Login

Post Reply 

Thread Tools  Search this Thread 
10062012, 07:48 PM  #199 
Enlisted Member
0
Rep 42
Posts 
This power vs. torque debate can go on and on... because neither side is wrong.
If you could only find out one thing about an engine, the HP is definitely more important than torque. It's just physics, and HP includes torque. However, to really get to know an engine, it's the area under the torque curve that matters. (it's still some form of average power, since width of the area is rpm and height is torque) We'll just have to wait and see what the graph for the new M3/4 looks like. 
Appreciate
0

10062012, 07:56 PM  #200  
Lieutenant General
262
Rep 10,343
Posts 
Quote:
Consider two equal masses m, a distance d apart, say equal to a wheelbase. This is an idealized two point mass "car". This results in 50:50 weight distribution and the polar moment I(about cg) = m d^2/2. Alter the weight distribution radically so 100% is on one axle. Weight distribution = 100:0, total weight unchanged. Now here I(about cg) = 0. For the more general case you can solve for the generalized I about the cg for any value of m1 and m2. I = (m1 m2 d^2)/(m1+m2) Again even though m1+m2 is fixed change their ratio and you can compute exactly how I changes. Through this simple example you should be able to see that axle weight distribution and polar moment are intricately linked. You can extrapolate to the case of a continuous variation in mass along the length rather than two point masses. In both cases though change one and you change the other. In the simple cases there will be an analytical solution, in the general case certainly not. Good book by the way!
__________________
E92 M3  Space Gray on Fox Red  MDCT  CF Roof  RAC RG63 Wheels  Brembo 380mm BBK   Vorsteiner Ti Exhaust  Matte Black Grilles/Side Gills/Rear Emblem/Mirrors   Alekshop Back up Camera  GP Thunders  BMW Aluminum Pedals  Elite Angels   XPEL Full Front Wrap  Hardwired V1  Interior Xenon Light Kit  

Appreciate
0

10062012, 08:14 PM  #201  
Lieutenant General
262
Rep 10,343
Posts 
Quote:
I not so sure if the area under the torque vs rpm curve has any particular meaning. Sure more is better but that is almost the same as saying more power at all rpms is better (or more torque...). What does have a meaning is the integral in time of torque x rpm (well in SI units torque x ω, but the same thing if you have consistent units). This gives the total work done by the engine or equivalently how much change in kinetic energy has been given to the vehicle. From that you get exactly how fast it is going. The key thing is you need to plot torque vs. time, not torque vs. rpm. There is a key shortcut here as well though because power is just torque x ω (again basically torque x rpm but in SI units). Thus you can time integrate power instead of messing with torque and rpm! Power is fundamental! Knowing power is absolutely more fundamental as it does not matter what the gear/final drive ratios are. But of course if you know the full torque curve vs. rpm then you can get the full power curve. You just CAN NOT use crank torque alone in many equations to determine speed or acceleration where you can use power alone! That's the power of power.
__________________
E92 M3  Space Gray on Fox Red  MDCT  CF Roof  RAC RG63 Wheels  Brembo 380mm BBK   Vorsteiner Ti Exhaust  Matte Black Grilles/Side Gills/Rear Emblem/Mirrors   Alekshop Back up Camera  GP Thunders  BMW Aluminum Pedals  Elite Angels   XPEL Full Front Wrap  Hardwired V1  Interior Xenon Light Kit  

Appreciate
0

10062012, 09:52 PM  #202 
Registered
0
Rep 2
Posts 
As a e92 m3 owner I understand that it's early so I'll wait and see what the final numbers are but what they are coming up with now in my opinion is not enough coming from a e92 m3 I'm not the tuning type and if the new m3 is around 80k and has the same hp I won't be trading in my e92 for it I
Feel like its not enough to make me trade or upgrade to it. I understand that the m3 is about more than straight line performance I love my m3 but if the figures stay true to this thread then I don't need the new m3...... 
Appreciate
0

10062012, 09:57 PM  #203  
Registered
0
Rep 2
Posts 
Quote:


Appreciate
0

10062012, 11:20 PM  #204 
Captain
7
Rep 618
Posts 

Appreciate
0

10072012, 12:43 AM  #205 
Major
45
Rep 1,106
Posts 
Same hp but much lighter weight? Phock yeah!!!
Please deliver that promise!
__________________
My e90
Frozen Grey. Dry CF Roof. Vorsteiner Double Sided CF Boot Lid. OSS. BMW Performance Aero Kit. BMW Performance CF Diffuser Cover. BMW Performance CF Trim. Robson Design Steering Wheel. Advan RS. Michelin PSS. H&R Touring Cup Kit. BMW Performance Brakes. Eisenmann Race Rear Muffler. ESS Directflash 
Appreciate
0

