spec-cie

Its a nice analogy that shows why you cannot say its one or the other. The analogy does sum up the difference between power and force well. It also describes why a good total 'applied force over time' (i.e. the area under the curve) is THE most important thing in determining how fast you car will get from min to max speed in a given gear.

But the power is not what's acting on the pad: its the force. The force is actually what makes things happen, the power is a metric of force over time. So it is correct to say that its torque that makes the car accelerate (F=ma if you remember your O-level or GCSE physics) and BHP is an indication of the torque produced at a particular engine (or wheel) speed.

LG John

Spec-cie is of course right. Torque is the actual force the drives the car. However, my post was in relation to those that say, "it's torque that makes you go"

They are not wrong....it is. But its the context in which they mean it its not enough to simply say, 'my cars got xlbft and therefore it's quick' For acceleration and maximum speed purposes peak bhp is the more useful measurement than peak torque.

It's certainly the torque you feel in the car though. That savage kick you get from a good turbo car is the sudden change in torque quickly altering its rate of acceleration but its also works the other way. My STI kicks hard at about 3500rpm and really knocks you into the seat but at about 6000rpm its dropped off a lot - this is because the boost is sliding back and with it the torque. Your instinct is to change gear because it feels like you are going slower but that's not the case, you are still making good power and the car is still pulling hard and continues to do so for a bit longer.

Taking our weedy guy and beefcake again it may well be the case that if they both really work out hard they can each burn 2000 calories an hour on average. But who'd you rather take a punch from?

Another way to think of it is converting the energy to something other than motion. Heat. Sitting with your cold hands on a hot radiator for 30 minutes won't do you any harm - in fact, it's rather nice. But if you used a blow torch on you hand for 15 seconds you'd perhaps apply the exact same amount of energy to your hands but in this instance would melt them off your bones!!

spec-cie

Agree with everything you've said there (and I like your analogies) but I think this is becoming less true. As I said before, just knowing peak BHP was useful when the vast majority of road cars were NA petrol engines with carbs ... the capacity for tuning and changing the curve is much more limited and so peak BHP is all you need to know to get a good idea of how fast the car will accelerate.

But things have changed. The new diesels, higher tolerances (for higher revs), variable valve lift and timing, etc. all mess with the 'standard' shape of the curve.

As a counter-example to your tractor: my wife is a biker who knows some guys that drag race. One has a highly-tuned Hyabusa with about 220 bhp - incredible from a 1.3 engine. It achieves this by revving to a much higher level than standard but it doesn't have much more than the standard torque (about 100 lbft I think). Stick that in a standard WRX (which has the about same power as standard) and you'll get much worse performance. The weedy torque will be overwhelmed by the resistance of the AWD and the weight of the vehicle and the car will be slow as old boots.

Both engines are tuned for different extremes: the tractor's for high peak torque over a short range to move those massive wheels even when dragging a large trailer full of horse**** over muddy ground; and the bike engine for low peak torque deployed over a large range to propel a guy on a lightweight bike up a drag strip at high speed.

In both cases the engine has been tuned to sacrifice one aspect (excellent torque at the low end vs. excellent torque at the top end) where as something like a scoobs engine has been tuned to balance both ... which is where the real performance comes from.

LG John

Fair point It sort of brings back the old saying for me: "there's no replacement for displacement" If you want a really fast car you want a big powerful engine that makes lots of torque across the whole rev range. Smaller engines that rev high or have F.I. are a compromised solution to try and achieve similar results from a smaller, lighter and in some cases cheaper unit.

Like everything in engineering it's a balancing act where compromises usually have to be made. For each application there is probably a sweet spot which is why tuning a car can sometimes reduce the quality of the vehicle by moving it out of the sweet spot (i.e. the FWD car with too much power).

What can't be ignored is the Evo OP talks of probably was 400/400 and that's not to be sniffed at no matter what application you have in mind

dazdavies

iTrader: (22)

My mates got one of these its stock standard. My classic shaped scoob is quite highly modded except it was still running the VF28 at the time. Can't be arsed to list my mods but click view my scooby they are all there.

Off the mark we were neck and neck but once i get into third gear I started gaining on him 4th gear is my strongest gear and carried on pulling by 5th and 6th it was all over i just kept going.

Considering his was standard and cost him £26K mine was not and cost me an awful lot more I wish i had my time again. I'd buy one of these and start modding that. Out of the box they are an awesome bit of kit.

If I had one of them and spent the same kind of money on it as i have the scoob I would have a trully awesome motor.

That said I love how mine drives and what it sounds like on full chat.

Daz

Hoppy

I think there's confusion over the word torque here. You can apply it to anything that turns with some force, from wheel nuts to brakes.

What I am saying here, and I think others too, is that it is 'power at the wheels' that makes a car go. That is normally measured by BHP and the more of that stuff you have available at any given time, the faster you will go. There are a lot of variables that effect that figure, but ultimately it's BHP and BHP alone that propels you. End of story.

