I tried to explain this to you over a year ago, why not look up the definition of HP. That would be a good start.

 

I think that once you know exactly what hp is by definition, you can then understand you can not have it without torque. It is a unit of measure describing torque, over time and distance. Without torque, hp can not be calculated...

 

Definition: A unit that is used to measure the power of engines and motors. One unit of horsepower is equal to the power needed to lift 550 pounds one foot in one second
How do you measure torque?

 

I just did, you refuse the accepted way of calculating hp by definition, torque is measured not calculated.

 

Hmm, it seems you are arguing a technicality.

OK, there are inertia dynos which use an accelerometer and these are often claimed to calculate the power directly by using the changes in rotational energy. Well, the SI unit of energy is the Joule and one Joule is equal to the energy expended in applying a force of one newton through a distance of one meter. The last time I checked, Nm is the SI units for torque.

Don't believe me? Do you believe Wikipedia? Look here;

http://en.wikipedia.org/wiki/Rotational_energy

Read the paragraph after the equation. "The instantaneous power of an angularly accelerating body is the torque times the angular velocity." This says that when you are calculating changes in the power in the drum that you most certainly do use the torque.

You have to first understand this concept. The torque I'm talking about above is the torque in the drum system, not the torque as you see it plotted on a fancy curve curve with the rpm across the bottom. Everything rotating that is accelerating or decelerating has a torque involved. Further, the losses in a rotating system mean you even have to apply a torque just to keep it rotating at a steady speed.

Taking the torque and burying it inside a more involved formula doesn't mean it's no longer there. I now think the fact that the dyno doesn't expressedly calculate torque but rather uses it buried inside the equations as it calculates HP is the technicality you're trying to argue.

 

Yes, I have read claims that an inertia dyno calculates the power directly. So what's your point about this exactly? Do you believe that me reading something that was written on the internet means I agree that it's true? The flip side of that kind of riduculous arguement is that you've read my statement that the dyno calculates torque and power which means you have agreed that it's true and that I am right.

I also find it very funny that you would read into my acknowlegement of reading certain claims as your proof even when you have no clue where I read it or who wrote it.

I'll give you one more simple example. You can google this formula very easily. The time for a frictionless rotating object to accelerate is given by the following formula;

time = inertia x dN/(308 x torque)

You re-arrange this to get his formula;

torque = inertia x dN / (308 x dt)

where
dN is the change is speed and dt is the change in time. You can also think of dN as the speed change and dt as the per unit of time each speed change is measured.

OK, using a precision clock and a measurment of the acceleration rate using an accelerometer to get the speed change the torque can be calculated.

However, instead of using the above formula on it's own, lets put the torque formula inside this formula;

hp = torque x rpm / 5250

and we get

hp = inertia x dN x rpm / (1617000 x dt)

Cool, now I have derived a formula that calculates HP directly from some other measurements. Does this mean torque wasn't used? No, it sure as hell does not. The torque calculation is now just part of this larger formula.

The dyno does more complex math than the above. However, the torque is still in the math.

Just FYI, I use the above formulas at work all the time. There is no need for me to even Google it.

 

OK boys..... here is a website which posts all the info you could want in a comprehensive article on "How to build an Inertial Dyno"

http://www.dtec.net.au/Inertia%20Dyn...gn%20Guide.htm

It is a design for a motor cycle dyno, but all the design criteria and physics calcs supplied are applicable to car inertial dynos as well.

Interesting info which clearly states the physics behind the determination of power in an inertia dyno. Lots of pics too.

I'll let you boys digest the info.....
........................................ ........................................ .....

I'm sure both points of view will find argument to support their case, but in essence, an Inertia dyno does determine power without measuring torque as we understand it in foot lbs (Nm)

In fact, the only first principles measurement used is time for each rotation of the flywheel (roller) load.

All other units are calculated, including torque AND power.

 

I will once again repeat that I have always stated that power is calculated, not measured. I have also stated that the torque is also calculated and further that the torque needed to accelerate the drum is always used as part of the calculation for power.

Ok to start back in post #80, I came up with this formula;

Now, this website that is being accepted as "proof" that the dyno calculates hp without using torque has two formula for delta Erot and the Power. I'm going to combine these into a single formula.

P = (inertia x (second w² / 2)) – (inertia x (first w² / 2)) / dt


Now, the astute person would be able to look at the two above formula and see that they are the same. However, some people may not see this, so I will do some changes to my formula.

First, I'll convert to SI units and get rid of the numeric conversion factors the English units cause.

P = inertia x dN x rotating speed / dt

Next, I'll elaborate on the dN

P = inertia x (N2-N1) x N / dt

where
N2 = final rotating speed
N1 = initial rotating speed
N = rotating speed

Next, N is the rotating speed but the speed varied from N2 to N2 so I need to do something about this. I'm going to use the average rotating speed or (N2 + N1) / 2.

P = inertia x (N2 - N1) x (N2 + N1) / 2 /dt

Next, simplify a bit;

P = (inertia x N2^2/2 - inertia x N1^2/2) / dt

Odd, but it that looks very similar to the equation from the quoted website. In fact, it looks identical. I started with the equation for the torque required to cause the change in speed of a rotating inertia, plugged it into the standard formula to calculate power using the torque and rotating speed and ended with the formula used by an inertia dyno as described in the link posted by OzzyTom.

To conclude, I'm really not sure how anyone can say the torque calculation isn't involved or isn't embedded into the math that is used to calculate the power. The torque is most definately part of the calculation, you just have to look for it.

