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12162009, 06:22 PM  #221  
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Thinking more about this I think I have it figured out a little bit more concisely. But not much... You must speak about what is typical because the physics does not rule out bad designs that may break the "rules". This explanation covers both gearing and torque vs. horsepower. Single in gear peak acceleration levels across the entire rpm or speed range (not typically how you drive when at full clip obviously!) are governed by peak torque to weight. This is simply a result of Newtons 2nd law when you work out the math. However, the length of time you can remain in any given gear and the peak speed you will obtain is more governed by peak power since a high redline (in a roughly acceleration optimized gear set) is always accompanied by larger gear ratios, more torque multiplication and therefore also speed multiplication from them. But when driving for peak multi gear performance this potential maximum in gear acceleration (referred to in the first sentence in this section) is never actually even felt (except in 1st gear). This is evident since a graph of drive wheel force vs. speed for all gears (again Newtons laws repeated for each gear) mandates operating well past peak torque and in the upper rpm ranges near or past redline (just pick a speed and follow the graph of force by gear up vertically to find the maximum force). It will always mandate the lowest gear which forces the highest rpm and hence the peak power (again AT THAT speed, not the engines peak power). You can also see this from a = P / (m x v). In short at any given speed using the lowest gear that does not exceed redline both maximizes acceleration and power (total common sense there and consistent with the math). This just so happens to be right along the lines of Bruce A's old and lengthy "essay" on this topic. In short to "prove" power to weight is more important than torque to weight you need, Newtons law, F=ma The definition of power slightly re arranged, a = P/(m x v) An understanding that ICE cars are pretty much designed with gearing that is matched to their engines to maximize acceleration You can't really just say a = P/(m x v) So maximum potential in gear instantaneous acceleration is governed by peak torque to weight whereas multigear overall acceleration is governed by power to weight.
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12162009, 06:33 PM  #222  
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12162009, 06:52 PM  #223  
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12172009, 03:29 AM  #225  
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This discussion is too heavy for holidays, but here's a quote from BMW Exec
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12172009, 04:28 AM  #226 
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Let me show you what BMW and others that manufacture in Europe will have to get their overall emissions down to by 2015 and might go some way to explaining the switch to smaller FI units instead of maintaining the larger capacity motor of old.
2015 European Co2 emissions regulations So what I am saying and have been saying for heaven knows how long is that the switch is coming whether you like it or not and with the M3 is more of an issue because of the sales volume than than it is will other rival brands. 
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12172009, 04:53 AM  #227 
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I love my ///M
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12172009, 08:17 AM  #228 
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12172009, 09:57 AM  #229 
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I thought I should mention here what the advantages of a normally aspirated engine over a forced induction engine are, or more to the point a flat, wide torque curve vs. a peaky, high, narrow torque curve. Just so everyone is on the same page.
In either case the designers and engineers will match the gearing to the engine's characteristics. So, in the end both types can be equivalent in a drag race. However, with a wide torque curve with a lower maximum torque, the whole drivetrain need only be designed to handle this lower maximum torque. Therefore, it can be smaller and lighter. That's not only lighter but it is rotating mass that is lighter. This has benefit for engine response and control, and even handling. The difference might seem small but for those who really want to maximize all aspects of a cars performance then it adds up. Another aspect of a high revving, flat torque engine is that this type of engine is usually associated with racing, since you spend more of your time near redline in a race. Although the M3 (and all the M cars) are street cars first, the whole M concept started out as racing cars that could be put on the road. As time went on the disparity between what is required for a road car and a race car grew wider  airbags, cats, etc. are required for a road car but just add weight and complexity to a race car. So its really not feasible to make a car just for racing and have it be roadlegal (and give most drivers what they want for daily driving) too. Especially if you're considering selling a lot of them. I'm sure BMW still wants to make money. BTW, an M3 still makes a good car for tracking (among the other roadlegal cars). But I see the M3 as rather a road car that is exceptional at practically everything anyone would ask of a road car. So, that is why many prefer a normally aspirated engine with a wide, flat torque curve over an engine that has the same horsepower but more (maximum) torque. I hope our two experts here (Mr. Swamp and Mr. Pearce) can agree.
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12172009, 10:36 AM  #230 
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I'm no expert, but it isn't clear to me that a forced induction engine can't have a relatively flat powerband of 5,000+ rpm. Of course it would start 1,000rpm lower. Actually I think the M engineers will solve both this and the "linear throttle response" fairly easily. More torque at the wheel, on the other hand, is a real challenge in terms of both traction and the effects of weight transfer under maximum acceleration. This is why I'm guessing that the F30 M3 will feature an active differential.

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12172009, 10:50 AM  #231  
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The linear throttle response is another wishful thinking. Porsche has not been able to remove that because it is a nature of how turbo works and they are currently the leader in terms of getting better throttle response using variable turbine geometry. Even by changing the angle of attack of the turbine blades, it still requires reasonably more time to spool up than say a completely N/A highrevving GT3, which is instantaneous. Still the best throttle response in Porsches are in the GT3 and the older Carrera GT. None of the turbo Porsches come even close. Yet, in the SUV market, they have been more successful than BMW since the throttle response, although laggy, in the Porsche Cayenne Turbo S is still WAY better than the X5 M because the Turbo S uses Variable turbine geometry while the BMW does not.
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12172009, 10:54 AM  #232 
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Sorry but it's a shame you can't hear the laughter I am having right now. FI engine's don't have flat torque curves I heard you say, ARE YOU SERIOUS.
As a percentage of over the entire rev range you will find that FI units offer a bigger percentage of 'flat' torque compared to any N/A engine, even the M3 which I might add is truly brilliant at this. 
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12172009, 12:32 PM  #233  
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Do you really want to get me started again on this?
If you understand how physics works and what torque and HP crossover point means, you will know a car that makes 400 HP and 400 ftlbs@ only 1500 rpm will have its torque curve nose dive past 5252 rpm and will not be making more than 50  70% past 5252 rpm since at this cross over point, that is the maximum a car can achieve in terms of HP. Unless the peak torque in the turbo car somehow gets shifted past 5000 rpm in order to achieve a flat torque curve using different internals, turbos and tune, which is irrelevant to this discussion. Did you forget how you kept bragging about your beloved Audi and how the supercharged S4 torque curve is flat and after doing some investigation, I burst the bubble when I posted a multitude of dynos of the new S4 where the torque curve plummeted like no tommorow past 5000 rpm? Quote:
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12172009, 12:35 PM  #234  
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12172009, 12:39 PM  #235 
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12172009, 12:46 PM  #236  
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Would you like me to explain using some example of dyno of your "awesome" ~380 whp 335 dynosheet?
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12172009, 12:54 PM  #237 
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An artifact of the units chosen and the presentation.

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12172009, 12:58 PM  #238  
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BCY, C5 RS6: 331 kilowatts (450 PS; 444 bhp) @ 5,7006,400 rpm; 560 newton metres (413 ft·lbf) @ 1,9505,600 rpm BRV, C5 RS6 Plus: 353 kilowatts (480 PS; 473 bhp) @ 6,0006,400 rpm; 560 newton metres (413 ft·lbf) @ 1,9506,000 rpm 

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12172009, 12:58 PM  #239 
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What is that supposed to mean? I simply offered to pick a random dyno sheet of a 380 whp 335 and explain what the cross over means. If you are unable to make arguments that justify your beliefs, why resort to sarcasm?
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12172009, 01:04 PM  #240  
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Can I please look at an actual dynosheet? I would like to see how the curve looks across the entire rev range.
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12172009, 01:23 PM  #241  
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Somewhere I have the factory dyno chart for the standard RS6 engine. I wanted to post it, because it is very impressive. 

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12172009, 01:34 PM  #242  
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I don't doubt it is flat from 1950  5600 rpm. It is much like the Porsche 911 Turbo 997 that delivers 500 wheel torque across a similar range. Still that is just a 3600 rpm range.
It was the entire curve across the rev range, cross over points and how the torque curve looks like at the top end of the car that I was interested, which is why I requested the dyno. Quote:
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