FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: jayb on March 26, 2019, 08:24:06 PM
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There's a lot of talk out there about dyno results that are bogus, and I must say I've seen quite a few of them myself. As an engineer I'm quite concerned with getting accurate and reliable results from my dyno, so today before running Jim's engine I checked the calibration on the torque sensor of the dyno.
This is not a difficult thing to do with Superflow dynos because they provide you with a steel bar, called the calibration bar, that bolts onto the dyno's absorber and activates the torque sensor when weight is applied to the bar. The decal on the calibration bar is shown in the picture below, and basically gives you a geometric idea of what is going on when you measure the accuracy of the torque sensor:
(http://fepower.net/Photos/Posts/dynocal1.jpg)
At the upper left on the decal, the dogbone shaped device is called the torque link, and it is what measures the torque that the engine is producing. The calibration bar hangs in such a way that it applies torque to the torque link, and weights can be hung on the calibration bar, in the notch shown at the right in the decal. The notch at the upper left of the calibration bar is the same notch. This notch positions the weight exactly 3 feet away from the center of rotation of the dyno's absorber, so that if you hang one pound of weight on the bar, you are applying 3 lb-ft of torque to the torque link. Also, the calibration bar itself has weight, so with only the calibration bar attached, the torque output from the dyno should read 17.5 lb-ft.
On my dyno there are studs coming out of the absorber, which allows you to bolt the bar into place. See the picture below, with the bar installed:
(http://fepower.net/Photos/Posts/dynocal2.jpg)
Once the bar is installed, you can start checking the calibration. The torque output on the computer screen of the dyno should ready 17.5 lb-ft at this point. Next, I have a hanger that I built to hang weights on the calibration bar. Basically what I did was buy a free weight set and hacked it up to make up this hanger, and then used the weights for calibration. After welding up the hanger, I took it and the weights to the local UPS store and weighed them on their certified scale, to an accuracy of 0.05 pounds. I labeled the hanger and each of the weights with their exact weight, based on the reading from the scale. The hanger weighs 29.85 pounds, so once it is hanging on the 3 foot long calibration bar, the dyno's torque reading should increase by 3 X 29.85, or 89.55 pounds. A picture of the hanger is below:
(http://fepower.net/Photos/Posts/dynocal3.jpg)
Next, more weights in pairs are hung on the hanger, to increase the torque on the absorber. Here's the first step, adding two 45 pound weights which together add 270.30 pounds of torque:
(http://fepower.net/Photos/Posts/dynocal4.jpg)
Here more weights have been added; see the tag on the 25 pound weight, giving its exact value:
(http://fepower.net/Photos/Posts/dynocal5.jpg)
Finally here are all the weights hanging on the bar:
(http://fepower.net/Photos/Posts/dynocal6.jpg)
Here are the results from my calibration test today:
Actual Weight Dyno Reading
17.50 lb-ft 17.50 lb-ft
107.05 lb-ft 106.50 lb-ft
377.35 lb-ft 377.70 lb-ft
523.30 lb-ft 523.55 lb-ft
673.15 lb-ft 673.50 lb-ft
880.75 lb-ft 880.70 lb-ft
As you can see, the torque sensor is extremely accurate. Any reputable dyno operator ought to have a set of calibration weights, and be able to show his customers exactly how accurate his dyno's torque sensor is. It is a simple matter to change the dyno's software to increase the torque reading by 5%, 10% or more, and some disreputable dyno operators have certainly done that. If you go to the dyno and want to be absolutely sure that you are getting accurate numbers, in addition to confirming the weather information you should have the dyno operator calibrate his dyno for you. It will cost a little extra because you are paying for the dyno operator's time, but you will have peace of mind about the test results.
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Thank you for the play-by-play. I was generally aware of what it took, but you have made it very clear.
KS
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Interesting topic, Jay. We have a few local engine dynos in my area, there is 1 shop that is notorious for its dyno sheets having much more generous power numbers than my buddys dyno (both are Superflow 901s). Of course the engines with the puffed up numbers never seems to produce the ET slip numbers such HP figures should produce. Amazing how much twirling a few knobs on the control panel can affect the readings, like rate of acceleration , and test conditions in the dyno cell, and of course, the calibration of the dyno.
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First thing I thought, just seeing the subject line and not reading anything was, "how many different weights do you test?" :)
Even when I test a simple pH meter I have to test several data points, to get a curve or slope. Make sure the measurements are linear. If they are supposed to be linear.
Sounds like you have it covered. Thanks for description of the process. I haven't been to a dyno in years, but I do remember watching that. I doubt it was done as accurately and thoughtfully. Getting the exact weights confirmed seems like a really good idea.
paulie
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I think the biggest reasons for bogus dyno results don't involve the calibration at all. Every dyno I've ever used is always calibrated (or at least checked) before hand. However, that would lead you to believe that you should be able to cart an engine from state to state, hitting various dynos and getting the same results, and we know that's not true, or even possible.
There was a gentleman a few weeks ago that dyno'd a BBF engine at the dyno I use and made about 1050 hp. He packed it up and took it down south to a very well known builder's shop and made 87hp more without making any changes. How does that happen?
I think the biggest reasons for dyno discrepancies revolve around the air flowing in and out of the dyno cell, along with the location of the weather data collection.
The bottom line of all of this is that dynos are a necessary evil. However, in order to provide a solid piece to customers, it has to be done. The best plan of action is to use the exact same dyno consistently so that you can see differences between engines without changing that variable. If you bounce around from dyno to dyno, you'll never know what's good or bad.
