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Old 01-31-2013, 10:42 AM
RambleJ RambleJ is offline
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Location: Las Colinas, Tx
Join Date: Oct 2008
Posts: 420
Mein Auto: 2012 535i M-sport
Originally Posted by K-A View Post
From what I've "learned" in Dyno testing, RWD Auto cars generally see 15-20% drivetrain/etc. loss by the time it gets to the rear wheels. 10-12% is very efficient (not sure if that's even possible on an Auto?).

There's a dyno sheet I recently saw that said a "general rule is to divide by .xx(number). When I did that, it came out to roughly 20% drivetrain loss.

Also, if you look at the 535i's consensus trap speed in the 1/4 mile, being around 100 MPH (give or take depending on various tests), and weighing at about 4,000 lbs, it shows that it should pin it right at around 340 HP.

A good basis I like to use is a car I used to be involved with in terms of HP/Track times, which is the SN-99 Mustang GT. It was a very pure basis of how RWHP and track times equate.

For example: The 5 Speed Manual 2004 GT made 225 RWHP on the money, which would put it at 260 HP on the money. The Auto version of the same car sucked out an additional 10-15 RWHP, putting them generally at around 210 RWHP. Those cars weighed at around 3200-3300 lbs.

The Manual GT did exactly a 14 Second 1/4 at 100 MPH, those two numbers perfectly aligning with each other as a good power/weight ratio VS track time/MPH ratio (same exact numbers the 535i generally does).

So, if you have a 3200 lb GT making 225 at the wheels with only little drivetrain loss due to being a Manual and doing a 100 MPH trap speed, and then add 700-800 lbs to it (i.e now making it a 535i) then you'd have your trap speed dropped to 92-93 MPH (1 MPH for ever 100 lbs). Therefore, in order for the 535i to recapture that 100 MPH trap speed AND taking into consideration it's an Auto tranny which means it has even more drivetrain loss, you'd need at least 80 more HP, putting it at 340 crank HP (since the GT has 260 crank HP and does the exact same numbers).

Sorry if that's confusing but hope it makes sense in terms of being a look into my crazy head.
Wow!! That's some hardcore rain man math.. Sorry but our cars are not pushing 340 at the crank.
2012 535i M-Sport

Previous BMW's: 87' 528e / 04' 325i / 07' 335i / 08' 335i / 08' 535i
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