Horsepower versus torque
- Thread starter Frank
- Start date
"The V8 commodore, for Wife bashers, alchoholics, and general road-idiot speeders who can't seem to obey the law, love merge lanes, and have to to keep edging forward centimeter by centimeter at the red lights just desperately waiting for them to turn green".
Ugh. AFAIC, practically every car with high power is occupied by a strange-in-the-head driver who has to do everything wrong. Why can't they save it for off-road or desolate places. Damn holden, ruining Australias image since...whenever it was invented.
What was the thread about again?
Ugh. AFAIC, practically every car with high power is occupied by a strange-in-the-head driver who has to do everything wrong. Why can't they save it for off-road or desolate places. Damn holden, ruining Australias image since...whenever it was invented.
What was the thread about again?
I was reading the Audi R10 TDI Le Mans handbook recently. Pretty good example of what a "thoroughbred" performance diesel engine can achieve these days.
Guden Oden said:Hum, I'm pretty certain that example of yours isn't as much a matter of tuning, but rather one of cubic inches...
Rest assured, a turbocharged diesel with the same volume's likely to have 2x the torque of your oldsmobile cast iron stove, easily. Heck, I read over ten years ago about a D.A.F. trailer tractor engine that delivered 2000Nm of torque, the only gas powered engine I know of that comes close is the Bugatti Veyron's W16 quad-turbocharged monster at what, 1100-something?
Now, I wonder what Volvo's 600HP truck engine's delivering in torque, certainly a LOT more than 2000Nm...
It's all a matter of how the car is tuned. Fuel and Induction have nothing to do with it.
An engines actual power is determined by only 3 factors. The amount of explosive force within the cylinder, the amount of mass in the piston and crankshaft it's moving, and the distance the piston and crankshaft have to travel.
If you want high torque but low overall horsepower you go with high compression which creates lots of explosive force, and large heavy pistons that travel a long distance.
What fuel is used to create that force is irrelevent. Fact is, the power isn't generated until the fuel burns. It's the burning and expansion that creates the power, and gasoline is just as capable of creating a large degree of expansion as diesel is.
mjtdevries
Newcomer
DiGuru said:
Ok, since you don't want to read the information about torque and horsepower I gave you, let's proof you wrong with your own example.
Take the BMW 5 series:
530i: V6 3,0l 258 HP, 300 Nm at 2500 rpm.
525d: V6 2,5l 177 HP, 400 Nm at 2000 rpm.
The consumer would buy 530, but you would buy the 525d.
Let's see who is right:
Performance specs from BMW:
- top speed
525d: 230km/h
530i: 250 km/h (electronically limited to 250)
- Acceleration 0-100 km/h
525d: 8,1 seconds
530i: 6,5 seconds.
OUCH! That consumer just left you in the dust...
Ready to admit that maybe it is you, who doesn't know the relevance of torque?
But as I said before, it's not fair to compare a 3l engine with a 2,5l engine.
So let's take the less powerful 2,5l gasoline engine to make it easier for your diesel powered car.
So now we have the 525i vs 525d. Sounds fair?
525i: V6 2,5l 218HP, 250 Nm at 2750 rpm.
525d: V6 2,5l 177 HP, 400 Nm at 2000 rpm.
Let's see if you win this time:
- top speed
525d: 230 km/h
525i: 245 km/h
- acceleration 0 - 100 km/h
525d: 8,1 seconds
525i: 7,5 seconds
Again you loose.
So maybe that silly consumer didn't make such a bad choice after all?
I think I have proven beyond doubt that you should not just look at torque when you want to know which car will accerelate better. I have also proven that if you have to decide solely based on either HP or Torque, that HP will give you a better indication of performance.
Now go back to my first post in this thread and this time read it with an open mind. And then you will also understand WHY this is the case.
mjtdevries
Newcomer
That was(is) true for older diesel engines, which were far less complex than gasoline engines, and which could run on just about anything.
The problem is that gasoline and especially LPG are more refined fuels which don't polutte the engine itself as much as diesel does. And especially the modern turbodiesels in consumer cars suffer from that.
Note that I spoke about the economical lifespan of that turbodiesel engine.
Because you use a less refined fuel you need to do very extensive and costly maintenance around 250000 km. So costly that it's not worth the money anymore.
That isn't needed for an engine that runs on LPG.
(Nobody in the Netherlands will run 250000 km on a gasoline engine, because of tax reasons, so I don't have experience numbers on those, but according to the car manufacturers those wouldn't need that costly maintenance either)
Sobek said:"The V8 commodore, for Wife bashers, alchoholics, and general road-idiot speeders who can't seem to obey the law, love merge lanes, and have to to keep edging forward centimeter by centimeter at the red lights just desperately waiting for them to turn green".