The Hyabusa drag bike and the tractor are interesting extremes. The bike engine has low torque, but will have high-lift cams to maximise and shift peak torque as close to peak revs as possible. And it revs very high. A close-ratio gearbox will keep the motor at peak 'power at wheel' as long as possible. Result a 220BHP rocket.

But in theory, you could use that same motor to create one very powerful tractor indeed. It would burn rubber (mud ) and pull wheelies and tree stumps with equal ease. But you would need a very close-ratio gearbox to keep the thing spinning at maximum revs and it would be very hard to control as a farm tool, ie when ploughing it would probably stall every time you changed gear

That's why we have different engines with different characteristics for different applications, which produce and deliver their power in different ways, but ultimately it is 'power at wheels' ie BHP, which determines your rate of progress.

Regards,

Richard.

LG John

That simply isn't the case though - if by move you mean 'haul ***', 'smoke the guy next to you' then yes, bhp at the wheels is what makes you move in a non-literal sence.

Torque however is a measurement of a force (rotational in this case) and its that which physically moves a car from a-b. Take all the torque away and the car goes nowhere (largely cause it'll also have zero bhp).

TonyBurns

iTrader: (3)

Hmmmm, When I stated it was all about torque thats because people put BHP ahead of torque, what I actually mean is that torque makes drivability, BHP doesnt, Hence 25mph, 5th gear, 1500rpm and weighing 1350kg (but producing more torque) v 3rd gear on a car weighing 1260kg with 60bhp less producing more BHP (but less torque) at 3k rpm at 25mph. This isnt BHP that does it, this is the drivability that torque gives you.
Though im not saying BHP doesnt play its part, in most cases your not going to use your peak BHP but you will use your peak torque more often.

Tony

spec-cie

Right, I'm going to have one more go at this. Power at the wheels and torque at the wheels are directly related by a constant at a given engine or wheel speed. It is torque that actually makes you go: it may be 'just a turning force' but its just a turning force that turns the that turns the crank that turns the gears that turns the driveshaft that turns the axles (through the diffs) that turn the wheels that propel you along the road. Power is a derivation of the force at that speed (whether wheel, engine or any other rotational component) - it is measuring exactly the same manifest effect but in a different dimension. You cannot say it is all about power or all about torque because THEY ARE DIRECTLY RELATED!!!

This whole discussion started because people always quote the peak figure for torque or for BHP and claim that is the whole story. It isn't: its the area under the curve (pick one or the other, it doesn't matter, because THEY ARE DIRECTLY RELATED) and one figure, given the many different curve shapes out there now, doesn't tell the whole story. But knowing both peak figures gives you a pretty good idea.

Hoppy, your car, like mine, produces the maximum BHP at the wheels in top gear (sixth on mine) at high revs (7,200 on mine). If you really believe its all about BHP at the wheels we'll have a little race from the lights (what this thread was originally all about). You select sixth and rev to the limiter to get your maximum BHP, I'll select first and rev to 3,000 to get my maximum torque and we'll see who is ahead at the 1/4 mile. And to be honest I don't care if your car is a 600 bhp monster; I'll still win.

LG John

spec I understand everything you just wrote. I fear others don´t and its an important concept to grasp. As you say, and I as I said earlier the area under the curve is the data you really need access to but it´s hardly realistic to ask Joe Soap next to you at the lights what his "area under the curve is" The next best thing IMHO is to know power, weight and drive.

TB you are talking to the wrong man. I have two driving modes. Minding my own business snail pace or driving like I stole it. I have little need for torque beyond a decent amount to keep up with the general flows of traffic. My perfect car combo would probably be a strong TDI for the work hack and a supercharged S2000 for pissing around in

Martin2005

Really intersting discussion this. Here's a suplimental question:

Which car is faster, a car with 350bhp that develops peak power at 7.5k rpm, or a car with the same power but peak at 5.5k rpm?

I suspect that this comes back to torque again

swtmerce

Irrelevant unless you know the AREA under the curve.

Martin2005

Really? I thought the answer would be the car that produces it's peak at lower revs because it would have much higher peak torque, and loads of low down grunt

Of course I'm very likely to be wrong about this though

Hoppy

Lots of agreement here, and disagreeement on semantics, I would say. The important thing missing from most posts is the effect of the gearbox/clutch, which allows you to transfer all your BHP to the road in a useable manner.

If you're talking drag racing, you will find every dragger in the world will rev to peak BHP (not torque) and hold it there via a close-ratio gearbox for as long as possible, with optimum appropriate gearing. If you set off at peak torque, like 3,000 rpm as you quote, in whatever gear, you will get thrashed

Richard.

nisr227

SB - simple but fairly effective eh. Check it out in the next few weeks, as well as a complete new look and options for the site, my calculator will have a torque option to stop Diesel owners moaning at me at last, but its still kept fairly simple. Check out Virtualracer for proper results and all factors.

spec-cie

Not the (bike) drag racer I know. He goes for peak torque at about 6000 rpm and then goes to the redline (I think its at 13000). If he went for peak power at 12000 revs he'd have to change up immediately: what would be the point? He holds GB class records at a number of drag strips so I believe he knows his stuff.

