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01-24-2013, 05:21 PM | #89 | |
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01-24-2013, 05:22 PM | #90 | |
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I agree he might be taking his role as the devil’s advocate to a bit of an extreme here to chastise MKE_M3. It seems fairly simple to me that a linear power delivery is the smooth buildup of power across an RPM range. This is the product of a flat torque curve that increases horsepower at a linear rate across the RPM band in question, as contrasted with a rising torque curve that would result in power building in a curvilinear fashion (i.e. exponentially). That’s the best way I can describe what we’re talking about here, but perhaps Bruce can correct my mistakes. |
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01-24-2013, 05:35 PM | #91 | |
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A peaky torque curve will have non-zero jerk. Due to hysteris of the tire carcass, it can only tolerate so much jerk before the static coefficient of friction is overcome and the tires spin. While tires do have an outright torque maximum for given conditions, inducing enough jerk would be enough to spin tires that otherwise wouldn't from torque alone. Example: If I apply 100% throttle in my supercharged S2000 from 2000 RPM to redline in 3rd gear, I won't break traction. If I punch it at 6k, I will. The amount of torque applied at 6k is no different. The jerk is. Last edited by urBan_dK; 01-24-2013 at 05:41 PM.. |
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01-24-2013, 05:50 PM | #92 | |
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You were right about torque, no HP, being multiplied by gearing. Out of genuine curiosity, can you explain why, if gearing doesn't matter, a car can increase speed by 30mph faster in 1st gear (traction not an issue)than at any point in 3rd gear? If power is all that matters, is the only difference wind resistance? It seems like gearing must be playing a role here, as well. p.s. since you may ask, an unpredictable lump in torque could be due to turbo lag where different throttle input patterns can lead to peak torque being built at different speeds/rpm's in real world driving, say if you have to lift when getting cut off or as traction breaks coming out of a corner then roll back on to the throttle vs. flooring it from 2500 RPM.
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01-24-2013, 07:29 PM | #93 | |
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01-24-2013, 09:11 PM | #94 | |
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S62 vs s85, stock vs stock:
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01-24-2013, 10:22 PM | #95 |
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It appears that BMW could have went with the AMG philosophy and shoved the S62 - as Mercedes has done with the M156 (6.2L V8) and numerous models - into the M3. By basing the S65 V8 on its bigger brother the S85 V10 we are the recipients of the newer techhnology (for the time) a higher revving, higher horsepower, lower displacement masterpiece.
That being said the S62 is no slouch, and still very much relevant - http://www.e90post.com/forums/showthread.php?t=795884 3 S62 engined DP cars took the top three starting positions for this years Rolex 24.
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01-25-2013, 03:25 PM | #96 | |
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But listen, an engine with a building torque curve, even a steeply building torque curve, will create next to zero actual "jerk", as you say. I am not familiar with any engine on the planet that will gain significant torque within, say 25 or 50 rpm. Even a car gaining an outrageous 347 pound feet of torque over 1500 rpm (in the example given by pbonsalb) cannot be called a "jerk" in second and third gear. Your example of punching it from cruise in third gear at 6000 rpm is an example of jerk from the takeup of driveline lash when going from light-loading cruise to full power. Look, I'm not saying that the jerk factor is immaterial. What I am saying is that it's of little consequence in terms of a building torque curve. Of course, saying it's of little consequence is not the same as saying it's of zero consequence, so I guess, point made, at least to some extent. Bruce PS - Just got back from the auto show in Harrisburg. It ain't a big deal like Detroit, L.A. and new York, but I'm still a happy guy. |
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01-25-2013, 03:33 PM | #97 | |
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01-25-2013, 04:41 PM | #98 | |
