Complete explanation to Dynoing

[theOmni]

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?

[UncleBob]

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.

[keno7]

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.

[UncleBob]

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

[BeteNoir]

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.

[UncleBob]

Originally Posted by BeteNoir

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.

[termiflyer]

Originally Posted by UncleBob

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 ). You measure torque and rpm to get a torque and horsepower curve.

[UncleBob]

Originally Posted by termiflyer

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 ). 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

[UncleBob]

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

[sracing]

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 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 ) 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

[sracing]

Originally Posted by UncleBob

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.

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

[UncleBob]

Originally Posted by sracing

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

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

[sracing]

Originally Posted by UncleBob

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, 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...

Jim
SR Racing

[UncleBob]

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

[Alastor187]

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…

Originally Posted by sracing

Well true on that cheap DynoJet, 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?