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Is 1.21 too high? ...more (archive)

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Posted by MattH on April 30, 2000 at 22:20:35:

In Reply to: It's not 21% loss, it's factor of 1.21 (more) posted by PM3 on April 30, 2000 at 18:02:35:

Most stock M3's dyno at around 205RWHP. I don't know what they dyno at the flywheel but if we assume that the average is the factory speced 240HP then that gives a multiplication factor of 1.17, not 1.21. I realize that it's possible that the stock engine is putting out more than 240HP but the standard use of the 1.21 correction factor opens the door for some shady business practices. For example, say I make a magnetic fuel atomizer that gives 8 HP for only $50. I can even 'prove' my claims with dyno results.. I just dyno a car with my device on it and see that it's getting 205RWHP, which, when multiplied by the standard 1.21 gives 248 BHP, 8 more than BMW specs.

I guess my real question is who has done engine dynos on their car to verify the standard factor that everyone uses?


: This is where I used to get it mixed up too. When you multiply by factor of 1.21, it's the same as adding 17.5% back to flywheel. THEN take off the 17.5% at the fly, and you'll get your rear wheel HP..but you can't get the same # by multiplying by 1.175, and expecting to get the same Flywheel HP again.
: See, RW HP has to be same as (0.825) which is 1-17.5%, mulitiplied by the flywheel HP. That will give you the 17.5% LOSS to the WHEELS, to get the RW HP. Otherwise, just multiply by the RW HP by 1.21 (or divide by 0.825), and you'll get almost the same #'s to the Flywheel. CYA>PM3

: : That is due to the drivetrain, correct? How does the drivetrain dissipate that power? Friction? Is there anything that could be done to reduce that loss? Just curious.

: : Thanks,
: : Ken

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