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Fleshing that out is made simpler by going back to that trusty old torque-at-the-drive-wheels thing. Let's say we have a CVT in an M3 that allows power to peak at 40 MPH, which is (not altogether coincidentally) where power peaks in a stock 6-speed car. At that point (and discounting rotational inertia and driveline friction), the drive wheels are getting about 4095 pound feet of torque thrown at them (4.06 CVT ratio X 3.85 final drive ratio = 15.63 overall gearing, times 262 pound feet of torque at 8300 rpm = 4095 pound feet). Of course, from now on the CVT keeps changing its gearing so that the car stays at the power peak as speeds increase. At 100 mph, the CVT, still faithfully keeping the engine at the power peak, is now down to about a 1.62 ratio, times the final drive ratio of 3.85, giving a 6.24 total ratio, times 262 pound feet gives us 1635 pound feet at the drive wheels, which is (and what are the odds?) just 40% of the pound feet thrown at the drive wheels at 40 MPH. So, ignoring increased wind and rolling resistance at 100 MPH, we will need 250% of the torque (and of course power) to achieve the same acceleration at 100 mph as we enjoyed at 40 mph. Boys and girls, we'll need 1035 HP at 100 MPH to achieve the same acceleration that 414 HP gave us at 40. So, just as with a linear power curve , we will need linear increases in power with linear increases in speed in order to maintain acceleration. Therefore, since power doesn't vary gear to gear, but power needs increase with speed, first gear pulls your face off, and third doesn't. Bruce Edit: PS - All numbers rounded off on a 20 year old calculator that I got free with a bowl of soup. Last edited by bruce.augenstein@comcast.; 01-25-2013 at 04:54 PM.. |
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01-25-2013, 07:14 PM | #99 |
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I own both and they are flat out different. Not a great comparo as both are fantastic and never went head to head in the market. I do thank the S62 for being a distant parent to the S65 though. Remember when BMW knocked the S62 down to 4.0l for racing purpose? And sitting on pole for 24hr Daytona is the Telmex BMW dp, which power-plant is under the hood?
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01-25-2013, 09:57 PM | #100 | |
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01-26-2013, 10:15 PM | #101 | |
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Also, I would use for an example of a peaky power curve, the old Yamaha two stroke engines we use to ride. They would lift the front wheel when they came " on the pipe". Really something for an inexperenced rider. |
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01-27-2013, 12:42 AM | #102 | |
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01-27-2013, 01:27 AM | #103 | |
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I like your: The simple explanation is that speed eats power in a completely linear way. Very nice. Horsepower and torque are very interesting and cause much confusion. I need that beer klinking smilie here. |
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01-27-2013, 12:50 PM | #104 | |
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01-27-2013, 12:59 PM | #105 | |
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01-27-2013, 03:08 PM | #106 |
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im guessing just any 2 stroke yamaha like yz250 i had. amazed at how it wanted to kill you at certain rpms all of the sudden. And it was my first bike and quite an experience.
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01-27-2013, 07:08 PM | #107 | |
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Now figure about 1200 foot pounds thrown at it in first gear at the torque peak. Twist will likely be eyeball-measurement obvious. How about the half shafts getting around 4600 foot pounds thrown at them. Still say they won't twist? How about the sprung clutch hub? The dual mass flywheel? Hell, even the cranshaft will twist with each cylinder firing. I remember reading an SAE paper about that decades ago. My intent is not to rag on you, but did you ever hear about "Gallopin' Gertie"? "She" was a supension bridge in Washington State, video'd here. Hey, what's some wind compared to massive steel girders? Even stout things will deflect under load. Bruce Last edited by bruce.augenstein@comcast.; 01-27-2013 at 07:25 PM.. |
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01-28-2013, 12:13 AM | #108 |
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I do know that drivelines twist. I am at least a little familiar with the failure rates of twisting driveline from repeted stress. I know of at least one Sikorsky helicopter (S 64) that measures power by driveline twist. But all in all very small amounts we are talking about here.
I thank you for not ragging on me and my intention is not to antagonize anyone here but what about the idea that the twist may absorb some of the shock and spread it out? |
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01-28-2013, 12:16 AM | #109 |
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Yea, the YZ250. That silly thing almost ended my young learning cruve. And, yes, that was quite a while ago.
Last edited by White 13; 01-28-2013 at 12:23 AM.. |
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01-28-2013, 12:41 AM | #110 | |
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