rmbuilder

This is a thread that began in another forum and since have received numerous emails asking me to post this topic on OSO. I want to thank all the people that have already replied (with very relevant information) and welcome them to place that input here also for the members of OSO.
Thanks.

There has been much written, on this and other enthusiast forums, regarding the accuracy and interpretation of dyno sheets received by customers of engine shops and dyno operators. Dyno software (such as WinDyn), their inputs and correction factors, is subject to manipulation and the GI GO (Garbage in/Garbage out) factor, as is any analysis program. This often results in confusion and frustration to the end user, who often is not skilled in the acquisition and interpretation of this data. Baselines are lost as a result making the final performance evaluation of your project (Performance increase in direct A/B comparison in the car/on the water) difficult at best. When you pay good money for a complete engine or testing session, you pay for the right to examine all your data. Observed power/Corrected power/BSFC/BMEP/A//F ratio and most importantly the correction factor used and how it was arrived at. Absent all this information, a dyno sheet contains little relevant data. Below is an article that will hopefully help many avoid the potential pitfalls of dyno testing. I will add links to this post at a later date; with many articles that will clearly define and help simplify testing procedures/data outputs to better understand your dyno sheet.

Attached below is a direct transcript of a sidebar contained in the article (Dyno Days) published on page 122 of the July 2006 HRM. I would like to acknowledge the author as Mike Magda.


[Quote]
Fudging The Numbers
Dyno numbers released to customers often don’t include the actual test readings for the engine. Customers are given corrected horsepower and torque numbers that are computed from the raw numbers to reflect changes in temperature, atmospheric pressure and altitude. This correction factor provides a standard upon which testing can be equally compared under different weather conditions or from different locations. During this correction process, unsavory or ego-driven engine builders can fudge the numbers given to customers or magazines.

Tracy Dennis of Sunset RaceCraft Racing Engines dynos more than 400 race engines a year. On some days he runs eight engines as part of a sealed engine race program for a foreign stock car series. All of the 150 engines he has built for the series must leave the shop within a few ponies of each other. The engine are also dyno’d in the other country to confirm Sunset’s consistency, so Dennis is very meticulous about calibrating his DTS dyno and keeping a close eye on all the correction factor inputs to ensure repeatability.

Dennis says the first opportunity for misrepresenting the numbers comes during the calibration. At a minimum dynos should be calibrated once a week. This involves hanging a measurement weight from the calibration arm that is bolted to the dyno’s absorber. A 2-ft arm that weighs 7 pounds and is supporting a 150-pound ball at the end should factor in at 307 ft-lb. But if a 100-pound ball is hung from the arm (207 ft-lb), and the calibration baseline is still set at 307, the numbers generated by the dyno test will be higher because the absorber has a 100 ft-lb advantage.

Most dyno operators looking for an edge won’t mess with the initial calibration but instead will fudge the other inputs to throw off the correction factor. In simple terms, dynos are usually programmed to correct the raw number to reflect how the engine would perform at sea level with 60-degree air temperature and a barometric pressure reading of 29.92 inches of mercury. If the dyno requires the operator to input the air temperature, altitude, or pressure, these numbers can be fudged to give higher horsepower correction. So if the air temperature across the carb is 78 degrees but the operator tells the dyno its 95 degrees, a higher correction factor will boost the numbers.

Most dynos are linked directly to a weather station hung near the air inlet, but Dennis says the data acquisition software can be manipulated to juice the numbers.

“The dyno operator can program the computer to correct to a barometric pressure higher than 29.92 inches and get more horsepower", he says.

Using a recent dyno test on a small block Dennis showed us a peak of 577 raw horsepower that corrected to 677. By raising the atmospheric pressure point in the correction formula to just under 31 inches, the adjusted numbers showed more than 700 HP.

Nicky Fowler of Scoggin-Dickey Parts Center often hears complaints about questionable dyno tests. He suggests customers request that the raw torque and horsepower numbers as well as the atmospheric conditions be included on the dyno printout. If the correction factor seems extraordinarily high, customers can ask the dyno operator for clarification. Customers who question the test day’s weather conditions can check Weather Underground (www.wunderground.com) for a complete rundown of the weather history in any zip code But make sure you understand all the weather definitions before you go charging back in to the engine builders shop. Dynos may measure absolute atmospheric pressure, not the pressure readings corrected to sea level you hear on the news. Vapor pressure and density altitude are other factors often used in dyno calculations. Both are computations from weather conditions not always found in weather reports. If you have accurate weather information, numerous Web sites offer dyno correction formulas that consider different weather factors. One of the easiest is www.wahiduddin.net/calc/calc_cf.htm.

