More Dyno Discussion, and Photos from the Mann Engineering Dyno Day

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I know a few of you had some questions after seeing the dyno results from our E85-fueled BRZ and the accompanying video, and I was actually in the dark myself about what some of the terminology on the dyno results actually meant. So I reached out once more to Gary Liu of Mann Engineering and he was able to provide some more information:

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86DRIVE: So, we get a lot of different numbers on that MAHA dyno results sheet. Which one of these would be what we would consider the “crank horsepower” number?

GARY LIU: The Engine power figure is what is actually measured to be at the flywheel or at the back end of the crank before it goes through the transmission, differential, and wheels. The dyno is able to calculate this number because it knows all the rotational mass of everything and how much torque is needed to overcome a load that is put on the rollers. From that the dyno is able to accurately measure how that power is being transferred through the rollers. This is pretty much a “raw” measurement based on the measured torque load put through the dyno rollers as the car is accelerating. This raw measurement is truly what your engine is doing based on the ambient data for those specific conditions and all the factors are taken in to account to get your calculate the engine power.

86D: And what is done then to calculate the Corrected power figure?

GL: The Corrected power figure is what is calculated based on some of the ambient data collected through some of the sensors in the dyno room. However, if you look at the print out, there is ambient temperature, intake air temperature, relative humidity, air pressure, and steam pressure and [the system] puts the relevant numbers into the SAE J1349 equation to calculate the correction factor. There are other types of correction factors we can use but we use SAE because that is what 99% of the dynos in the US use, [but] in Europe and Japan, they use different correction factors. I know of some dynos that you have to manually input some of the ambient values to be able to get the correction factor, however, MAHA included the sensors in their dyno package because they are relevant to the final output, depending on which correction factor is chosen. I looked at other dynos and with some, you would have to purchase some of these sensors separately. The point of this factor is to standardize the readings.

86D: So what about Wheel power, what is the relationship of that to the engine power?

GL: Wheel power is calculated directly based on the engine power and drag power. Drag power is measured during the coast down process, “the green line” on the chart. The point of this is that things like gear oil, transmission movement. tire grip are measured as a system to really see how much power is being lost. Cold tires versus hot and sticky tires will behave differently and that is why MAHA always measures the drag power after each pull. We have a sensor that we attach to the front of the car where the fan blows the fresh air in to increase accuracy. We want to get the most accurate readings we can to make sure the numbers make sense. The wheel power is found by taking the engine power and subtracting the drag power to get your final wheel power. This gets you the wheel power because you have a loss of power from the engine due to having to overcome things like sticky tires in order to get the car moving.

86D: Seems like it’s a very complicated subject with lots of technical details. If our readers want to learn more, can you refer us to a resource online where we can do more reading up on the subject?

GL: Here is a website that does give an idea about corrected power: I do agree with most of the things this person writes, but not everything. We have more correction factors available that are not even listed on this site, like JIS.
http://www.dynamometer-info.co.uk/automotive-dyno-correction-factors.htm

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Our thanks to Gary and Michelle of Mann Engineering who were our gracious hosts for this fun and informative day at the dyno & car meet. We had a fun group drive in the morning leading to Mann’s shop, where food trucks and awesome cars awaited! Mann even had one of the new Subaru WRX STIs on display, and for some of us that was our first look at the new AWD machine. We can’t wait to see what cool packages they develop for the new car! Of course, we took some photos at the event and they’re finally up for your viewing pleasure! Click on the gallery of photos below.

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