Dyno Comparison Challenge Results
What We Did
We went around to all the local dyno facilities to gather data and see what and how big the differences are between them. Do all the dynos really tell a different story? Though lengthy, this is just a summary of the results found. We’ll expand on this with a full-length version in the near future.
I’m sure we’ve all been there. Someone asks how much power you make, and when you tell them the results of your hard work, the response is “Yea, but on what dyno?” It’s as if all your accomplishments are reduced to nothing more than a faulty piece of equipment spewing lies. Well, now there’s concrete evidence to back up or deflate your response.
How We Did It
We chose one car, a turbocharged Honda Civic with an H22 making ~600WHP. We chose it because with such high horsepower, variances in readings show up more easily and with more detail. The car’s owner built and tuned it himself, which means he had no ties to any one shop. The theory is since he has no ties, we’ll get honest readings that any general customer off the street should expect to get. (We didn’t want anyone purposely messing with numbers!) We tried to dyno the car at as many places as possible, on the same day when possible, to eliminate as many variables as possible. Because we were the ones hosting the testing, we decided to go first, to eliminate our messing with our numbers to match/raise/lower the results.
Generally, this is a subject that’s very controversial, with loads of speculation coming from both sides of the aisle. But really, should there be speculation? After all, a dyno is nothing more than a tool for measuring. You never seem to hear Stanley and Craftsman arguing over who has the most accurate tape measure, so why does the automotive world argue? Well, this should end the debate. A dyno is a tool that uses a series of mathematical calculations to produce an end number. That is, measure how fast a given known mass is accelerated (inertia) or by measuring the strain (torque) during acceleration with a resistance applied (or both). They really should all read the same. And for the most part, they do!
We measured five different dynos and came up with numbers varying from 487 to 617HP. In order to throw out the odd-ball highs and lows, we decided to use the median average from all the tests as our documented standard and came up with 598HP. This gives us an even measuring point as to how far each dyno is from what would be the “average” power measured.
· Average HP: Measured from three separate runs on each dyno.
· Repeatability: How repeatable a dyno is measured in percentage by the variance of difference between the three separate runs. (Anyone can hit the bull’s-eye in darts sometimes, but it doesn’t make them good at darts. Repeatability is how close to the bull’s-eye they can get on average.)
· % Difference: How “off” the dyno is from the measured median of all 15 runs from all the dynos. (Measured median: 598HP)
As this chart shows, if we assume the car has the median measured 598HP, all of the dynos are within +/-2 percent, or a maximum variance of 3.9 percent. Pretty close, even considering the one with the biggest difference (-2.07 percent) was documented to have tire slippage, slightly skewing the results. And all of these are right in par with the dyno this car did in Virginia (at sea level) with a measurement of 603HP. That is, of course, except for dyno number 5, but as the notes column states, a “different” correction factor was used (for whatever reason), so let’s take a look at that.
A correction factor (CF) is probably one of the most misunderstood properties of dyno testing on cars. It’s commonly seen as “Correcting for sea level” or “The power it would make if it were at sea level.” Well, it’s not!
In order to understand the CF, you must first understand why you’re using a dyno in the first place. The dyno prints a number on a piece of paper, which is a measurement. But what good is a measurement if you have nothing to compare it to? The whole premise behind a dyno is to compare numbers of one car to another (or to itself after changes). Therefore, you must compare on an equal playing field. If you measure a distance in yards and another in centimeters, the two measurements are useless without some sort of conversion factor (for the dyno the correction factor) to make them comparable.
Simply stated, that’s all a CF is: a calculation taking into account known variables to equalize the comparables. If you dyno your car on a hot day, then take it home, add a header, and re-dyno it on a cold day, the power numbers measured will change. But are they changing because you added a header or because the temperature was colder on the second dyno day? A proper CF takes this into account.
A CF simply calculates the changes for known variables: temperature, pressure, and humidity and calculates the measured horsepower into corrected power. The SAE (Society of Automotive Engineers) has come up with predefined constants for these (77degF, 29.234inHg, 0% humidity). Now that we have constants, we can adjust accordingly. These adjustments are made all over the U.S. on every dyno (usually automatically by the dyno software). This now makes a level playing field where you can compare your numbers from a dyno at 6000 feet in Colorado on a 32-degree day, to someone at 0 feet in Florida on a 102-degree day.
This, however, does not mean that if you take your car to sea level, these are the numbers you will produce. If, for example, you max out your injectors at 6000 feet, when you travel to sea level, the air density changes, and you have to use more fuel, fuel that’s not available. Therefore, your car will not produce the corrected power numbers. (In fact, it will probably run lean and explode!) It’s simply a way for people to fairly compare different setups during different conditions.
