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Correcting for a Changing World

    There are many conflicting stories and opinions when running a query on the internet.  One has to be careful about digesting information and using it to form a plan of attack.  There are just as many people out there who say vaccinations will kill your child as will cite the statistics for eradication of disease.   So how do we know we are getting good info?  The truth is, it's hard, but each of us must find an expert and seek their guidance for things we wish to know.  It doesn't matter if you're looking for a second opinion on surgery or who makes the best camshaft for your motorcycle.  It's wise to find an impartial person with a good deal of experience.  It's also wise to double check the experts on your favorite forum.   I'm not certain, but it's hard for me to comprehend how a guy with 20,000 posts in a forum has any time to build, test and tune his way into expert-dom when all he clearly does is spend 16 hours a day on forums.

    I've read on more instances I can count, how the only way one can compare two sets of dyno numbers is to have them done on the same dyno, on the same day.  In some cases I believe that is true.  One case would be if the dyno was so old, it had no means of electronic environmental data collection for correction factor calculation, or if it was one where the dyno operator had to manually enter the weather conditions.  In one case the user would end up with a chart that detailed the power made that day under those conditions and only comparable to a test performed at the same conditions.   In the other case, he could have a little human error (intentional or not) factored into the conditions.   Another case might be where the dyno is located so far above sea level or so far below sea level or be operated at such incredibly high or low temperatures as to have huge correction adjustments that are too large to be trusted due to rounding errors and other factors.  Additionally, different brands of dynos read differently.  This shouldn't be the case, but it's a nasty fact of life.  Dynojet is the worst offender here.  Not only are their numbers inflated compared to all other brands, they are not consistent from unit to unit.  If your a Dynojet owner, you surely have seen the power reading change with software revisions.  If you own another brand of motorcycle dyno you most likely have a selection of power output in real value or DJ values.   How and why Dynojet became the dyno with higher readings than everyone else is well documented.  You will find a link here soon with more background on that issue.

    Other than the cases mentioned above, the correction factor is something one can trust to ensure that the changes in power output they are observing are due to engine and drive line changes and not due to environmental factors.  Also, consider that when an engine manufacturer makes a change, they must be certain  the new camshaft, cylinder head, air box, exhaust or what ever else they are testing changed power in the manner they believed.  Therefore, the dyno is calibrated with a dead weight (OEM manufacturer's do not use inertia-only dynos for development) to calibrate the torque load cell.  If they can believe the weight and they can believe the rpm reading, they can believe the horsepower reading.  The air pressure is measured, the temperature is measured and the humidity is measured.  Each of these affects the power produced.  Lower temperature air is denser and contains more oxygen, so as a result for each 10 degree drop in temp power goes up approximately 1%.  Barometric pressure forces air into the engine.  As it rises, more air is ingested with each intake event and thus power rises.  It drops as baro pressure falls.  Humidity is the least significant of the factors, but it should be recognized that each cubic foot of air can only contain so much of any substance.  Air that is full of water contains less oxygen than dry air.  As a result, power drops with humid air. 

When tested, an engine's power is measured and the weather conditions are compared to standard.  The temperature, pressure and humidity level are entered into a formula known as a correction factor and that formula applied to the power numbers.  A correction factor of 1.00 means the power measured has been multiplied by a factor of 1.00.  That means the numbers measured that day have not had any adjustments what so ever.  A factor of 1.04 means the number has actually bean increased by 4% since the conditions were less favorable than the standard.  These standards must be used because engineers need to know what works and what doesn't. 

Yes, there are many internet experts that will say you have to go back to your tuner on the same day or else the results are not valid, but that would mean that every auto manufacturer had to do every camshaft change and every cylinder head port and valve change, and every compression ratio change , intake runner length modification, exhaust diameter change, header length modification, catalytic converter volume test and everything else they could do during development on the exact same day.   Since everyone with an IQ over 60 knows that's not possible or practical, we can come to the realization that correction factors (CF) allow development over, days months and years.  And yes, the manufacturers sometimes have more than 100 dynos in their test lab.  The technician is not always using the same dyno for every test.  They are calibrated and verified to be repeatable from unit to unit, usually within less than 1%.

Poplular correction factors are SAE, ECE, DIN and STD (sometimes called STP).  They each have their own base value for temperature, pressure and humidity that each standard considers to be normal.  That doesn't mean optimum, just what each governing body determined would be a good value or base line to compare all measurements to. 