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I tend to agree with Brent on the weather sensor locations and cell ventilation - you can always tell if I forget to turn on that big fan before a pull.
On the weights it is - as you noted - significant to weigh the weights. I also found that the common weightlifting weights are noticeably off from the cast in values. I think its like ladies dress sizes - done on purpose to make you feel like you are doing better than you really are....
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87 HP - thats a lot, even for weather. But I can see it. Here we were running mid 3000s DA this weekend which was a great weekend of weather for us. Track is 1719' MSL, so anything in the 3Ks is great. A week ago at Ennis, which is 515 MSL they had negative DA. Right now this minute, their DA is 24.48 and ours is 2088.35. The temp + RH numbers are nearly identical. We are only 204 miles from Ennis. In summer, we can easily be as "high" as Denver. Why I paid nearly $1000 for a new Altronics trailer weather station and new Crew Chief software for use at the track. The car actually picked up 3 numbers on Saturday, which is totally backwards from normal.
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Superflow says that the airflow over the engine during the dyno pull is important, and they have specifications for where to put the air vent in the ceiling, and the exhaust fan, to make sure that isn't a big factor, or at least its a consistent factor. But the other thing that varies quite a bit from dyno cell to dyno cell is the dyno's exhaust system. Every one is different, and when you hook the headers up to the exhaust system you are introducing another restriction. I have a pressure transducer in my exhaust system to monitor this, and I've seen readings as high as 0.7 psi, which would certainly steal some power. When it gets to that point I will usually run one pull with the dyno's exhaust system disconnected, and rely on the fan at the back of the dyno cell to expel the exhaust. They sure are loud that way, and probably for a lot of shops that's not a practical test. The neighbors might complain ;D They sure sound good like that though...
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After seeing my local shop get their upgrades done a year or so back, the information from the Superflow guy was he knew who's dyno was accurate and which ones were a bit "skewed" on numbers.
Seems you need to do some weather calculations in the cell and enter that information so that it compensates. That should make every dyno read the same no matter what elevation. But like anything, garbage in=garbage out.
The dyno should just be a tuning tool. But you know how some guys are always after "a number". ;)
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Battlestar, I think most of us are "Looking for a number", the numbers that mean the most to me are on an ET slip, more important to me than a Flowbench or dyno sheet. ;D
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My main reason for dynoing this motor is so I can give the info to the converter company and hopefully get the best converter for my bracket car. Jim
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Superflow says that the airflow over the engine during the dyno pull is important, and they have specifications for where to put the air vent in the ceiling, and the exhaust fan, to make sure that isn't a big factor, or at least its a consistent factor. But the other thing that varies quite a bit from dyno cell to dyno cell is the dyno's exhaust system. Every one is different, and when you hook the headers up to the exhaust system you are introducing another restriction. I have a pressure transducer in my exhaust system to monitor this, and I've seen readings as high as 0.7 psi, which would certainly steal some power. When it gets to that point I will usually run one pull with the dyno's exhaust system disconnected, and rely on the fan at the back of the dyno cell to expel the exhaust. They sure are loud that way, and probably for a lot of shops that's not a practical test. The neighbors might complain ;D They sure sound good like that though...
Nice explanation Jay!
My question is a follow-up to the above: when the backpressure with your exhaust system on the dyno gets to that 0.7 psi mark, what does it roughly drop to when it's disconnected?
In other words, what's a good number in-car to shoot for (or, modify an existing system if say it does approach 0.7 psi)? I recognize that every in-car system will vary widely but......
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Bob, I have seen some cases where removing the dyno's exhaust system results in a gain of 15 HP, and other cases where there is zero gain, all with 0.6 to 0.7 psi with the exhaust system connected. Some engines seem to tolerate the back pressure better than others.
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My main reason for dynoing this motor is so I can give the info to the converter company and hopefully get the best converter for my bracket car. Jim
Excellent - you'll be way ahead of many others by doing that.
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Battlestar, I think most of us are "Looking for a number", the numbers that mean the most to me are on an ET slip, more important to me than a Flowbench or dyno sheet. ;D
Luckily I've been to poor to worry about "big" ET numbers. Only one I worry about is the one I write on the car before a round.
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Bob, I have seen some cases where removing the dyno's exhaust system results in a gain of 15 HP, and other cases where there is zero gain, all with 0.6 to 0.7 psi with the exhaust system connected. Some engines seem to tolerate the back pressure better than others.
Very interesting and thanks Jay!
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Battlestar, I think most of us are "Looking for a number", the numbers that mean the most to me are on an ET slip, more important to me than a Flowbench or dyno sheet. ;D
Luckily I've been to poor to worry about "big" ET numbers. Only one I worry about is the one I write on the car before a round.
It's fun chasing a lower ET number, and I'm guilty of that, it's kind of addicting. But something tells me that Larry's approach is far more sane.
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Sanity<--->FE Oil<--->Vinegar Doesn't work unless you shake 'em up.
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Battlestar, I think most of us are "Looking for a number", the numbers that mean the most to me are on an ET slip, more important to me than a Flowbench or dyno sheet. ;D
Luckily I've been to poor to worry about "big" ET numbers. Only one I worry about is the one I write on the car before a round.
It's fun chasing a lower ET number, and I'm guilty of that, it's kind of addicting. But something tells me that Larry's approach is far more sane.
You have the power, you just need to work on the chassis part. It took me years(23) of tinkering to get mine to where it is now. Still not "fast" compared to many others, but I just want it "fun". There is always someone faster.