Login


01072007, 09:30 AM  #89  
S0THPAW
985
Rep 5,821
Posts 
Quote:
yes. no doubt. If you want to be fastest off the line(330i) 1st gear too. Reverse gear not off course. lol. 

Appreciate
0

01072007, 12:31 PM  #90 
Major General
137
Rep 9,216
Posts 
You can calculate optimum shift points quite easily. This spreadsheet will help you do it:
http://www.datsuns.com/ftp/shiftpoints95.xls You plug in the data from your engines torque curve (i.e torque for each increment of rpm) and your gear ratios. (n.b. this data goes in the light blue cells). The lower table shows the rpm your engine will jump to after a shift. So in the example if you shift from 1st to 2nd at 5600rpm you will wind up doing 3327 rpm. The optimum shift points are where the output shaft torque (in the top table) would be greater in the next gear than the one you are in (or the redline if this is never the case). In many cars it's always the redline, but in diesels and petrol turbos it often isn't. I'd be interested in seeing the results for the 335i BTW I just reread some of the earlier posts in this thread, Just remember ... peak torque and horsepower figures tellyou very little. Torque and horsepower curves tell you quite a bit more. But you still can't compare on vehicle to another without looking at the gearing. Torque and horsepower are effectively expressions of the same thing. I personally find torque simplest to comprehend because unlike hp it is expressed independantly of rpm. In fact the simplest way to look at it is to realise that torque IS an expression or horsepower without the rev's being factored in (and vice versa). I also firmly believe that a highish torque engine is better onthe road than a high revving and hence high HP version. So far this has all been about straight line acceleration. But on the road or even a track the breadth and exploitability of the 'powerband' is what matters.
__________________
Last edited by NFS; 01072007 at 12:47 PM. 
Appreciate
0

01072007, 02:48 PM  #91  
Your resident 4um troll
50
Rep 2,021
Posts 
NO NO NO
in to ask wehre to shift, but where your car would have the highest acceleration and need for speed, most of the examples and discussions have been under the assumption of as close to the same circumstances as possible (gear ratios for instance) you guys are right that torque creates acceleration ive said it a dozen times, but how fast your car actually accelerats depends on how often that torque is applied for gods sake Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 03:10 PM  #92  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
now to help you understand ill say this, if your engine just went through ONE rotation hell yes your car would accelerate more at 1k rpms than 6k what you guys seem to keep missing is that at 6k rpms although the force is about 50 pounds less its pushing the car 6 times more often, so yea while the car is accelerating less for each rotation of the engine its rotating 6 times more, WAY more than enough to make up for the loss now thats not very accurate to how an engine actually works, its abstract to help people understand if you honestly think an engine with like 100 torque @ 15k rpms wont out accelerate a civic....
__________________
RAWRrrrr
Last edited by teknochild; 01072007 at 03:33 PM. 

Appreciate
0

01072007, 03:56 PM  #93  
Major General
137
Rep 9,216
Posts 
Quote:
For me torque is an easy concept  it is a constant 'twisting' force. Forces create acceleration  so far so simple. Torque can be MEASURED. Horsepower is an extrapolation of torque which takes into account the rpm at which torque is being generated. HP is not MEASURED it is calculated from MEASURED torque and rpm data. Importantly  because I think this is where you are going wrong  horsepower is NOT the rate at which torque is generated. It is simply a way of expressing 'power' which takes into account both torque and rotational speed. In any event ... as we will come to later, because of gears the engines rotational speed is not a key issue when it comes to a cars performance. If you know your torque for any given rpm then you can calculate your horsepower. Acceleration is the 'rate of change' it is the rate at which speed increases. I got the torque curve data from here: http://www.e90post.com/forums/showthread.php?t=218 and the gear ratios from here: http://www.conceptcarz.com/vehicle/z10690/default.aspx Which has enabled me to produce the following shift point data for the 330i: What this shows is important. If you have a gearbox you might as well stop thinking about the engine torque. The output shaft torque is what matters. The car will be accelerating hardest (i.e the rate of change of it's speed will be greatest) when the output shaft torque is greatest. In this case this is at 4000rpm in 1st gear. Engine speed and wheel speed are irrelevant here. This is nothing to do with speed  it is the rate at which you INCREASE speed (ie acceleration) that we are looking for. So why hang onto a gear after the torque starts dropping ..? Well to answer this you need to look at what will happen when you change up. For the 330i the output shaft torque in the next gear up will ALWAYS be lower than that for the gear you are in. So when you change up a gear your rate of change of speed (ie your acceleration) will ALWAYS reduce. So to maximise the rate of change at all speeds you should change at the redline (sorry Robin Hood  that's the way the numbers seem to work). This all makes sense when you think about the practical way in which a car accelerates. The greatest acceleration is off the line ... which is when the output shaft torque is highest. As you climb up through the revs and through the gears the output shaft torque decreases and hence you rate of change of speed (acceleration) also decreases. Further .. the peak output shaft torque is generally at 4000rpm .. and in my experience at least there is a definite kick in the back at this point in any gear (in my car at least). Going back to your multiple choice graph ... the rate of change of speed (ie the acceleration) will be greatest where the torque is greatest assuming the driveshaft torque describes a similar curve as the engine torque this would be between D and B. However, because our cars have gears and because changing up a gear reduces the amount of torque generated at the wheels you would not be maximising you rate of change of speed if you changed gear before the redline (at least in a 330i)
__________________


Appreciate
0

01072007, 04:05 PM  #95  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 04:08 PM  #96  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 04:10 PM  #97  
Major General
137
Rep 9,216
Posts 
Quote:
Not to mention the the fact that a 330i stall at 10 mph in 4th. However, if I did short shift into 4th and accelerate up to the redline the greatest change in speed (i.,e. peak acceleration) would be at 4000rpm. The acceleration  i.e. the rate of change of speed  is highest when the output shaft torque is highest. This is around 4000rpm in in first gear (and this is the peak in the other gears also). Think about it .. at 4000 rpm a 330i is doing say 25 mph. It gets to 35 mph (the redline) extremely quickly .... the rate of change of speed is lower at all other points in it's speed range.
__________________


Appreciate
0

01072007, 04:12 PM  #98  
Major General
137
Rep 9,216
Posts 
Quote:
As I have said in earlier posts the car kicks at 4000rpm in all gears ... and funnily enough this is where the output shaft torque is highest in all gears.
__________________


Appreciate
0

01072007, 04:24 PM  #99  
Major General
137
Rep 9,216
Posts 
Quote:
Acceleration is the rate of change of velocity over time. When the car 'JERKS' it is because it is gaining speed very quickly  hence it is ACCELERATING hard. Acceleration = jerk. Diesels have very high levels of low end torque  so they can produce this jerk at low engine speeds ..... Your own experience should confirm this to be correct ..... and as I have shown the reason we don't shift up at low rpm is not that hp is a measure of the rate at which torque is applied. It is because gearing means that the output shaft torque will reduce when we change up a gear, so we should delay changing until it is absolutely necessary (ie redline) or until the output shaft torque would be greater in the higher gear.
__________________


Appreciate
0

01072007, 04:25 PM  #100  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
HP is not meant to be the rate at which torque is output, but it IS, if the torque in an engine was totally flat @ like 300tq, each set of combustians would be generating that 300 torque and accelerating the car at a=f/m, but as i said before at 6k vs 1k that torque is being applied 6 times as often (theres 6 times as many combustians) so even if torque has gone down at 6k rpms your car will most likely be experiencing a larger average acceleration over a second than at 1k, as i already stated, yes at 1k each set of combustians will accelerate your car more, but if there are 6 times as many combustians to combat a 1/5 drop in acceleration per combustian then your car will be accelerating faster 300 torque applied to a car that weighs 3000 pounds will accelerate it at a rate of 2.18 mph 200 torque applied to a car that weighs 3000 pounds will accelerate it at a rate of 1.5 mph if you apply this force 6 times as often as you do the 300 torque your car will accelerate over 4 times as fast
__________________
RAWRrrrr


