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Dynamometer to Dynamometer ?

Posted: Sun Jul 24, 2016 3:18 pm
by OldGuy
Seeking Professional Opinion.

In an online motorcycle forum I stated that only a back to back dyno pull on the same dyno, same motorcycle and preferably same air density would give another member license to call a Vance & Hines exhaust pipe junk and declare brand X pipe to be the best. Mr Trasher responded by stating that simply wasn't true because the curves would show all that need be known to evaluate the two exhaust pipes. Furthermore he stated that the dynamometers did not even have to be the same brand.

I quit drag racing back in 1980 so I'm not current, however, back in my day you could gain horsepower and torque by simply hauling your engine around to different dynos until you achieved peak numbers. Was I wrong when I stated: same dyno, same motorcycle and preferably same air density.


Re: Dynamometer to Dynamometer ?

Posted: Sun Jul 24, 2016 5:02 pm
by Kevin Johnson
Sounds like you have a good handle on it. That (air density) was first on the list if you want to try to compare results between different dynos. ... no-testing
When it comes to dyno testing, think consistency. 1. Comparing Different Testing Locations

If you like fiction, you’ll like this adage, “It makes no difference where I test my engine, the air density correction factors will make my engine numbers comparable no matter where the testing took place.”

Look closely at all of the various accepted air density correction formulas. I can count four that I work with on a daily basis (DIN 70020, EEC 80/1269, ISO 1585, SAE J1349), and who’s to say which one was used with which brand of dyno?

They all state if a variance is greater than 3 percent of what you’re trying to correct to, it is suspect. So, say you want to correct to SAE J1349—atmospheric conditions to air temperature of 77 degrees,29.235 inches- Hg (990mb) actual pressure and 0-percent relative humidity, you’re testing in Denver and you want to compare results with a buddy in Charlotte.

Re: Dynamometer to Dynamometer ?

Posted: Thu Jul 28, 2016 11:12 am
by Zmechanic
Yes, in any kind of A/B test, best practice dictates you change as few variables as possible. Obviously, the preference is to only change ONE. Regardless of the notion that, in theory, the dynos should give the same results. That is their GOAL, but in practice is a different story. The air correction is not perfect. Also, when was the last time the calibrated their load cell? Have they overloaded the load cell before and hurt its linearity? When was the DAQ system cal'd? And on, and on.

Back in my younger days, I witnessed a rather unscrupulous person dyno a bracket motor, then fudge the air correction to give it a nice little peak HP boost to pump up the dyno sheet before he advertised it for sale. [-X