Power And Torque Again

[jengis 0]

Noticing how easily this issue snowballed, off-topic, in a recent thread, and noticing how there is so much confusion with it I thought I would try to give an explantion without too much maths. I don't want to start a war so please don't post if its not constructive.

 

Q) Is it power or torque that makes a car accellerate quickly (i.e "feel" quick)?

A) Ignore the engine for a second, think about what is happenning at the wheels. It is a force (torque) through the driveshafts turning the wheels that makes the car accellerate. The more torque, the better the accelleration. The size of the wheel will alter the amount of force, but lets stick with a normal size wheel...

 

Q) But doesn't the engine produce the torque at the driveshafts?

A) Not directly, no. The gearbox has different ratios. The gears multiply the force (torque) produced at the engine's crakshaft by different amounts, depending on the specific gear ratio and the differential ratio. Of course, the engine produces the force in the first place but that force is increased by the gearing (massively in 1st gear and less so in 5th, for example). The point is, you can get a gear ratio to give you as much torque as you want! The wonders of newtonian meachanics!

 

Q) Hang on! Why can't you just have lower gears? Or, why not just one reeeaaallllly low gear???

A) We are talking about a rotational force - everything is spinning. As the wheels spin faster so does the engine. The engine has a limit to how fast it can spin. The different gears allow the car to travel at different speeds relative to the engine speed so you need to have a series of gears to get a good speed range. The lowest gear gives most accelleration but speed is limited, top gear has the least accellerative force.

 

Q) So where does engine torque come into it? You haven't mentioned Horsepower yet either!

The engine needs to produce some amount of torque in the first place, of course. An average road engine will produce good torque from just above idle right up to 5 or 6 thousand RPM. In a 205 1.9 Gti the torque will start to drop rapidly after 5 or 6k RPM. At these high engine speeds, torque is reduced so much that it will be necessary to swap to a higher gear, sacrificing the benefit of the low gearing to get the engine to back into its torque producing zone. Its a trade-off. This trade-off can be described better with a power figure... but hold on just a sec...

 

Q) Yes, but my engine doesn't loose torque at 6k RPM! its a tuned 16v and it makes its best torque at over 6k and revs to over 7k. Its better, right?

A) In a word, Yes! It is! Even if its maximum torque is no greater than a standard engine, the fact it doesn't loose out at the top end will enable it to stay in a lower gear for longer, keeping the benefit of the low gear multiplying the forces. So if you max-out the engine accellerating through the gears it will be faster.

 

Q) So whats power?

A) The trade off between engine torque and the low gear. It's a theoretical figure but by multiplying engine speed with torque you have represented this trade-off. If the engine is spinning at its maximum power RPM it will be at the best engine speed for accelleration at that road speed. In a standard 205 1.9 GTi it's at about 6k RPM.

 

Q) So is power the most important factor?

A) Yes, if you can make the engine spin at that speed most of the time. If not then perhaps its a missleading figure. If you have an engine that has poor torque but produces it at 9k RPM it will need lower gears to make best use of its potential. If the engine can spin fast it can run lower gearing for the same road speed. If it is geared too high/long and you don't rev the engine to 9k RPM then you won't get the benefit of the power and the engine will feel sluggish.

 

In summary:

Torque is the actual force produced by the engine but when multiplied by the speed it is spinning it gives a figure that will reflect how much force will be available at the wheels once the gearing has done its thing. You can't get something for nothing. You need revs and torque to produce power. Without either you have zero power. If that power is produced through higher RPM you need lower gearing to make use of that potential.

[DrSarty 90 1 Cars]

 

Kindred spirit.

[jamiej 0]

Couldn't have said it better myself, it is all too easy to see how the confusion starts though !

[Cameron 16]

All very true, but I think most of the confusion in that thread came from when people talked about a preference for torque or power. When someone says they prefer an engine to have more torque its an empty statement. What they mean is they prefer an engine with peak torque at low rpm.

 

Also, in line with your first statement, people need to remember that there is only so much force that the tyres are actually capable of transmitting to the road before they begin to slip. So an engine that produces more torque won't necessarily make you go faster. It will to a point, but then once the wheels spin any extra force you're making is wasted.

That's where RPM is important, as if you make a lower amount of peak torque but at a faster rate (i.e higher revs) you are able to take advantage of the lower gearing for as long as possible. What's more if the torque doesn't exceed the capability of the tyres then nothing is wasted.

 

I agree with what you're saying, but I think you could do with a slightly better summary and it may help people understand a little better if I add a bit, or it may confuse the hell out of people. Either way, I'll have a go.

 

In order to make the car move you need two things, you need a force to overcome the inertia (resistance to move) of the vehicle, and then you need to keep applying that force at a certain rate to keep the car moving. If you want the car to move faster, you either need to either increase the force, or increase the rate at which you apply it.

The force that you are applying is the torque produced by the engine, and this force is applied to the road surface by your tyres. So by increasing the force you will increase the rate at which the car moves forward. Great, but once you exceed the force that the tyres can transmit they will start to slip and so will be wasting effort. So once you have reached the point at which you can't apply any more force without the tyres slipping (wheelspin) your only other option is to increase the rate at which you apply it, i.e. increasing the RPM.

So, since torque is a figure representing the force an engine is making, and power is a figure representing the rate at which the engine is capable of applying it, both are equally important when it comes to understanding the performance of an engine. Just as you can't have a figure for power without an applied force, you can't have movement without applying that force at a certain speed.

In an ideal world you would have an engine that produces an amount of torque equal to the tyres' ability to apply it, and keeps applying it until infinity RPM. Since that is impossible to achieve due to the laws of physics, you have to have a compromise.

 

That went on a bit more than I had planned! Hope it actually helps.

[welshpug 1,657]

All very true, but I think most of the confusion in that thread came from when people talked about a preference for torque or power. When someone says they prefer an engine to have more torque its an empty statement. What they mean is they prefer an engine with peak torque at low rpm.

 

AKA the 8v vs 16v argument,where the 8v FEELS torquier, due to the fact that unlike the 16v that comes on song over 5k rpm, it just runs out of puff and is nowhere near matching the 16v engines torque or power, in standard form....

 

Also, in line with your first statement, people need to remember that there is only so much force that the tyres are actually capable of transmitting to the road before they begin to slip. So an engine that produces more torque won't necessarily make you go faster. It will to a point, but then once the wheels spin any extra force you're making is wasted.

That's where RPM is important, as if you make a lower amount of peak torque but at a faster rate (i.e higher revs) you are able to take advantage of the lower gearing for as long as possible. What's more if the torque doesn't exceed the capability of the tyres then nothing is wasted.

 

I think Jeremy Clarkson illustrated it very well with an AMG Mercedes a few series ago on Top Gear, the thing has sooo much torque all it wanted to do was smoke the tyres

 

But of course that was down to the application, the Articulated trucks I used to work on were only 420 bhp, but they had somewhere in the region of 2000Nm/1450 LBft's of torque, I'd love to see that AMG Mercedes engine trying to haul 44 tons as effectively as that engine could!

[AdamP 0]

What you have said is correct. However, there are many ways to think about it!

 

Power = the amount of engine transferred in a given time period, or lb/ft per 1/5250th of a second in the case of HP.

 

Obviously, the more energy that is transferred in a time period, the faster you will accelerate. The energy doesn't change as the rotation is geared up or down (conservation of energy etc etc), so the faster the engine is turning, the more energy is supplied and the faster you will accelerate.

 

So power does cause acceleration.

 

That makes sense to me having done a whole coursework on it, however i'm not the best at explaining.

Edited December 24, 2008 by AdamP

[projectpug 0]

I like the good old turbo diesel example very good low down torque but for only a short period of time.

http://www.superchips.co.uk/pdf/powercurve/psahdi90.pdf

 

Nice example there how even with high torque you only get a small hp figure due to the fact it tails off very quickly.

[Cameron 16]

Yeah that AMG represents it quite well, they were limited by RPM so had no choice but to up the boooost and make the thing churn out mountains of torque to get the power figure they wanted. As for the lorries, they have 4 (or more) driving wheels and a hell of a lot more weight over them (more weight on contact patch = more grip) so they can easily get that power (ahem.. torque) down. Remember the Top Gear boys trying to drift their lorries? Edited December 24, 2008 by Cameron

[DrSarty 90 1 Cars]

I'm afraid the last few posts I can't agree with.

 

Remember that torque is THE rotational force an engine makes; it makes NOTHING else. Torque is the measurement of that force, and it changes across the rev range. It has a peak figure (and I for not one solitary minute am saying highest is best, & I never have).

 

I have always said the spread of the torque is what is important in conjunction with where it peaks, which is directly related to the application for the engine. A nicely spread delivery and a high average is what you're wanting for a typical road car, with a peak tailored to arrive at the typical, optimum RPM the engine will expect to be operating at. This is why I mentioned with regards to F1 cars, that their engines need to be operating at such high RPM for their 200mph+ track application. Their 900bhp 'rating' is simply a representation of the engine's turning ability at such 18,000+ rpm figures. It's just a visualisation whereas torque is a pure and simple measurment of the engine's rotational 'power'.

 

Again, I have always said that horse power is purely a calculation derived FROM torque; it is simply not the other way round. Sure, with one you can work out the other, or even the RPM given torque & BHP, but the torque always exists as it is the measurement of the engine's ability to rotate a crank. You must have torque to generate a BHP figure, and again (I'm sorry), not the other way round.

 

It's not an argument really as to which one is better, as one simply doesn't exist without the other.

[Cameron 16]

100% agree, but the engine also turns at a certain speed, making power an important figure too. If the engine generated a torque but didn't rotate then you wouldn't go anywhere.

The thing is, if you break it down like that then essentially it can be broken down further. The torque is essentially a function of the cylinder pressure, the area of the piston, and the length of the crank. So really we should be talking in BMEP. The thing is BMEP isn't exactly layman's terms!

This brings me to the point I've been making, an engine doesn't JUST generate torque, it generates torque at a rotational speed. Ergo, torque and power are equally important figures.

 

Hope that makes sense.

 

Ps, Merry Christmas!

[jengis 0]

I think Jeremy Clarkson illustrated it very well with an AMG Mercedes a few series ago on Top Gear, the thing has sooo much torque all it wanted to do was smoke the tyres

 

But of course that was down to the application, the Articulated trucks I used to work on were only 420 bhp, but they had somewhere in the region of 2000Nm/1450 LBft's of torque, I'd love to see that AMG Mercedes engine trying to haul 44 tons as effectively as that engine could!

 

I'm sure The AMG engine would be fine hauling a truck if had the right gearing (i.e very low). In a truck there's lots of gears, stacked very closely together, and it will go through 3 or 4 changes before 30mph. This is because it has a very limited spread of torque. The said AMG engine would need to be geared even lower to generate the forces to haul the truck (as it makes its power at higher revs, compared to a diesel truck engine), but with more power and a nice spead of torque through its rev range it would probably enable the same tonnage of vehicle to accellarate more quickly! That the engine has lower torque is of little matter, so long as the gears are suited to the application.

 

The reason big diesel engines are used for trucks is because they make the required power as cheaply, reliably and, (most importantly) ecomomically as possible.

 

Quote Cameron...Also, in line with your first statement, people need to remember that there is only so much force that the tyres are actually capable of transmitting to the road before they begin to slip. So an engine that produces more torque won't necessarily make you go faster. It will to a point, but then once the wheels spin any extra force you're making is wasted.

That's where RPM is important, as if you make a lower amount of peak torque but at a faster rate (i.e higher revs) you are able to take advantage of the lower gearing for as long as possible. What's more if the torque doesn't exceed the capability of the tyres then nothing is wasted.

 

Whilst I see what you are trying to say (and agree with you) I think the idea about an engines torque being too much for the tyres to cope with is a genearalisation and a slightly missleading argument - there are plenty of ways to look at it, and it all gets rather confusing as everyone has their own way of reasonig it.