So, how do we know when our forum expert needs a refresher course - or a an introductory course - in dynamometers and chart reading?  As soon as you read something like "SAE smoothing 5" you know to start discounting his expertise.   There is no SAE smoothing.  What our many failed internet experts are doing is reading the top line of the Dynojet dyno plot which reads, "CF: SAE  Smoothing 5"  The line is telling you "Correction Factor is SAE" smoothing factor applied to curve is 5."  Thus the shop used the latest SAE correction factor and applied a smoothing to the curve of 5.  The 5 only tells us that they employed the highest possible smoothing so that chart didn't come out so bumpy.   It's very typical of Dynojets with their slow processor (comparably to other dynamometers) and single trigger finger on the drum to suffer signal aliasing resulting in bumpy charts.   Usually the operator is instructed to apply maximum smoothing to make it look pretty.  I can't say for sure how they are smoothing but often the math will involve averaging the surrounding data points.  At data point 10 for example, we will include all the samples from data point 6 through 14 and average them to create the smoothed point 10.  Data point 11 includes points from 7 through 15 and so on.  The nasty spikes go away and a pretty chart is the result.  I can assure you it has absolutely nothing to do with SAE correction factors.  There is no SAE Smoothing.  Type that and you'll be alerting everyone on the net that you have yet to learn how to read your own dyno chart.

I've taken a bunch of text below from SuperFlow's dynamometer literature.  It's from a dynamometer usage and correction factor guide.   It is quite possibly the best written piece I've ever come across.  Kudos to Superflow.  All credit to them.

The power output of an internal combustion engine is significantly influenced by barometric pressure, ambient air temperature, and air humidity.

Lower ambient barometric pressure reduces the density of the air, thus reduces the amount of oxygen filling the cylinder for each cycle, resulting in lower power output. Conversely, higher barometric pressure increases power. 

Lower ambient air temperature results in increased density of the air, thus increases the amount of oxygen filling the cylinder for each cycle, resulting in higher power output. Conversely, higher air temperature reduces power output. 

Lower air humidity (less water vapor) leaves more room for oxygen per cubic foot of air, thus increases the amount of oxygen filling the cylinder for each cycle, resulting in higher power output. Conversely, higher air humidity reduces power output. 
 

Several Standards organizations have determined methods for estimating engine power under reference conditions. The best known organizations are:

ISO (International Standards Organization), worldwide 
SAE (Society of Automotive Engineers), USA 
ECE (European Community), Europe 
JIS (Japanese Institute for Standardization), Japan 
DIN (Deutsche Industrie Norm), Germany 

There are power correction standards for gasoline and Diesel engines, for applications in road vehicles, stationary engines, or marine engines, etc. For a motorcycle dynamometer, relevant standards are those generally intended for gasoline engines in road vehicles and those specific to motorcycles. 
 

Power correction standards try to estimate what engine power would be under reference conditions. They cannot actually calculate exactly what power output would be. The greater the difference between the ambient conditions during the test and the reference conditions, the greater the error in the estimate. Most power correction standards include limits on their applicability. This limit is typically +/- 7%. This means if the correction factor is greater than (>) 1.07 or less than (<) 0.93, the corrected power numbers are not officially considered to be acceptable, and the test should be performed again under conditions which are closer to the reference conditions. For private applications this is less of a problem, and the corrected power numbers are still the best basis for comparisons. However, please keep this into consideration when comparing test results obtained under considerably different test conditions.

Power corrections are only valid for Wide Open Throttle (WOT) tests. You should disregard corrected power numbers for any test performed under partial throttle conditions. The default configurations supplied with the CycleDyn system include power corrections to the following standards: SAE, STP, ECE, DIN.

SAE -- The SAE standard applied is a modified version of the SAE J1349 standard of June 1990. Power is corrected to reference conditions of 29.23 InHg (99 kPa) of dry air and 77 F (25°C). This SAE standard requires a correction for friction torque. Friction torque can be determined by measurements on special motoring dynamometers (which is only practical in research environments) or can be estimated. When estimates must be used, the SAE standard uses a default Mechanical Efficiency (ME) value of 85%. This is approximately correct at peak torque but not at other engine operating speeds. Some dynamometer systems use the SAE correction factor for atmospheric conditions but do not take mechanical efficiency into consideration at all (i.e. they assume a ME of 100%).

STP -- The STP (also called STD) standard is another power correction standard determined by the SAE. This standard has been stable for a long time and is widely used in the performance industry. Power is corrected to reference conditions of 29.92 InHg (103.3 kPa) of dry air and 60 F (15.5°C). Because the reference conditions include higher pressure and cooler air than the SAE standard, these corrected power numbers will always be about 4 % higher than the SAE power numbers. Friction torque is handled in the same way as in the SAE standard. 

ECE -- The ECE standard is based on the European Directives. Power is corrected to reference conditions of 99 kPa (29.23 InHg) of dry air and 25°C (77 F). Friction torque is not taken into consideration at all. In 1995, a new Directive (95/1/EEC) regarding test methods for motorcycles was published. 

DIN -- The DIN standard is determined by the German automotive industry. Power is corrected to reference conditions of 101.3 kPa (29.33 InHg) of dry air and 20°C (68 F). With the advent of European legislation and standards, national standards such as the DIN (formerly widely used) are now less significant. 
 

