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At the crank and at the wheel...


03112007, 04:41 AM  #1 
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At the crank and at the wheel...
Hi guys ,
So please excuse my stupid question but i'm not very knowledgable with the terms. I would like to know what the difference is when you guys compare hp and tq at the crank and at the wheel. When bmw says the 335 for example has 300 horsepower is that at the crank or at the wheel? Reason i ask is because some people say the PROcede gives about 333hp at the wheel and then 380+ hp at the crank. What i want to know basically is when my PROcede is installed and my friend's for example ask me how much hp and tq I have what do i say? Thankx again for answering my noob question... 
03112007, 04:51 AM  #2  
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The ratings on this forum are done by people placing their cars on a dyno. http://en.wikipedia.org/wiki/Dynamometer Dynos measure the power that is actually at the wheel (hence the term wheel horsepower or whp). This is a much lower number than would be reported at the crank because of losses due to the transmission and rear end. BMW estimates that the losses are about 13% on the 335. I have heard estimates as high as 1820% drive train losses for other cars. Either number that you report is fine. Keep in mind that different dynos report different amounts of horsepower. 

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03112007, 05:20 AM  #3  
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Thank you very much for the explanation... I suppose that's why when the 335's are Dynoed initially they put out numbers like 270hp instead of th 300 hp claimed by bmw... 

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03112007, 09:28 AM  #4 
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right, but if u apply that 1315% correction (15% for the step, according to bmw), then its actually higher than 300.
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03112007, 11:03 AM  #5 
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Also, take into account that the 335i is rated using the NEW SAE engine output measuring system. Now, I'm no engineer (nor do I play one on TV), but, from my understanding, this system takes into account the engine accessories (alternator, A/C Compressor, and a few other whosits and whatsits.)
As a rough comparison the difference between the "new" SAE spec, and the old one is 10%. This means that if your engine was measured using the old system, you'd be pulling around 330hp, at the crank (M3 territory, which makes me happy). 1215% are the common numbers I've heard tossed around for powertrain losses in RWD cars. Coralred's guesses seem pretty good to me. One thing that needs to be considered is that the engine was a "ringer" when it was tested. Perhaps the test engine had more drag on it, or 91 octane fuel. This means that given the right conditions (installed in the car, not the engine dyno, 93+ octane, etc), it will produce a touch more power. This would result in the higher than 300 when the chassis output is corrected back to find the engine output. Shiv from Vishnu has put the 335i's STOCK output on a dyno (specifically the AMS Dyno) at around 275hp. This means a rough at the crank number of 316 (and some change) hp. Corrected to the OLD SAE spec (using my seat of the pants 10% value) puts your car, with the steptronic transmission, at 347.8 HP. Now, is it just me, or does that make anyone else retardedly happy? Also, just for giggles sake, lets see what a Procede'd 335i is doing at the crank. We'll use 320hp as our untuned value for a procede (Shiv's numbers from a few weeks ago). Correcting to the crank 320*1.15=368hp Correcting to the OLD SAE spec 368*1.10=404.8hp ...yea... Adam 
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03112007, 11:05 AM  #6  
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Wow, that's a really intelligent post. Didnt know that about the new rating.
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03112007, 11:53 AM  #7  
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SAE net ratings, while more accurate than gross ratings, still represent the engine's power at the flywheel. Contrary to some reports, it does not measure power at the drive wheels. SAEcertified horsepower In 2005, the Society of Automotive Engineers introduced a new test procedure for engine horsepower and torque. The procedure eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as "SAEcertified". Many manufacturers began switching to the new rating immediately, often with surprising results. The rated output of Cadillac's supercharged Northstar V8 jumped from 440 hp (328 kW) to 469 hp (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZFE V6 fell from 210 hp (157 kW) to 190 hp (142 kW). The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from 500 hp (373 kW) to 505 hp (377 kW). 