 

I assume you mean in first or second gear? Fair enough but if I made my point about gearing well enough it should explain that the car should be geared suitably so the resultant torque produced at the wheels will be not be massively over-working the tyres. In this instance you would use taller gearing, especially in first and second... so then it all comes down to how much power there is again. Once up to higher road-speeds (50mph + or so) you would need VERY large amounts of engine torque to be a problem.

 

 

Glad this discussion didn't bring about the slanging match i anticipated. Happy xmas! Time for beer!

[Cameron 16]

Ah yes, that's true about the gearing.. but only if you choose it around the engine's output. If you're modifying a production engine as we all are, it's a bit of a different story.

[jengis 0]

Ah yes, that's true about the gearing.. but only if you choose it around the engine's output. If you're modifying a production engine as we all are, it's a bit of a different story.

 

Yes, but we can, and do, alter the gearing don't we? Gearing for out-and-out performance is mostly chosen depending on the engines best operating range and the resultant power to weight of the vehicle it is in. The character of the engine (i.e peaky and lacking midrange torque or broad spread) is important too. Then there is the amount of gears needed vs the time wasted swapping cogs... if theres a specific speed range chosen...the minor factors are many....!

 

I'm not dissagreeing with you Cameron. It's clear form everyones posts so far that we are all of similar ideas. It's explaining the idea and how it comes accross that's different.

 

I just had had issues with the idea that really big torque at the engine will be a limiting factor and have uncontrollable wheelspin - it won't if it has long gearing to make use of the engines particular character (modern big torque turbo diesels manage...)

 

When people quote the maths its (usually!) all perfectly clear cut. And for those who fully understand it it makes perfect sense and how can anybody argue with it. But most people don't want the maths. After all, we like cars not calculators, and how things feel is the most important thing but hardest to describe in words.

 

Problem is, myself and yourself, who are trying to put into laymans terms what we believe to be the real world application of said maths can sometimes inadvertantly give people new to the terms and ideas the, frankly, wrong information.

 

These things are true and it seems we all agree on them-

 

1. You cannot have power without the engine producing a turning force
   
2. The turning force is useless unless it is actually rotating, "doing something"
   
3. By multiplying the force by the speed you get a rating of POWER (this is not a force in itself)
   
4. Power can be calculated for any point in the engines operating range but Peak/Max BHP (as is quoted) only refers to one particular engine speed.
   
5. The power available at any engine speed can be utilised, through gearing (gearbox or wheels size etc), to produce a usable force that will accellerate the vehicle.
   

 

I guess I'm just trying to get it more accurately described than it can (or needs to) be!

Got to let it rest now I've put my own opinion down as best I can.

[Cameron 16]

Well said.

[DrSarty 90 1 Cars]

You're totally correct about BMEP which we mentioned in the other thread. But like you say, it's not exactly layman's terms, and I would argue the same about torque; it's really not a tangible quantity, whereas BHP - as has often been stated 'sells cars' - is not only commonly accepted, for example a bit like ABV for drinks rather than specific gravity, but is proven when you say:

 

Yeah that AMG represents it quite well, they were limited by RPM so had no choice but to up the boooost and make the thing churn out mountains of torque to get the power figure they wanted.

...

 

because in the sales wars it's easier and likely to appeal to a wider audience to have a BHP figure which is higher than a competitor's car to encourage people to buy it.

 

People often say car x has ___horsepower & ___torque (in that order) to try and rationalise for fairly how the engine performs. Please remember that if we're rating or grading an engine, you need to remove gearboxes, diffs, transfer boxes, vehicle weight, wheel size and tyres from the equation, as ultimately in this 'argument' it's that which is truly confusing people, as it trashes the whole notion of horsepower 'being king' when a car with 20bhp less wins a straighline drag.