Changing the CF (half CF)
The claim is that on boosted (super/turbocharged) cars, the CF should be only half of what it is for naturally aspirated cars. This is about the most ridiculous thing you can do! Not only is it incorrect, but it creates numbers that are absurd that cannot be compared with any other dyno (oftentimes not even with itself!), and back to the basics, isn’t that why you’re using the dyno in the first place – to compare? Think about it, if we’re correcting for humidity because the water molecules in the air are displacing the oxygen molecules, does that mean that on boosted applications half the water is simply going to “leave the air” before being sucked into the intake? No! So why are we only correcting for half of it? There are some applications where SAE J1349 states not to use a CF. (We will address this issue in the full review but even then, it’s not some made up number of half!)
Again, these numbers are not correct(ed)! Many people like to brag using “uncorrected” numbers. Well, were those numbers measured on a hot day or a cold day? One car on a hot day may show less power than another on a cold day, even though the actual results are different. The only thing an uncorrected number is good for is to show what that specific car would run at that specific time, temperature, altitude, humidity, and pressure if you were trying to calculate what it should run in the ¼ mile right then. Tomorrow, if the conditions change, the results will also change.
So Why the Differences?
As you can see from the chart, all of the dynos read within +/- 2 percent of each other except one, but why? If four out of five agree, you really have to start questioning the 5th one. Is it possible that it’s just a poor dyno? No, not likely. If some company produced a dyno that consistently created numbers out in left field, the general public would eventually catch on, and the company probably wouldn’t sell very many dynos.
Chances are it’s probably the dyno operator. Whether he mis-calibrated the dyno, is untrained in exactly how to use it, or is just downright shady, one can only speculate. But if you take a moment to look at the numbers, and listen to what the dyno operator is saying, you can often pick up some clues! Dyno #5’s operator used a trap speed calculator with this car since it has been known to trap 130mph in the ¼. Based on that and the weight, he comes up with a number of 488HP, pretty close to the numbers he already has from the dyno. Convenient? No, not really, it’s pretty simple math here. What the dyno operator does not realize is that the trap speeds are based on the uncorrected numbers, not the corrected numbers! Why? Because that’s what your car is actually outputting to the ground, at that exact given moment, and that’s exactly how it will perform. But mind you, we’re still at 6000 feet, so those numbers would do no good to compare with anyone else’s. What this seems to equate to is though the dyno operator claims a correction factor of 1.15, it appears there really is no correction factor being used! Either he mis-calibrated the dyno itself, or he’s an untrained user of the software. Either way, clearly the numbers are wrong!
How Can We Adjust for This?
In this case, it seems really quite simple. Though the dyno operator claims to be using a CF of 1.15, it appears there is, in fact, no correction factor at all being used (based on our analysis of the trap speed calculation). It’s really as simple as multiplying in a proper CF. If you measure the temp, pressure, and humidity at the dyno facility (we did!), there are plenty of online automated calculators you can use. In this case with the measured readings we took (66degF, 10 percent Humidity, 24.21inHg) we came up with a CF of 1.23 (or 123 percent). So, if we multiply the 489HP (Dyno #5’s average) by 1.23 we come up with 601HP! Now that sounds a bit closer to what it should be, now doesn’t it?
Let’s for a second entertain the theory that dyno #5’s operator was in fact using a correction factor of 1.15. If that was the case, then by dividing the result, 489HP by 1.15, you get an uncorrected result of 425HP. Now, let’s multiply that by the accurate correction factor for that specific moment in time of 1.23, and you’re left with 522HP. This is still a significant variation from the other four dynos. In statistical mathematics, we call this anomaly an outlier. And what do we do with outliers? We throw the data away because clearly there was either human or mechanical error in the calculation.
Too often people have a complete misunderstanding of dynos, correction factors, and how they all work together. Because of this, there seem to be a lot of disputes that can easily be resolved if people just take the time to sit down and really look at the facts. Not what they want to hear or see, but what is, in fact, the truth. As the owner of a dyno, I would love to sit here and tell you why my dyno is better than XYZ’s, but I can’t, because it really isn’t true. At the end of the day, all the dynos pretty much read the same. Oftentimes what makes the biggest difference isn’t the dyno itself, it’s the operator running it.
We will be expanding on this in the near future to include hitting more dynos with the same car (in the Denver area), as well as diving further into how the dynos work, what other factors can vary dynos (tire pressure, load, etc.), and fully expanding with all the info from each of the data logs from each run (time through pull, measured boost, temps). So, as always, stay “tuned!”
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