posted on August 30, 2011 22:50
Don't Trust Strangers! How Dynos Lie and When to Believe One
by Khiem Dinh
Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing. All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.
Ah yes, the dyno plot and power numbers. The dyno plot is the subject of many forum pissing matches, the holder of false prophecies, the teller of partial truths… Reading the forums, you would be led to believe that everyone had their car dyno'd on a "low reading" dyno. Well, except for in South Florida; there must be some magic octane booster in the air down there. Dynos are a great tool for tracking performance changes, but trying to compare across different dynos on different days with different cars is borderline meaningless. We’re here to tell you why. We'll also tell you the best case for getting results you can compare.
First, we should give a very very (very) brief overview on dynos as related to cars. There are two main categories: engine dynos and chassis dynos. Big companies and race teams use engine dynos in highly controlled test cells that enable the operators to have extremely high accuracy and repeatability. Eric posted up a little bit about the engine dyno test cell at Cosworth here and here. The dynos that most people in the aftermarket industry are familiar with are chassis dynos where you attach a car and go. Because a chassis dyno measures at the wheels or hubs, additional rotating masses and frictional losses are taken into account.
To show how power is calculated, I did this quick little derivation. Now you also know why horsepower and torque always cross at 5252 rpms on a dyno plot (assuming horsepower and torque are plotted on the same scale axis). Did I mention that math and science are cool? I think I have before.
|Power is equal to energy per unit of time. Energy is equal to a force times distance which just happens to be the same as torque. Because our distance is a wheel spinning, we have to throw in the 2* pi radians to convert revolutions into a distance value. I started off with SI units because they actually make sense.
|Why the heck does 1hp = 550 lb-ft/s? Why are there 12 inches in a foot or 5280 feet in a mile? That's why I like working in SI units. Anyways, after doing all the unit conversions, you can see the silly 5252 down there.
Let's go into some of the reasons that you can't compare numbers between different dynos. #1, there are a bunch of different types of dynos! Trying to compare absolute numbers between a Dynojet, Mustang Dyno, Superflow, Dyno Dynamics, Dynapack, and the other handful of dyno types just doesn't work. Do the weight scales at the doctor's office, gym, grocery store, and your bathroom all read exactly the same? I don’t think so. #2, even if they are the same type of dyno, their calibration could be off. In a manufacturing, scientific, or laboratory environment, every measuring device has to have its calibration checked often and certified to ensure its accuracy. How many dynos do you know of that get calibrated often? #3, the setup of the car can alter the dyno reading. If the tire pressures, type of tire, or alignment are different from a previous dyno session, the results will be altered. Even if the car is strapped down differently, that can alter the readings and strapping down the car to varying degrees can alter the alignment as the suspension compresses.
|The guys at Edmunds Inside Line strapped down a Lexus LFA to an in-ground Dynojet chassis dyno at MD Automotive.
Posted in: Magazine
Wednesday, August 31, 2011 1:52 AM
Haha octane booster in the air in Florida? I've never found anything but heat (bad) and humidity which I have seen articles stating that humidity works similar to water injection. Maybe that's the magical octane booster? That or the fact that our pumps actually carry 93 octane unlike the 91 octane pee water they use for fuel in California!
Wednesday, August 31, 2011 2:20 AM
I think he's making a joke about how dyno readings taken by tuners in South Florida (Miami) are somewhat notorious for inflated numbers. This is down to the sheer quantity of trailer-queen horsepower pissing matches that go on down there. Patrons of tuners pay big bucks to have the highest numbers so the tuners have incentive to inflate those numbers artificially.
Also, you can only get 91 octane in California!? That's sad. I thought there where all kinds of crazy high-performance cars in Cali? What do they do for fuel? You can get 94 octane pump-gas (Ethanol Free!) here in Indiana quite easily. Circle Oil Company tends to carry it as well as many of the stations affiliated with big grocery stores (Meyer and Baesler's). Of course there are also a lot of stations around here that carry race-gas as well. You can get 104 octane right out of the pump! Not exactly street-legal though. That certainly doesn't keep the turbo Honda guys around here from filling their cars 3/4 full with 93 octane and then spiking it with a 5 gallon jug of the good stuff :) Suppose that isn't much surprise given that VP Fuels has their Midwest distribution center right here in Terre Haute. I guess there are some benefits to living in "The Arm-Pit of America" after all!