 

If you're going to rate an engine's performance, the fairest explanation is not a unique figure of either torque or horsepower, it is best represented by the graph area under the torque curve. Note how I said the torque curve yet again, as the BHP curve is purely a mathematical calculation derived from it. This area under the graph in my opinion is representing something along the lines of BMEP, which is showing the engine's efficiency at generating rotational power. Revs are absolutely a factor if you want to go fast, hence higher revving track engines and F1 cars revving much higher.

 

As a result though they will (perhaps just like a diesel engine) have a relatively narrow torque band which the driver must work a little harder to remain in or near to, to stay competitive. The only time 'we' get caught out by TDi rep mobiles on the roads is when they're in that sweet spot of torque, meaning they accelerate better - which is a genuine application of force - whereas engines tuned for higher power which is more likely than not tied in with higher RPM, perhaps have to again drive harder to get the thrust they need to go quickly.

 

Now to go quickly isn't that difficult. Christ a Ford Ka can probably muster over a 100mph. But to go quickly, quickly, i.e. to get there as quick as possible, requires the engine's rotational power to be there early on AND STAY THERE (i.e. the flat, fat torque curve), which is what requires an efficient engine. However 'efficient' isn't sexy, and to most, neither is torque. But BHP like ABV has become an easy representation, although it doesn't really tell the whole story, which is why this argument exists. As soon as one person says (like they did) 'oh HP is much more important', and then dangerously says 'torque is irrelevant', then it shows they've been suckered into the BHP sales patter. I'm not saying it's meaningless, I'm just saying it's another way of explaining in sexy terms what engines are primarily doing, which is creating torque.

Edited December 26, 2008 by DrSarty

[jengis 0]

If you're going to rate an engine's performance, the fairest explanation is not a unique figure of either torque or horsepower, it is best represented by the graph area under the torque curve. Note how I said the torque curve yet again, as the BHP curve is purely a mathematical calculation derived from it. This area under the graph in my opinion is representing something along the lines of BMEP, which is showing the engine's efficiency at generating rotational power. Revs are absolutely a factor if you want to go fast, hence higher revving track engines and F1 cars revving much higher.

 

If you plot the BMEP curve it will match, exactly, the shape of the torque curve. They are effectively the same thing - the averaged forces from all the cylinders acting upon the common crank. Its just BMEP, Brake Mean Effectieve Pressure, is used as a measure of the engines efficiency accross its rev range and is often expressed as a ratio.

 

To "measure" the performance... Well, If you are sticking with the one particular gear then you would want to use the area under the torque curve that will relate to the speed over which the car will be accellerating. If you are going through the gears then how do you know when the best gear change point is? Looking at where BHP peaks shows where the best accelleration will be gained. Unfortunately you can't be there all the time in real-world driving. Thats the issue I guess.

 

As a result though they will (perhaps just like a diesel engine) have a relatively narrow torque band which the driver must work a little harder to remain in or near to, to stay competitive. The only time 'we' get caught out by TDi rep mobiles on the roads is when they're in that sweet spot of torque, meaning they accelerate better - which is a genuine application of force - whereas engines tuned for higher power which is more likely than not tied in with higher RPM, perhaps have to again drive harder to get the thrust they need to go quickly.

 

Yes, exactly. By driving a little harder I presume you mean having the right gear to keep the engine up near peak BHP. A diesel driver would be doing the same thing - trying to keep near peak BHP its just that in a diesel the BHP peak is less "peaky" and is not at high RPM so is easy to reach if you are in too high a gear.

[DrSarty 90 1 Cars]

Yes, exactly. By driving a little harder I presume you mean having the right gear to keep the engine up near peak BHP. A diesel driver would be doing the same thing - trying to keep near peak BHP its just that in a diesel the BHP peak is less "peaky" and is not at high RPM so is easy to reach if you are in too high a gear.

 

Now that is interesting. I'd like to know if people when they are driving harder, are actually trying to stay nearer peak BHP or peak torque. Try that one on for size.