10072012, 01:28 AM  #206  
Major General
1123
Rep 7,432
Posts
Drives: 2015 M4 DCT
Join Date: Jan 2012
Location: Montreal

Quote:
The flaw in your example resides in the fact that you consider the masses limited to be directly over the axles. Take your example of the two mass car but instead of combining both masses over one axle, combine them right in the middle of the two axles. You would also reduce I to zero but would maintain the 5050 weight distribution. As another example, let's assume a car of a given weight and polar moment. If the axles are positioned at equal distance from the centre of gravity, the car has a 5050 weight distribution. Moving the front axle forward (assuming negligeable mass for the axles themselves) will shift the weight distribution towards the rear axle, but the polar moment around the CG remains unaltered. In both examples one parameter could be changed without impacting the other. Last edited by CanAutM3; 10072012 at 03:50 PM. 

Appreciate
0

10072012, 04:32 AM  #207  
Grease Monkey
82
Rep 2,512
Posts
Drives: 2011 E90 M3,1994 Euro E36 M3/4
Join Date: Apr 2006
Location: Fort McMurray, Alberta, Canada

Quote:
That is why torque by itself is useless........ Torque is a static measure of the amount of weight the crankshaft could lift if it had a lever 1 foot long on the end of it (I know you know this but many people don't) hence the unit Lb/Ft. Torque can't do work or accelerate an object unless it it delivered over time. In my main area of knowledge, diesel engines, horsepower is the way all engines are rated, the torque numbers are mentioned of course, but in reality, the amount of horsepower the engine produces determines the amount of work it can do, not the amount of torque it produces. Everyone knows that diesels make huge torque numbers. For example, a Detroit Diesel Series 60 14L engine makes 1650 Lb/Ft @ 1100 RPM yet only makes 515 HP @ 1800RPM in it's most aggressive factory rating. http://www.demanddetroit.com/pdf/Eng..._S60_specs.pdf So the moral of what I am saying is that torque is good but unless it can be maintained across a broad RPM range you can't make much horsepower with it. You can't make horsepower without torque, but torque itself can't perform work. A torquey engine is nice, but unless that torque curve is broad, the average horsepower output of the engine is many times not as good as an engine with a lower peak torque number and a broader/flatter torque curve.
__________________
Last edited by BMRLVR; 10072012 at 03:10 PM. 

Appreciate
0

10072012, 11:40 AM  #208 
Captain
58
Rep 918
Posts 
Not only that it appears to be the last NA BMW engine period  at least for now
__________________
2012 650cic Space Gray/Vermillon Red/Blk.Top(retired)
2011 MINI CooperS BRGII/Lounge Green/Sport/Prem/Connect/Black Xenon/Black Conical Spokes/ACS springs/ACS exhaust/Alta Shorty/Continental Extreme DW 2012 M3 AW/FR NDH2/2MK/ZPP/ZCP/ZCW/752/6NR/OEM CF splitters/OEM CF Mirror caps 2012 X3 35i Titanium Silver/Black ZAP/ZPP/TECH/APPS/Breyton GTS/Continental Extreme DW 
Appreciate
0

10072012, 01:29 PM  #209 
Lieutenant Colonel
188
Rep 1,860
Posts 
Torque is physical characteristic of the engine (at the output axle). as is rpm, but as a 'number' it is only relevant WITH rpm, because 'torque' is simply the physical force of the engine at its output, which can be manipulated any way you want, with gear/levers.. hence it MUST be stated WITH rpm (as HP, or power) otherwise it it truly meaning less.
Why it SEEMS relevant, is because turbo engines give high torque at low rpm, which is the equivalent of high HP at low rpm (the two are identical). The same with diesel engines. So it is a characteristic of the physical setup (NA vs turbo, diesels) which give higher power at low rpm (=high 'torque' at low rpm). That said, it is a misleading number, as it leads people to draw wrong conclusions  but it is given simply as a statistic of the engine, for engineers to use. So if you want to talk about something that is accurate, you should talk about HP@rpm (=torque*rpm@rpm), or if you want to see how fast a car is, you also consider how fast it revs. 
Appreciate
0

10072012, 06:02 PM  #210  
Lieutenant General
262
Rep 10,343
Posts 
Quote:
__________________
E92 M3  Space Gray on Fox Red  MDCT  CF Roof  RAC RG63 Wheels  Brembo 380mm BBK   Vorsteiner Ti Exhaust  Matte Black Grilles/Side Gills/Rear Emblem/Mirrors   Alekshop Back up Camera  GP Thunders  BMW Aluminum Pedals  Elite Angels   XPEL Full Front Wrap  Hardwired V1  Interior Xenon Light Kit  

Appreciate
0

10072012, 07:12 PM  #211  
Grease Monkey
82
Rep 2,512
Posts
Drives: 2011 E90 M3,1994 Euro E36 M3/4
Join Date: Apr 2006
Location: Fort McMurray, Alberta, Canada

Quote:
Check out the dynographs in the link and you can see how lots of torque can deliver small horsepower numbers, or rather, do little amounts of work. There are software/fuel injector/pump/turbo upgrades (aftermarket and OEM) available for small, and medium sized diesel engines. Many of these upgrades aren't aimed at increasing the peak torque numbers too much (they sometimes do increase a little bit but are controlled by the computer to a max number), but are aimed at making more torque higher in the RPM range. By not increasing peak torque numbers greatly, you are not increasing the BMEP's or IMEP's that the engine components are subjected to beyond design specs. Because of the multiplication effect that RPM's have on torque's ability to produce horsepower, holding that same peak torque higher into the RPM range allows lots more horsepower to be delivered without stressing the engine too greatly. Torque is force, and force can't achieve any work unless there is movement measured over a time. This is what everyone needs to understand. Even when an engine is "torquey" the acceleration it achieves totally dependant upon the amount of horsepower that the torque develops at a specific RPM. 

Appreciate
0

10072012, 10:06 PM  #212  
Second Lieutenant
7
Rep 289
Posts 
Quote:
__________________
2015 435i Xdrive Black on Black with loaded


Appreciate
0

10082012, 01:14 AM  #213  
Colonel
42
Rep 2,693
Posts 
Quote:
__________________
E92 335  Space Gray  Saddle Brown Dakota  Dark Burl Trim  ZPP  ZSP  AT  Idrive  6FL  FBO  Dinan CAI  Quaife LSD  STG3 PROCede.


Appreciate
0

10082012, 09:33 AM  #215  
Private First Class
1
Rep 133
Posts 
Quote:


Appreciate
0

10082012, 12:30 PM  #216  
Lieutenant General
262
Rep 10,343
Posts 
Quote:
Arbitrary changes in the density (or mass) distribution in a vehicle will change its axle weight and will change it polar moment as both are sums (integrals) of the density distribution. Also, a more even axle balance (at a given fixed weight) produces a lower I whereas a more unbalanced axle ratio will produce a lower I (that comes from the simple formula I provided prior). 

Appreciate
0

10082012, 07:25 PM  #218  
Lieutenant General
262
Rep 10,343
Posts 
Quote:
1. Stock to modded, always a lame comparison. 2. More apples to apples might be a software only modded F82 M4 vs. software only modded N54 3er. The M will destroy it. 3. Another comparison might be on the track vs. a drag race. The cars are more designed for the former. 4. Crank torque is meaningless. Power is what dictates acceleration at any speed in any gear. 5. The E9X M3 puts more torque to the wheels than many modified N54 3ers and this beats them in races. That is because it makes more power. Sure extensively modded N54s can out drag an E9X M3, no big surprise there.
__________________
E92 M3  Space Gray on Fox Red  MDCT  CF Roof  RAC RG63 Wheels  Brembo 380mm BBK   Vorsteiner Ti Exhaust  Matte Black Grilles/Side Gills/Rear Emblem/Mirrors   Alekshop Back up Camera  GP Thunders  BMW Aluminum Pedals  Elite Angels   XPEL Full Front Wrap  Hardwired V1  Interior Xenon Light Kit  

Appreciate
0

10082012, 09:58 PM  #219  
4 Star General Twilight Zone
22
Rep 1,025
Posts 
Quote:
I can slap a Hayabusa 1300cc motor into a smart car and it will spank your precious little 335 all day long > 060, 1/4 mile etc.. How about an STI with a chip and down pipe compared to your stock 335? We can make other comparisons? Does any of that make sense yet?
__________________
'12 Ducati Diavel
'06 E46 M3 (sold)  '97 325is  Dinan (sold) '02 Z3 3.0 coupe (sold) Past  '84 Ferrari 308 GTSI  '88 Ferrari 328 GTS  '05 Dodge Viper Copperhead Edition #180/300  Audi S4 

Appreciate
0

Post Reply 
Bookmarks 
Thread Tools  Search this Thread 