There is a tendency for all those standards to converge. The only worldwide power correction standards at this time are the ones determined by ISO. For internal combustion engines in road vehicles, this is the ISO 1585 standard. The current SAE J 1349 and ECE standards are nearly identical to the ISO 1585 standard.

It should be noted that even with modern dynos, there can be ways to manipulate the data.  In the old days one could either write the data down with a fudge factor (on manually recorded dynos) or whack the water brake control valve wide open causing a sudden load that uses the inertia of the system to create an artificial power spike.  Today we have automated systems that prevent this, however it is often seen that unscrupulous shops will change correction factors run to run.   Since the old STD corrects to a lower temp and higher pressure it reads about 4% higher than the current SAE standard.  So yes, using the STD value will make your sportbike display 6 more hp on it's chart than the 150hp SAE corrected chart.  What's worse is that just a few years ago, one dynamometer manufacturer had an ambient temp probe that was a small hand held unit.   It was nice as it allowed the tuner to place it near the vehicle's intake so any engine heat that affected the air into the airbox could be accounted for.   Unfortunately the sneaky tuner who did not gain any power, but perhaps even lost power, could make base line runs with the temp sensor on his desk.  After the disappointing tuning session he could throw the temp sensor on top of the engine or even just behind the exhaust fan of his desk top computer. The resultant 20 to 50 degree temperature rise would give a 2 to 5% power increase over the initial tests.  "Wow!  What  great tuner I have."  Yes, temperatures rise in the dyno room as engine and exhaust heat affect the area surrounding the bike.  This should be taken into consideration when tuning and it's OK if your chart shows a temperature rise as the tuning session progresses.  It may fall, it may rise, but there shouldn't be a 30 deg F rise from baseline to best power run.

It's unfortunate, but dyno chart manipulation is common enough that I can speak to knowing of the break up of a business partnership.  Yes, that temp probe trick led to the demise of a partnership in Colorado.   A computer science guy and motorcycle enthusiast joined forces with a popular head porter form Louisiana.  They did a great business together until the engineer partner noticed that his porting and engine building partner's dyno tests had recorded a 120 deg F ambient temp during the final runs.  No one believed the 120 degree temps in a Colorado December.  Especially not the managing partner.  When your own head porter can't provide better or more power without falsifying power data, who can you trust?

There was once a dyno manufacturer (actually, he still sells dynos) that touted that there was no such thing as STD.  He had a rant on his website denouncing another manufacturer's product as junk because they allowed the user to print out charts with STD correction factors on them.  His claim was that there was no such thing and that the company was simply allowing the tuners to print uncorrected data out and hand that to the customer resulting in unrepeatable junk data and the ability to swindle customers.  I found it surprising that this guy, the manufacturer of a dynamometer, never heard of STD, that he had no idea that STD was also known as STP, and that it was a valid (if outdated) standard.  It also helps to know that the manufacturer doesn't really make his own dynos. They are built to his specs by a large automotive dyno manufacturer.  As a result, he wasn't really knowledgeable regarding correction factors.  The scary part is that he sold the damn things.   If he doesn't know what he's talking about, how can you trust any other yahoo on the internet? 

The truth is, my own dyno allows me to use SAE, ECE, DIN, JIS and I can even display uncorrected.  The CF is displayed on the print out, so everyone can tell if I print an uncorrected chart.   Professionals may want to read the uncorrected data.  Sometimes it's good to know.  I've also stopped to show customers the difference on more than one occasion.  It's fun to be able show a guy a Street Glide that just make 99 hp compared to the 66 it was making before he hot rodded it.  Then to show him that it's a cold dry day and it really high pressure day too.  In all reality his actual power output was 102 to 103 ponies!  Being happy that your customer technically broke the ton on his big twin is kind of fun.  Yes, the SAE CF brings him to 99, but that day at that time he broke 100.  In the example case I just explained to him that the 99hp is what he should use to compare other future modifications.  But it's great to sometimes see the raw data - why a manufacturer would rant about such a feature is beyond me.  When doing normal tuning and printing the resultant graphs I always use the latest SAE standard.  STD is outdatated and reads 4% higher than SAE.  It really shouldn't be used.  If you're in Germany or Holland DIN makes the most sense.  Elsewhere in Europe ECE is popular and I would use that if most shops in my area were using it.  In Japan and many parts of Asia JIS in the default.  That said, if your dyno guy is going to BS you, you need to find another dyno shop.  Hopefully he's knowledgeable and excited enough about tuning your bike as to WANT to show you different charts.  When you see this you'll know he knows what he's talking about.  His enthusiasm displays his investment in your motorcycle.  When you find that tuner, keep his business card on your shop wall.  He's the one who will deliver quality tuning regardless of the correction factor he chooses.

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Last modified: 01/31/16