Wednesday, August 31, 2011 5:22 AM
Yes, cars tested on the dynos in South Florida often magically hit really high numbers relative to what is more commonly seen across the rest of the nation.
91 octane is the highest you can get in Cali. Even then, it sucks. I've heard a tuner say that cars that do not knock on 89 octane in Oregon WILL knock on Cali 91 octane. I've also heard 91 octane in Arizona is better than Cali 91. No direct experience myself, but that's what I've heard.
Wednesday, August 31, 2011 8:04 AM
Also in the armpit that is Indiana and I have 4 e85 stations in about a 10 mile radius... And the BMV didn't bat an eye when I registered my Evo VII... I love this place sometimes!
Wednesday, August 31, 2011 10:31 AM
"Reading the forums, you would be led to believe that everyone had their car dyno'd on a "low reading" dyno."
....and "conservative tune" ;)
Wednesday, August 31, 2011 11:20 AM
In Montana the highest octane we get is 91 as well. Knocking on 91 in Cali and not knocking on 89 in Oregon could have as much to do with temperature, humidity, and pressure (altitude) as the actual gas itself.
Good article. It seemed to be pretty free of bias like many articles that try and promote their favorite brand of dyno, or bash some other brand the author doesn't like.
I have always thought repeatability is the most important thing in a dyno and the operator is the biggest contributor to that.
Wednesday, August 31, 2011 2:22 PM
Good reading spracerut.
Here in California there are some tuners that "manipulate" HP readings with gear ratio, so the customers are happy, and post their pissing-contest dyno sheet on their favorite forum and the drama goes on.
Wednesday, August 31, 2011 10:25 PM
Khiem - I don't know where California sits but Oregon and Arizona have let ethanol creep in. Oregon is up to ten percent now, but I'm seeing a lot more 92 octane up here as well.
Thursday, September 01, 2011 9:08 AM
Good info. Was a nice read spdracerut.
Thursday, September 01, 2011 10:35 AM
CA 91 octane is the fail.
But, that's not the highest you can get out of the pump, as there's a gas station fairly close to me that sells 100. Technically, you're not supposed to pump it directly into your car, but that's what the 5 gallon "water" jugs I have are for (those fuel jugs that are legal everywhere else aren't legal to be sold as "fuel" jugs here, so they're sold as "water" jugs).
Really, CA fuel and emissions laws in general are the fail. :D
Thursday, September 01, 2011 7:17 PM
What is your guys take on the MaHa LPS Dyno:
It claims it can account for drivetrain loss and accurately calculate crank horsepower. It still runs into all the problems you've stated in this article, I just wanted to see your take on the crank horsepower claim. Thanks for all the article btw, this is one of my favorite informational sites!
Thursday, September 01, 2011 11:14 PM
@skilit, interesting that the MaHa dyno can do a constant speed and also uses an oil temperature probe with that data logged along with the power/torque info.
In Europe, generally the dynos are set up to report an estimated crank horsepower. Of course here in the US, they just report wheel hp. That difference actually created a whole lot of forum talk once because the guys at Top Gear UK dyno tested some American muscle car (if my memory serves me correctly), and most of the people here in the US were saying it was impossible that the car put down that much power. Well, that's because the guys in the UK report estimated crank HP numbers and the majority of Americans don't seem to know this.
Anyways..... there is some coastdown procedure they use on the dynos which gives the estimated drivetrain losses. The dyno then uses that data to estimate a crank horsepower value.
Friday, September 09, 2011 10:07 AM
estimating crank from wheel HP is kinda another silly game like adjusting for altitude and such.
The less manipulating you do to the number the better, I agree consistency and repeatability is the main goal in Dyno operation.
MotoIQ Proudly Presents Our Partners: