Below are some more tips that may help those that are try to look at dyno graphs and evaluate an engine package or component.

There is more to dyno testing than buying a dyno and then spending a few days at the dyno manufacture orientation and training class for new dyno owners.  I talked to a couple of guys that were engineers and the company they worked for sent them to the new dyno owners class.   After talking to these fellow engineers, I decided not to waste any money attending the new dynos owners class.  The two engineers I talked to have been designing and developing engines and operating other brands of dynos for years said it would be a total waste my time and money.   

The two guys said the general theme of the class was how conduct dyno test and try to impress your customers with the new expensive piece of equipment that you just bought.  The instructor used the classes as another opportunity  to try to sell your more of their overpriced equipment.  Another theme (sales pitch) of the classes was: (You can go from zero tuning experience to tuning engines like the professional tuners if you spend another few thousand dollars for our Air fuel testing equipment, another 4K or so on our eddy current option and another 1200 per year to rent the tuning software)     

Anyone with considerable two stroke tuning experience will tell you the same thing about trying to jet a two-stroke using an O2 sensor especially one where you are using a sniffer tube to send the exhaust sample to the air/fuel meter.   You cannot tune a two stroke the same way you tune a four-stroke using an O2 sensor.  The sad thing about a large percentage of the new dyno owners that attended the dynos for dummies class, are tuning customers two strokes using the methods that were taught at the classes. 

Understanding the limitations of your test equipment (dyno and related test equipment) and how the test methods affect the engine being tested is one of the keys to getting repeatable and meaningful data from your dyno test.  They do not teach this at the dynos for dummies class.  Getting meaningful and repeatable test results is learned while attending and studying at the University of Hard Knocks.  A degree from the University of Hard Knock may be earned in a few years or less for some and others attend daily and study at this world famous University for a life time and never get close to earning their degree. 

The engine and pipe temperatures being the same at the beginning of a series of back-to-back dyno test is very important on a high performance two stroke.  The length of time the engine is under load (dyno run time) needs to be different for different engine applications.  Drag racing 300', 1/4 mile drag racing, hill racing, short course, flat track, road racing, etc. need different dyno run times to simulate the internal engine and pipe temperatures.  One of the things taught at the dynos for dummies class is the run time on the dyno should be the same as the time the engine is under load in the real world.  This makes sense to laymen but those that have an in depth understanding of thermodynamics and have done extensive testing do not subscribe to or support this test procedure of “run it on the dyno like you race it”.

Another important variable in a testing procedure is the amount of rest or time that elapses between back-to-back dyno runs.

Comparing back-to-back dyno runs from different engine setups is another VERY IMPORTANT area of data analysis that is often not used correctly.  Especially when testing two stroke engines.

Four stroke engines usually require 3 or more 5 to 10 second runs for the oil and pipe temperature to reach their optimum temperatures before the engine will produce maximum power.

Two strokes sometimes will show their highest torque reading on the first dyno run and the highest power run in runs 2 through 4 when back-to-back dyno runs are being made.  If you are doing back-to back testing ALWAYS MAKE THE SAME NUMBER OF RUNS for each engine setup being tested and maintain the same amount of time between runs.

When analyzing different engine set ups or comparing totally different engine packages ALWAYS COMPARE LIKE DYNO RUNS.  This means compare the 1st run of one setup with the 1st  run of the other engine setup.  Compare run 2 of one setup with the 2 nd run of another engine setup.  You see the trend compare run 3 on one setup with run 3 on a different setup, so on and so on. 

I usually throw away run #1 and sometimes run #2 from a set of 3 to 5 back-to-back test.  Run #1 seldom repeats where successive runs 2,3,4 etc do repeat if you are testing a well developed and well tuned engine that is not trying to hurt itself. 

For two strokes, run # 1 usually has the highest peak torque and the most low-end power on a set of back-to-back runs.  The last run of a set of back-to-back runs will usually show the highest power after the power peak (what most tuners call the over-run or over-rev portion of the power curve.).  Comparing run #1 on one set up with run #3 of another setup or dyno run 4 on one setup with dyno run 2 of another setup is MEANINGLESS but often done. 

Always look to see if the type of correction factor is the same on all dyno runs being compared.  Types of common correction factors are STD, SAE, DIN, JIS, ECC.  Most of the shops advertising there products usually use the STD correction factor because it make the numbers the largest.  I like to use SAE because it more closely corrects the power numbers 

Dyno runs made at sea level will often have correction factors in the .97 to 1.03.  Dyno runs at 3000 ft elevation will usually have correction factors in the 1.08 to 1.14.  It is common for Dyno runs made at around 5000 to have correction factors of 1.20 and higher.  The unfortunate fact about correction factors is the math is correct for calculating the correct change in air density BUT TWO STROKE ENGINE POWER DOES NOT RESPOND to the air density change like the corrected air density multiplier (correction factor) predicted.  The correction factors were primarily developed to predict the power output of a 4 stroke aircraft engine at different altitudes while be tested in a dyno cell at ground level.

Dyno runs made at sea level with sea level correction factors should not be compared to dyno runs made at 5000 ft with correction factors around 1.20.  The Society of Automotive Engineers (SAE) says any dyno runs that have more than a 4 % difference in the correction factor cannot be compared because the engines do not respond to the air density change like the correction factor predicts.  This is the reason why back to back testing is so important.  The correction factors will usually vary more that 7 percent from winter to summer in the same test cell.

Engines should be developed and tested at the altitude (air density) where they will be used.  Be careful buying cutting edge engine products from companies that are located and do their testing at altitudes that are 2500 ft and higher unless you race at similar altitudes where the engines were developed.  Most engines have about a 2000-foot elevation window where the engine will produce the power that the correction factor predicts and can be tuned for max power without detonation being a problem. 


Example:
If an engine was developed at 1000 ft elevation, it will make good power and respond to tuning in a predictable manner from sea level to about 2000 ft.  If the ports, pipe and head was developed at 1000 ft it can usually be tuned for max power at sea level without detonation provided it was a well developed engine package.  If an engine’s port, head and pipe was developed at 3000 ft it will usually tune and run reliable from about 1000 ft to 4000 ft elevation detonation free.  If the ports, head and pipe was developed at 3000ft it will probably encounter detonation at sea level if you try to jet it for max.  If it was developed for max power at 3000 ft you can usually jet it for max power at 2000 to 4000 ft elevation providing it was a sound engine package that could be tuned at for max power at 3000 ft without detonation issues.