Appreciate
0

01072007, 04:26 PM  #101  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
oh for christ sake man, get a clue, jerk is the rate of acceleration of your acceleration, it has a huge factor in pushing you back in your seat, thats why its named "jerk" i should also note that even when jerk is negative, its still possible to be accelerating at a faster rate than when jerk is positive i should also note that your jerk rate is directly related to the slope of the HP curve this is why you FEEL like your accelerating at a faster rate where torque is peak, even though in the world of reality your rate of acceleration is highest at peak hp because after peak hp, jerk goes negative; becasue your rate of acceleration is actually decreasing, although its still a HELL of a lot higher than at peak torque
__________________
RAWRrrrr


Appreciate
0

01072007, 04:34 PM  #102  
Major General
137
Rep 9,216
Posts 
Quote:
And who discovered 'jerk' ... I don't remember this from any of my physics lessons and what on earth is negative jerk? You have 2 things  velocity (aka speed) and acceleration. Speed is a constant linear concept. For instance ... you are traveling at a steady 100mph is talking about speed. Acceleration is the rate at which speed increase over time. So if you increase your speed from 100mph to 150mph in one minute, then you are accelerating at an average 50mph per minute. Acceleration of acceleration just means higher levels of acceleration. So in this example you may have accelerated gradually to begin with and much more at the end. Your peak acceleration will be when the rate of change of velocity over time was greatest. I think this is what you are calling jerk.
__________________


Appreciate
0

01072007, 04:35 PM  #103  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 04:35 PM  #104  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
holly shit, please go back to physics class man http://en.wikipedia.org/wiki/Jerk
__________________
RAWRrrrr


Appreciate
0

01072007, 04:39 PM  #105  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 04:42 PM  #106  
Major General
137
Rep 9,216
Posts 
Quote:
If an engine is generating 100 nm of torque at 1000rpm and 200nm of torque at 5000rpm it is not generating the torque 5 times more often at 5000rpm. It is generating 100nm and 200nm of torque in each case ... and that's that. In both cases this torque is simply a rotational force which is turning the driveshaft and hence the wheels. The speed at which the driveshaft is turning is not the issue, nor is the speed at which the engine is turning. The torque simply increases the rotational speed of the drive shaft and the rate of increase in this rotational speed is directly in relation to the amount of torque applied. In short it is the amount of torque that matters not the engine speed at which it is generated. The latter only becomes relevant when we introduce gearing or try to plot a torque curve This would be much simpler in concept if we didn't have to worry about the rotational bit and we could just call torque a force. The harder you push something the faster it accelerates is a pretty simple concept to grasp.
__________________


Appreciate
0

01072007, 04:45 PM  #107  
Major General
137
Rep 9,216
Posts 
Quote:
So if we can agree that the jerk is the point at which the rate of change of speed (i.e acceleration) is increasing fastest (ie jerk), then what makes you believe that this would not also be the point at which acceleration is greatest?
__________________


Appreciate
0

01072007, 04:46 PM  #108  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
__________________
RAWRrrrr


Appreciate
0

01072007, 04:49 PM  #109  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
jerk is not a point, as i already said jerk is directly realted to the slope of the HP curve, why is this? because the more HP the higher the obtainable ROA so as HP is increasing rapidly so is your ROA as ROA is increasing rapidly jerk is high, as you get to peak HP your potential ROA will be highest and jerk will become 0, as you start to lose HP past peak your potential ROA starts to fall and jerk again increases (i express as "negative" jerk) what needs to be noted though is that even as your ROA is decreasing after peak HP, and even though your at peak HP your jerk is 0, your ROA is still higher than at lower HP points
__________________
RAWRrrrr


Appreciate
0

01072007, 04:53 PM  #110  
Your resident 4um troll
50
Rep 2,021
Posts 
Quote:
your car is accelerating at a rate of 100mph/s (lol i wish) steadily, jerk is 0 your car is accelerating at a rate of 10mph/s, increasing its accleration by 10mph/s/s, so even though jerk is higher here, your car still has a lower ROA
__________________
RAWRrrrr


Appreciate
0

Post Reply 
Bookmarks 
Thread Tools  Search this Thread 

