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      01-23-2008, 04:07 PM   #14
Expert Road Racer

Drives: 07 335i e90, 09 335i e93
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Originally Posted by InJapan View Post

Unless someone has more information about how the test was done. You must assume that these numbers were at the wheel hubs. I'm guessing that Eric Hsu just mis spoke / typed.

You can also get a hint from what he wrote... 5% higher than other dynos... Dynopacks in general only lose about 10% compaired to 15% from a Dynojet because there are no rotational / frictional losses from the wheel and tire. If he meant flywheel, then he would have said 15% higher than other dynos.
I think the dyno already had a drivetrain-to-flywheel correction factor of 5% built into the report on the screen.

Here's an explanation from another forum:


The Dynapack measures power at the axle - one of the only machines that actually does - most machines measure what the roller did, which isn't ever the same thing. The reason we have a "flywheel" screen is because most chassis dynos for years have been incorrectly displaying measured torque. We show the measured torque, because we can - we actually measure it directly.

If you have a car making 300lb/ft at the flywheel and run it through a drivetrain that has a 4:1 gear reduction, the gears will multiply the torque and divide the rpm. So your actual "wheel torque" or axle torque will be something like 1200lb/ft - minus a little bit of efficiency loss. So why do most if not all roller dynos display the torque as 300 minus a little loss? Beats me. It is completely incorrect, but those are the numbers people are used to seeing. It is probably because most roller machines are measuring the roller and back calculating everything from there - and dividing it by the engine rpm, instead of the axle RPM. Their torque numbers are calculated several steps away from the original measurement. By comparison, we measure torque at the source, and RPM at the source, so it is direct measurement. Because people aren't used to seeing 1200lb/ft of axle torque (even though it is correct) we have the "flywheel" screen. The flywheel screen takes our measured torque number and divides it by the vehicle's gear ratio to get it back to a "flywheel" torque number that people are expecting to see. True, it isn't actual flywheel torque because we aren't measuring it there - but it is much closer to being flywheel torque than wheel torque. After seeing the explanation, I think you'll see that calling 300lb/ft the "wheel torque" is ridiculous - but it has become the standard. We show you the real numbers as a default and have the Flywheel screen to show the numbers the other way if you want them. Because of all of this, most of our operators use the flywheel screen to give the torque numbers that they are expecting to see.

The TCF function is there if you want to use an estimate for drivetrain loss and add it to the "flywheel" numbers. If it is at 1.0 - nothing is done. If it is at 1.15, then the measurements are multiplied by 1.15 on the flywheel numbers only. Also, the flywheel screens show the TCF number - to discourage people from being sneaky about using it.

Sorry it is so long winded, but I think it is important to show why we do what we do so people understand it. If you have any other questions, just let me know.


John Card
Dynapack USA
Phone (559) 292-3800
Fax (559) 292-4900


Sent: Tuesday, December 18, 2007 10:31 AM
Subject: RE: Dyno Graph Questions??

Hello John,
Thank you for the detailed response to my questions. So the dyno sheet I sent shows a TCF of 1.00, does that mean that the results displayed are at the hubs, or an estimate of flywheel power with the Dynapack software adding a correction factor in?


RE: Dyno Graph Questions??
Date: Tue, 18 Dec 2007 10:55:11 -0800


1.0 means that no correction has been added in - so it is as measured at the hubs.



IMHO the dyno graph shows the hub number with a 5% torque correction factor built in.


501HP * 5252/6515 = 403 ft-lbs TQ after 5% correction factor which = 384 ft-lbs MEASURED HUB TQ (@ 6515 RPM before 5% correction factor) = 476 HP actual measured at the hubs with NO drivetrain loss consideration.

Last edited by sdiver68; 01-23-2008 at 04:48 PM.