pretty well in steady state brake mode. :grinno:

Jim

That is an interesting point.

I disagree on that.
Most vehicles are dyno'd in a straight through gear (1:1) to minimise losses through the gearbox.
So the only things necessary to convert from wheel force to flywheel torque is the diff ratio and tyre size. Both are easy to find without needing a rpm feed from the engine.

The torque and HP curves at the wheels will give exactly the same shape as the torque and HP curves at the flywheel. They just need scaled, which depends on the gearing.

I didn't say its impossible, I said it requires more info. If you don't have the exact wheel circumference, and the exact final (total) gear ratio, you won't have the exact RPM, and without the exact RPM, your torque numbers will be wrong.

You will have a curve, but without accurate numbers, its not terribly helpful

Easy.
Power = Force * Velocity.
can also be written as
Power = Torque * rotating speed

The torque at the rollers is measured. The speed they are rotating at is directly measured also.
From this power is calculated.

exactly what I was refering to. The person I was responding to said it wasn't possible

I do not believe it is possible to find HP without knowing a RPM. It could be the RPM of the Engine, or of the dyno, as long as you have the Calculations right. KevintheNerd did a decent explanation on how RPM and torque change though gearing here:

http://www.saturnfans.com/forums/showthread.php?t=103238

For Engine dynos, the engine is connected directly to either a water or electric (a.k.a. eddie current) brake. This brake puts a load (torque) on the engine. Because of Newton's third law, this load against the puts a equal and opposite torque on the housing of the brake. The torque on the brake is measured often by a strain gage at a known distance from the break. ex: 100lbs @ 2 ft from the center of the rotating shaft equals 200 ft*lbs of torque. This torque that you just found is the torque that the engine is applying to the brake.

For chassis dynos, there are a few ways to do this: one, take the drums that your wheels turn and hook up a break to them, but this complicates things, so probably not used. Two, the drums' RPM can be constantly measured. Using numerical analysis of the drum's rpm over time, one can calculate the angular acceleration of the drums. Using:

Torque = m*r^2*a/2

where "m" is mass of the drums, "r" is the radius of the drums, and "a" is the angular acceleration. Now you know the torque applied to the drums


If the engine is connected without any gearing to the dyno, the torque found above is the engine output torque. If there is gearing between the engine and dyno, then by using the gear ratios, one can calculate the engine output torque. If you recorded what RPM produced what amount of torque, then you can create a Torque vs RPM graph.

As for the HP, using the torque vs rpm data, you can calculate HP using this equation:

Power (hp) = Torque (ft*lb) * Rotational Speed (rpm) / 5252

That equation is from kevinthenerd, see the above link for an explanation of how to get that formula.

I hope this helps, and i believe it is accurate.

thats rear wheel torque, not engine torque. There's a large difference between the two

thats rear wheel torque, not engine torque. There's a large difference between the two

The difference between the two is the drive ratio.:wink:

you can calculate engine torque from rear wheel torque, right? given you know all the correct ratios.

input RPM = x*output RPM

where "x" is the gear ratio.

input torque = output torque/x

this is true because torque and rpm are inversely proportional to each other when dealing with gears.

it won't be exact because you have friction loses and will be extremely difficult to account for the angular acceleration of all the rotating mass in the car. But if the drum that the wheels turn have a large amount of mass compared to the the rotating parts in the car, those losses will not throw your data off too much.

yes, if you have the added info. As I already said, you need the added info, or a tach lead, before you can calculate the engine torque

Engine HP doesn't require it