There are other tricks to fudge the numbers, even if the customer is in the control room watching. The classic move is allowing the air-inlet-temp sensor to rest on the intake manifold instead of reading the actual air temperature. The hotter temperature tells the computer to generate a higher correction factor

Bottom line: Find reputable dyno operators who will work with you, be open with all the numbers, and answer all your questions.
[End Quote]

Bob

SB

iTrader: (1)

Good info, as always RM.

A good article (not great, but good) to read if one does not know much about what factors/parameters go into a dyno test I suggest reading this quick article from 'Car Craft' magazine. Remember, it's a quick article, but will make one start to understand there is more than push a button and 'it is what it is' .

http://www.carcraft.com/techarticles/986/

articfriends

iTrader: (1)

There has been ALOT of motor builds on websites this spring with BS dyno numbers from "overly generous" correction factors.For anyone that doesn't get real technicle this is the long and short of it:Correction factors use the weather conditions at the time and day of the motor being dynoed combined with the altitude/location of dyno facility to standardize and give a consistent output so the results can be repeated at any location and weather condition,nothing more/nothing less. There has been a pattern of FLY BY NIGHT/UNSRCUPULOUS engine builders who crawl out from under a rock touting UNREPEATABLE results from engine builds no different than others have been doing for years. The biggest way they INFLATE their dyno results is by taking intake air temp on a hotter spot on the motor.I recently seen one of these so called "experts"post dyno results from a 540 that made 667hp,he inflated the hp numbers to 740 by raising the intake air temp sensor reading from the 60 degree ambient he had ducted into the motor from outside to 130 plus degrees. The motor was getting the outside air but temp sensor was sensing heat off the motor.The correction factor went from the 2% it should have been (which would have put the motors corrected output at 680) to 11% which INFLATED his numbers to 740 hp (60 more hp than what motor was ACTUALLY making). After doing this he proudly put the motor up for sale for 26,000$ claiming he has a "super secret" engine building "formula" along with porting tricks that his "team of secret head porters" have perfected. The person who stands to get skrewed in a deal like this is YOU,the average boater who is writing the check. Correction factors are important,if they are manipulated you may end up buying a motor or having a motor built that doesn't perform up to what you expected,your boat will have a dissapointing increase in speed and you will be wondering why its not turning as tall of a prop as you expected (the ultimate test of power increase),Smitty

offthefront

iTrader: (1)

Kudos Mr Builder ..........

tomcat

It can work both ways. We have to watch out for the correction factor when testing centrifugal blowers. Due to their mounting position these blowers can have their intake air heated by the hot dyno headers, while the dyno's air temp sensor is measuring room temperature. If the operator "corrects" with the room temp instead of what actually enters the blower, the corrected HP is low.

articfriends

iTrader: (1)

So tomcat,on a innercooled blower motor where would the most accurate place be to measure intake air? probably mount the intake sensor as near the air horn as possible?. It does make me wonder,when I dynoed my motor at Crocketts last year I don't know where he measured the air temp or what my exact correction factor was but his cmi dyno headers are water cooled.The observed hp numbers and corrected numbers were almost identical,from what I can figure correction factor was less than 2% but it was 60 degrees out so it should have been,Smitty

Whiteknuckle

Great Info RMBuilder!

tomcat

Smitty: You wouldn't have much of a problem with water cooled headers, but the temp should be measured at the inlet of the supercharger. Sometimes the dyno has the sensor in the big bell inlet for carbs which obivously doesn't fit the blower inlet, sometimes the sensor is in a weather station mounted elsewhere. To prevent any confusion we put a thermocouple right at the blower inlet and correct the observed numbers ourselves.

articfriends

iTrader: (1)

I'll keep that in mind for next time,thanks,Smitty

Vinny P

Hey Bob,

As usual, great information!! It almost seems to me that our phone conversation and my dyno sheet I sent you somehow influenced this thread.

I sent you the information you requested about my current valve springs.

Vinny