 

WOWW!!!!!!!! I think what the op needs to know is that there is no international calibration standard for dynos. The numbers coming off a dyno are for reference only and mean little, as the make, style, operator, settings, temperature, humidty and phase of the moon will change the readings, ok I made up the last one! AS has been stated it is a tuning tool only. The ops first pull identified a bad fuel pump, that was changed yielding better numbers. You get a baseline, that is only good for that exact dyno w/ that exact setup and operator and then you try to make improvements! I have had operators increase my numbers on my last ( third ) pull by 20% so I could "mess" w/ my car nut friends who think the numbers have huge importance and the operator new what I was requesting because he gets the same request all the time. Its just a tool, no magic, just reference!!!!

 

OK Guys... ... I'm not sure what it is about "tech/performance" that makes these debates turn ugly, but I would prefer to keep threads like this one open, with some great exchanges of information, regardless of who thinks they are absolutely right.

I would imagine no one will be getting any "apologies" or admissions of giving up any time soon, either, so maybe we could not hold our breath on that.

I don't want to spend any more time cleaning up this thread. If I do, I'll just close it and proceed directly to the infractions for everyone involved.

Attack ideas, not people.

 

I will apologize if any of my comments were believed to be attacking a person. I was attempting to only comment on the ideas posted without attacking anyone.

 

Your arguments have been rational and intelligent without being belligerant. I can't say the same about those from a certain "forum expert".

 

Ralph,

Your previous dyno has huge spikes in it... not sure if that's from the missfires or from your transmission shifting during the pull, but those spikes should not be read as peak hp. Any numbers from a dyno run that looks like that should basically be ignored.

As far as our dyno, it is calibrated to read just like a Mustang Chassis Dyno which is the most accurate IMO. My reasoning for this comes from my software engineering days. Back in college when I was just getting into tuning, I developed my own road dyno software. My software took a log file from an in-gear pull and looked at the change in RPM over time. Along with gearing and tire size, the software calculated the car's on-road acceleration. Then using weight, frontal area, drag coefficient, and air density, the software calculated the power and torque required to create the measured acceleration. While this sounds somewhat complicated, the whole thing is based on a few basic physics equations.

Since this software worked purely off real-world on-road acceleration and basic physics, the results are very accurate as long as the input parameters are accurate, the road is flat, and there is no significant wind. After doing a lot of testing with the software and getting very consistent results, I decided to test the results against several chassis dynos. To my surprise, I found that the power measurement from my software was always within 1-3whp as compared to most Mustang Chassis dynos. Dynojets usually came in at 10-15% higher and Dynapaks came in at 15-18% higher.

Considering these results, I concluded that the Mustang dyno measures power most "accurately" as it comes the closest to a real world power measurement. So when we acquired our Dynocom Chassis dyno, I calibrated it to read just like a Mustang dyno. This may hurt some people's ego's, but at the end of the day its the most accurate measurement I've seen.

All this being said, the most important part about a dyno is consistency, not the absolute numbers. If a dyno can reproduce the same measured results consistently, then its a good tool for tuning no matter what the actual numbers are. So if a dyno reads high, it doesn't mean its better or worse than a lower reading dyno when you're using it as a tuning tool.

Thanks
-- Ed

 

I also tried to read through most of this thread and there is a lot of good info as well as some confusion here on how dynos work.

Most dynos do in fact measure power and in the absence of RPM data they can display a power curve over time or over wheel speed. Power is basically work. The dyno software knows the inertia and diameter of the roller and measures the speed of the roller over time (acceleration). It then calculates how much power it takes to achieve the measured acceleration of the roller at the given speeds. Using this data along with measured RPM, it can then calculate wheel torque. This is the basis of just about any dyno measurement out there.

The big misconception that dynos measure torque directly comes from load bearing dynos. This type of dyno uses a load cell and when people see the load cell, they assume the dyno is using it to directly measure wheel torque. In fact, the load cell is only there to measure the amount of additional load being applied by the eddy brakes to slow down the roller. This load is then added back into the traditional inertial type power calculation to get the final power number. So even in this case, the dyno software is first measuring the acceleration of the drum, calculating the power it takes to achieve this acceleration, and then adding in the additional load that was applied by the eddy brake.

Just about every dyno works this way. The only possible exception is a hub type dyno that has nearly zero inertia and primarily uses the eddy brakes to create load.

Thanks
-- Ed

 

Well well I don't think anyone can argue with that

 

BTW, I'll add to this that the whole concept of wheel torque is kind of a misnomer. Technically the torque applied at the wheels has a great deal to do with the gearing advantage and will vary greatly between different gears. What you see on a dyno chart is actually engine torque minus parasitic losses. And its only there as a calculation based off measured power.

To test this out, next time you're on a dyno, have the operator do pulls in a couple different gears. The resulting hp and torque will be nearly identical in each gear, but as we all know, you're actually applying a lot more torque at the wheel in a lower gear.

-- Ed

 

This is exactly what I was trying to get across

 

Thanks Ed.

How does the fact that you started measuring torque after it had already peaked figure in.

And why does my torque fall off so fast.

Thanks again Ed

 

I think you're referring to the second run (blue) where torque peaked very early in the RPM range. That is actually an artificial torque peak that can happen with automatic transmissions when the torque converter slips. On that particular run, the torque converter slipped a bit more and at that road speed your RPM was actually higher resulting in more torque at the wheels. Then the torque converter catches up and locks up bringing RPM back down. That is why torque drops off so quickly after that early peak. Generally that kind of torque peak should be ignored as its an anomaly from the torque converted slippage. Your true torque peak is around 240ft-lbs at around 4000RPM as is shown in the other two runs.

In general doing dyno pulls with an automatic transmission is a bit more difficult unless you can manually lock the torque converter and lock the transmission in a particular gear (neither of which was possible on your car).

Hope that helps.

Thanks
-- Ed

 

Good info, it really clears things up