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      10-26-2014, 02:46 PM   #1
jeffp1105
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Horsepower questions & calculations

When BMW quotes a HP figure in specs, for example 300HP for an N54 based Z4 35i, are they talking engine horsepower or wheel horsepower?

Does anyone have an estimate, or better yet, a measured loss rate between engine and wheel horsepower?

I just got a repeatable measure of 329 whp on my 35i with a COBB V3, Stage 1 Aggressive and a K&N drop in filter as the only 2 performance mods. I'm trying to figure out if I got a minor 10% boost of 329 over 300 stock or a much more impressive 29% boost if I assume a 15% loss taking the stock down to 255 whp so 329 vs 255 = 29% improvement.

Thanks for any insight into this and sorry if its in another thread.

Jeff
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      10-26-2014, 02:54 PM   #2
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I cannot imagine any manufacturer quoting HP at the rear wheel. I would bet money the 300 figure is at the flywheel, but I can't guarantee it.
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      10-26-2014, 08:08 PM   #3
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Yes. 300 at the fly. 15% parasitic loss is about right for 6mt.
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      10-26-2014, 09:03 PM   #4
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Quote:
Originally Posted by paris1 View Post
I cannot imagine any manufacturer quoting HP at the rear wheel. I would bet money the 300 figure is at the flywheel, but I can't guarantee it.
I'd tend to agree, but dynos of the S55-equipped F80 M3/F82 M4 are showing that those cars are putting out at the wheels the numbers BMW quoted at the crank. Pretty crazy.
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      10-27-2014, 12:50 AM   #5
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Quote:
Originally Posted by Hujan View Post
I'd tend to agree, but dynos of the S55-equipped F80 M3/F82 M4 are showing that those cars are putting out at the wheels the numbers BMW quoted at the crank. Pretty crazy.
I would say use dyno numbers to compare upgrades done on the same car, preferably on the same day, only. Look at the quarter mile trap speeds instead of dynos if you want to understand the actual horsepower available accurately.

What is fascinating about the S55 is not so much the peak power figure itself, but how it available at such a wide rpm range compared to a tuned N54.
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      10-27-2014, 10:16 AM   #6
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Thanks, gents. Dyno Day in 2 weeks.
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      10-31-2014, 02:46 PM   #7
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Use 18% losses to calculate whp from bhp for n54. Also choose your dyno service and operator on good advice, not convenience. Also, use 4th gear to acquire results you may compare with others.
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