Well, semantics means 'meaning' which is the problem we're having. What you seem to mean is different to my understanding of these concepts. Yes, we're ignoring the effect of gearbox and clutch but they are not relevant to the fundamental concept we can't agree on: that torque and power, at the wheels or wherever, are directly related at a given speed, and that power (BHP) is a derivation of torque so it can only be torque that makes you go.

Hoppy

From what you've said, in theory, both cars would have identical performance (given appropriate gearing) but you need to see the power graphs to be certain. However, we know that the 5.5k rpm car has significantly more torque and expereience suggests it will deliver it over a more useable range than the inevitably more 'peaky' 7.5k rpm motor. So in practise, the 5.5k car will be probably easier to drive quickly. However, top spped of both cars will be identical.

Richard.

Hoppy

Now we're getting confused with his launch technique, which is the great art of drag racing. If your friend is not to spin the rear wheel excessively, or bog down, or flip the thing, he needs to control take-off in this manner. But when he's rolling, got good traction and is still pointing safely in the right direction, he'll he clinging as close to peak BHP as possible and not bimbling along at 6000rpm.


I agree absolutely. Always have I guess the difference between us is you are talking theory and I am talking in practise, which of course has to involve clutch and gearbox tyres etc which has not really come into the debate before now.

Take the F1 race yesterday. We know these are the fastest cars on the planet, we know thay have relatively low torque, but rev to 19000k and achieve astonishing BHP. We also know that, like the drag racer, they have great difficulty getting cleanly off the line and use sophisticated launch control to get the optimum start. But we can see from the on-screen graphics, after that they hold revs between 16-19k at every opportunity because they are seeking to get as much BHP as possible for as much of the time as possible.

Yes, of course BHP is a function of torque, but the more BHP you can get on the road for as much time as possible (regardless of the torque figure at that engine speed) only then will you be making maximum progress.

Best regards,

Richard.

spec-cie

Now I really am confused because you seem to be distinguishing power at the wheels versus torque at the engine ... I've been talking about power and torque at the engine or power and torque at the wheel ... this is how you avoid having to worry about the effects of gearing.

But let's talk about gearing for a while. At launch you want to apply as close to peak torque as possible (without, as you say, bogging the engine down or excessive wheel-spin): that's where you get max acceleration even if power at the wheels is low. So you select first and rev to the peak torque point. As you accelerate torque drops off and so acceleration slows but you are still accelerating. When do you change gear? Well as near to the limiter as possible because, due to the gearing, you are still getting more torque at the wheel than you would at the equivalent engine speed in the next gear up (remember gears divide engine speed - and so multiply torque - to drive the wheels until top which is usually the 'final ratio': one rotation of the crankshaft is matched by one rotation of the wheels). If you car is ideally set up a change near the rev limit (which is also usually close to peak power - is this the point you are making?) will result in a drop in revs that takes you close to the peak engine torque ... i.e. you have the maximum possible acceleration for the gear.

Your own example for the F1 cars demonstrates this. Max torque on an F1 car is typically around 15,000 revs. So they rev to the max in, say second, because they are exerting more torque at the wheel at 19,000 revs than they would at the equivalent engine speed in third, then they shift up to third and the engine speed drops to 15,000 revs which is the point of max engine torque and so max torque for third and away they go again. This also explains why launch is tricky because gearing a car such that 14,000 revs at the engine results in a 'good' wheel rotation speed for getaway is tricky (basically its a fine line between getting bogged down and wheel-spinning, even with those big tyres).

Max power at the wheels is developed in top gear at the peak (engine) power revs. This is where the least torque is: you accelerate less quickly in sixth at peak power than at any other point in any other gear. So its always about maximising torque at the wheels.

Yes, this is theory. But any form of engineering is about getting as close to the theoretical maximum as possible within the constrains of cost, durability, materials, etc. Plus we really talking about the very basics of the theory here ... engineering to maximise torque at the wheels throughout the engine range and gear ratios is the fundamental principle of performance auto-engineering.

Must admit I'm not sure whether we're agreeing or not ... but I'm enjoying the discussion anyway

Hoppy

SC, I think we are agreeing but theory and practise involve confusing variables. Let me pose a purely hypothetical drag car with magical ability which eliminates the key variables as I see them.

This car has an infinitely variable automatic gearbox that will hold the motor at whatever revs you set, so much grip that it can never spin the wheels, and other clever controls to keep it going straight as an arrow. (I said this was a hypothetical car )

The motor produces peak torque at 4,000rpm, and peak BHP at 7,000rpm. For the fastest accelleration, what engine speed would you set before dropping the clutch?

Richard.

spec-cie

Well, given we have a magic car that eliminates all the other variables (I assume that's the point), I'd go for 4000 revs. Acceleration=Force(torque)/mass and peak acceleration will be achieved (and maintained with your magic gear box) at 4000 revs. It's kind of like doing a continuous launch ...

Hoppy

Then we disagree after all What about that magic gearbox and all the other impossible features I suggested in post #80 that allow maximum power at the wheels (ie BHP) to be deployed at all times?

Anyone else care to comment?

Regards,

Richard.