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03112007, 01:12 PM  #8  
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no, no it wasnt Quote:
do NOT do something stupid like compare new SAE corrected numbers to old ones by adding 10%, thats just dumb, some engines GAINED HP with the new rating procedure, SAE has ALWAYS taken into account all the crap on your car, that was the whole point, the only thing it doesnt include is driveline
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03112007, 03:14 PM  #9 
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My '04 TL was rated 270 HP when it was released. Under the new SAE standard the power "dropped" to 258 HP.
Also, when measuring the HP at the crank from the results of a dyno pull, you should take the whp number and divide by the difference of 100% and the drivetrain loss. Multiplying by 1.15 is incorrect. You should divide by 0.85. 300 whp / 0.85 = 353 HP, whereas 300 * 1.15 = 345 HP. 
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03112007, 05:57 PM  #10  
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SAE gross (used prior to 1972) is the engine output at the crank (no drive train loss) WITHOUT accessories and exhaust/cat (no one uses this one anymore and it's significantly higher than the next one, more or less 10%, because of the losses due to accessories). SAE net (used after 1972) is the engine output at the crank (no drive train loss) WITH accessories (this is the 300hp advertised by BMW for the 335 and the hp figure eveyone talks about when comparing cars). SAE certified is a new procedure that eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured (not very common yet and hp figures can go either up or down compared to SAE net figures). you also have brake hp which is commonly used, similiar to SAE net (at the crank no drive train loss), but instead of having ALL accessories tied to the engine, some auxiliary components (as the water pump) are NOT connected to the engine. The figure is very slightly superior to SAE net figure. When power is measured on a dyno it's w(wheel)hp. After compensating for drive train loss ( due to gearbox, diff, heat loss etc...) by dividing by .87 or .85 (man vs auto) in the case of the 335, you get approximately the SAE NET value (since the accessories are connected in your engine in your car). so supposing that the dyno is accurate all you have to do is divide by .87 or .85 to get the SAE net value. In the case of modding what is REALLY important is to dyno the car before and after the mod on the SAME dyno (the same day) because what is really important is the delta to measure effective hp gains by REDUCING the number of outside variables. And since not all dyno are as accurate you want to use the same dyno on the same day (to also reduce diff in temp and humidity which can alter the results). btw all this info can be found here: http://en.wikipedia.org/wiki/Horsepower I hope this answers your question. cheers Last edited by Barat; 03112007 at 07:49 PM. 

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03112007, 07:40 PM  #11  
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Cut and pasted 

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03112007, 07:49 PM  #12  
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Thankx Barat. You and I are going to have a few discussions in Albany You seem to know your stuff. 

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03112007, 07:56 PM  #13 
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Hey guys.
As I said, I'm NOT an engineer, nor do I even pretend to be one. That was information gleaned from the readings I had done over time. I thank you guys for taking the time to correct me. DrM and Barat, especially. Adam 
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03122007, 12:27 AM  #14 
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03122007, 12:45 AM  #15 
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Ya it's gonna be really crazy, especially since we're like 5 from Montreal then we'll probably follow each other which will be pretty cool as well.
And then on the way back ... Damnnnnnnnn i can't wait to see what it's (PROcede) all about 
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03122007, 10:51 PM  #17 
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Yes, that is true, WilyB. I don't normally use two different numbers when going abck and forth between equations, though. I read it as 13% to 15%, not 13% for one direction and 15% for the other direction.

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03122007, 11:18 PM  #18  
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If you experience a 13% drive train loss and you know the crank horsepower of your car, you figure out the wheel horsepower rating like this: 400hp (at the crank) * (10.13) = 348 wheel horsepower. so: Crank_hp * (1  loss%) = Wheel_hp Since we use chassis dynos, we know the Wheel_hp instead of the Crank_hp. So, if you wanted to estimate the Crank_hp you would do this: Crank_hp = Wheel_hp / (1  loss%) which is the same as Crank_hp = Wheel_hp * 1 / (1  loss%) Given that BMW estimates a 13% drive train loss on the manual cars, this would be 1/(10.13) = 1.149425287 So, if you knew the Wheel_hp you could simply multiply the Wheel_hp by 1.149425287 to get the Crank_hp. In the example above 348 whp * 1.15 = 400.2 ~= 400 crank_hp WilyB simply rounded this to 1.15. It is simply coincidence that it happens to be the value that BMW estimates as the drive train percentage loss for the automatic. 

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03132007, 12:07 AM  #19  
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03132007, 12:20 AM  #20 
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Ummm... I didn't disagree with WilyB, I stated I read it differently, I understand where he is coming from.. It's just wheel / .85 is easy to see 15% loss. wheel / .87 is easy to see 13% loss. So, if I knew crank and subtracted 15% for the first or 13% for the second, the math will work both ways in my head easier. My brain is just wired 15% differently.

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03132007, 02:39 AM  #21  
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But given that BMW estimates the losses at 13% on the manual transmission and that they estimate the car at 300 ft.lbs. that works out to between 40 and 45 horsepower taken by the drivetrain. That should work out to be roughly the same losses no matter how much someone modifies the horsepower of the engine. So, I agree with you. Adding 45 horsepower to someone's dyno number is probably a lot closer to reality than adjusting for the 13%. Shiv's car put 405 ft.lbs. to the ground so I would say his car is probably really making about 450. 

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