[welshpug 1,657]

aaarghhh!!! my heid hurts, all I want is a faster pug!!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[pip470 61]

Rich, I will be doing some math on this exact question shortly. On the drag strip every gear change has to be at the exactly the right time but what is the right time. well fastest acceleration happens when the torque is the most but that doesn't nessercerally mean that you should change just after peak torque to drop just under peak torque. This is all new stuff I'm learning too as last season I looked at my power and torque graph and also the revs dropped in each gear for the gearbox I was using. The biggest drop in revs was from 1st to second. I havn't got the figures to hand but will do a full write up soon. So with that in mind I held the car in 1st for longer than normal, 7000rpm. The rest of the gears were 6500 and that was with peak torque at 5100 and peak power at 6500. Now I think the correct way and im going to try and show this, is to work out the torque at the wheels taking into account gear ratio, final drive, tyre rolling radius and the torque at a given rpm. So basicly I work out what the actual torque at the wheels is at say 6000rpm in second gear, i look at the rev drop for the gear change (lets say 1000rpm) so putting all the data in for 3rd gear at 5000rpm to see if the figure is greater than in second at 6000. If it is then thats the time to change gear, if it isnt then i should hold onto the gear a little longer as the car is accelerting faster than it would if i were to change up a gear. I will try and do this for a couple of gearboxes I have and also a couple of wheel combinations I have. Can I just point out that this is all from what I have been reading and I do not claim this is right, but hopefully the maths will show the results.

[DrSarty 90 1 Cars]

aaarghhh!!! my heid hurts, all I want is a faster pug!!

 

And Mierion, if you really want your head to hurt, I was wondering what would happen in a 1/4mile drag sprint, in a Newtonian ideal world - no wheel spin, identical drivers, identical start & reaction times, identical wheels, tyres & gearboxes & same red line etc etc - if 2 x 1000BHP Nissan Skylines had a race, where whilst the BHP peak figure is the same (based mathematically on 'torque x rpm' being the same for both cars), one car had a higher torque figure elsewhere in the rev range than the other, i.e. a different shaped torque curve.

 

For example, car A may have a peak BHP of 772lb/ft at 6,800rpm, but car B has 1,050lb/ft at 5,000rpm. Both = max 1,000bhp

 

This would then be a test purely of the engine, although it could be argued that gearing matched to engine would be a factor. It is definitely possible to produce a set of calculations (some people like Josh have a downloaded calculator), to put the Skyline gearbox & FD ratios, wheel & tyre sizes into with a set of torque/bhp specs based on the two imaginary but fair dyno curves. On the latter, I'd like to see if the areas under the curves could ever be the same; I think they could.

 

And another question is what does an engine dyno measure? What nature are the actual measuring instruments inside at the crucial point? I mentioned earlier that Jas E's work is at Mahle Powertrain, and they always have engines on their dynos, and their technicians are always watching the torque readings, only choosing to look at the BHP readings to see if the customer is going to be happy, particularly if they've specified a certain figure, more likely than anything to 'beat a mate'.

 

What I am learning from all this together with me fiddling with engines, is that whilst there is no doubt some detailed and exact science behind how engines perform, there are sooooooo many factors (such as what makes people so different), that a degree of trial, error and perhaps some luck comes into it too. Sandy once said in a large debate about Puma racing, that you've kind of gotta get your hands dirty and try stuff. It may not work, and may not work how you postulated with hours and hours on the flowbench & calculator, and then a random 'gut feeling' change you just throw in pleasantly surprises you.

 

So just to clarify, I am not arguing, I am simply fascinated by this branch of science and engineering that in another life I'd definitely have gone into. However at the moment, I simply struggle with whole notion of BHP being an alternative or indeed competitor to torque, and for that matter even referring to it, other than in its simplistic 'sales' type context. And I am also not being awkward for the sake of it; I'm just interested.

 

Rich

[jengis 0]

Rich, I will be doing some math on this exact question shortly. On the drag strip every gear change has to be at the exactly the right time but what is the right time. well fastest acceleration happens when the torque is the most but that doesn't nessercerally mean that you should change just after peak torque to drop just under peak torque. This is all new stuff I'm learning too as last season I looked at my power and torque graph and also the revs dropped in each gear for the gearbox I was using. The biggest drop in revs was from 1st to second. I havn't got the figures to hand but will do a full write up soon. So with that in mind I held the car in 1st for longer than normal, 7000rpm. The rest of the gears were 6500 and that was with peak torque at 5100 and peak power at 6500. Now I think the correct way and im going to try and show this, is to work out the torque at the wheels taking into account gear ratio, final drive, tyre rolling radius and the torque at a given rpm. So basicly I work out what the actual torque at the wheels is at say 6000rpm in second gear, i look at the rev drop for the gear change (lets say 1000rpm) so putting all the data in for 3rd gear at 5000rpm to see if the figure is greater than in second at 6000. If it is then thats the time to change gear, if it isnt then i should hold onto the gear a little longer as the car is accelerting faster than it would if i were to change up a gear. I will try and do this for a couple of gearboxes I have and also a couple of wheel combinations I have. Can I just point out that this is all from what I have been reading and I do not claim this is right, but hopefully the maths will show the results.

 

Your method for calculating this is exactly right. I have an excell file that allows you to input all the vehicle data and it gives you the graphs for torque at the wheels in each gear. You can have a try with it if you like. (too much time on my hands obviously!)

[jengis 0]

Now that is interesting. I'd like to know if people when they are driving harder, are actually trying to stay nearer peak BHP or peak torque. Try that one on for size.

 

Easy. Peak BHP, without a doubt.

 

Most regular "non-sporty" car engines peak their torque at around 3k but delivering peak BHP at 6k (ish) RPM. Whats the point in all the extra revs if you don't aim for peak power RPM?

[pip470 61]

Your method for calculating this is exactly right. I have an excell file that allows you to input all the vehicle data and it gives you the graphs for torque at the wheels in each gear. You can have a try with it if you like. (too much time on my hands obviously!)

 

 

Jengis that would be great many thanks, Ill pm you my email addres if thats ok. Well I've just run through the first set of figures and I have temperarily baffled myself. The gearbox I have just used for the figures is a 1.6 with a 4.43 diff and a radius of 14(the website i got the formula from was a bit vaugue as to weather this should be rolling radius.) The formula is as follows: force at wheels = torque *gear ratio* final drive all divided by the wheel radius. Now where im a liitle bemused is no matter how far you take it up torque scale to the peak and back down the other side, the 1st gear results are higher than any point in second, That makes sence as the torque is multiplied by a bigger amount in the lower gears, however it still isn't clear where I should change gear. Going sleep now to dream about it some more. Phill

[DrSarty 90 1 Cars]

Easy. Peak BHP, without a doubt.

 

Most regular "non-sporty" car engines peak their torque at around 3k but delivering peak BHP at 6k (ish) RPM. Whats the point in all the extra revs if you don't aim for peak power RPM?

 

If that is the case, I'm not surprised, as the rotational speed of the engine - providing you are in the correct gear - will equate to higher top speed. However, if you have a 1.1litre Ford Ka, do you GET THERE quicker by driving to red line or trying to stay on peak torque in each gear?

[Cameron 16]

And Mierion, if you really want your head to hurt, I was wondering what would happen in a 1/4mile drag sprint, in a Newtonian ideal world - no wheel spin, identical drivers, identical start & reaction times, identical wheels, tyres & gearboxes & same red line etc etc - if 2 x 1000BHP Nissan Skylines had a race, where whilst the BHP peak figure is the same (based mathematically on 'torque x rpm' being the same for both cars), one car had a higher torque figure elsewhere in the rev range than the other, i.e. a different shaped torque curve.

 

For example, car A may have a peak BHP of 772lb/ft at 6,800rpm, but car B has 1,050lb/ft at 5,000rpm. Both = max 1,000bhp.

 

Good question! I think it would depend on the shape of the torque curve. Car B would have a much more "peaky" torque curve, as to keep to 1000bhp the torque would have to drop off fairly rapidly as the revs increased. Meaning you'd have a great surge mid-gear but then it would rapidly tail off. Car A would be a flatter curve, so I would guess that it would be marginally faster as you would get a better mean acceleration in each gear, if that makes sense.