Ugh. AFAIC, practically every car with high power is occupied by a strange-in-the-head driver who has to do everything wrong. Why can't they save it for off-road or desolate places. Damn holden, ruining Australias image since...whenever it was invented.
What was the thread about again?
I drive my 300C AWD very smoothly and only gun it to pass or enter the freeway.
ShootMyMonkey
Veteran
Really just depends on the size of the turbo. The smaller the turbo, the earlier it peaks.
Actually a lot of those particulate emissions can be dealt with by chemical means -- and of course, the right kind of diesel needs to be used. I believe biodiesels are very good on this, though I might be confusing it with something else. Urea injection is the basic method to eliminate NOx emissions, and supposedly, does a decent job on most other emissions. The other thing is to avoid over-rich conditions. Diesel power is pretty much controllable by the amount of fuel injected, so you can keep injecting fuel and you'll make more power -- but once you start injecting more fuel than can be oxidized by the air, you get incomplete combustion and loads of black smoke.
See, now this is why the American car buyer is hopelessly idiotic. Absolutely no sense of perspective. The point is that low-end power and torque is useful to all vehicles for normal driving. I don't see the value of 1/4 mile times to get from my apartment to work. Autocrosses don't get me to the grocery store. There isn't any value whatsoever in using those as your benchmarks and there never could be. If you regularly go to the track, then yipee for you, but comparing that to a commuter drive is stupid.
The moment low-end torque is brought up, everybody's examples is some 500 ft-lbs on a big-rig. Have you ever dared to think there's something in between making 500 ft-lbs at 2000 rpm and making only about 40 ft-lbs at the same rpm? A daily driver vehicle will probably never climb above 4000 rpm throughout its entire lifetime. So why should someone care what kind of power their Camrys make at 6500 rpm?
Doing engine maintenance on any gasoline engine is rather rare on the US. About the closest thing to that that is ever done is a cleaning of the fuel injectors. Basically, a gasoline engine outlasts pretty much all of the other components of the car. I've only ever heard of maintenance on the engine itself for people restoring very old cars.mjtdevries said:
ShootMyMonkey said:
The average american commutes over 30 minutes on drives that include significant stints on freeways. I average 25k miles per year in freeway traffic for business travel and regularly have to pass slower traffic on two-lane roads. Here it is somewhat helpful to be able to stomp the gas and get from 55 mph to 90 mph quick enough to pass before getting hit head-on by oncoming traffic.
I *am*, however, looking at getting a TDI Rabbit (golf) for my work commute (6 miles on surface streets).
My current car (Impreza 2.0R) tends to fall back when I just push the gas pedal down at low rpm. Little torque at low rpm, not helped by having awd. It only starts to accelerate above 3000 rpm, and around 5500 rpm it really starts to go. It goes plenty fast enough, and it accelerates fine, but I have to keep the rpm above 3000 rpm to do that. Of course, I could rev it and get launched pretty fast from the stoplight, but that isn't good for the engine and clutch, and consumes lots of fuel.
In a car with a (turbo-) diesel engine on the other hand, the above is never a problem. You have plenty of torque at low rpm. But to accelerate fast from 0 to 100 km/h (60 mph), you need to choose between falling back or shifting two times, instead of only once for a gasoline car.
Then again, for normal driving around as ShootMyMonkey said, you generally never rev your engine above 4000 rpm. And you might want to drive around the city in third gear, for good fuel efficiency. And in that case, lots of torque at low rpm make the car feel faster and much more responsive. You press down the gas pedal and it still accelerates. Which would require me to shift back to second gear.
So, while a gasoline car is better for things like 0-60 mph and 1/4 mile, a (turbo-) diesel engine is better for driving in the city. Or a V8 with lots of cubic inches, of course.
And on the highway at constant speed, it doesn't matter very much in either case. But then again: if you drive in fifth or sixth gear on the higway and want to accelerate to pass a car, you're still better off with a diesel engine, as with the gasoline one you would want to shift down to third gear.
So, yes, simply looking at high horsepower and using a gearbox that tries to use the full rpm range possible, you win the 0-60 mph and 1/4 mile contests. But you have to work harder when you're simply driving around and miss the low-end torque. Unless you like to keep the engine revved as high as possible whenever you want to be fast.
So, horsepower isn't a good metric for fluid driving, 0-60 and 1/4 mile aren't for the same reason, and while torque is a good metric for that, it isn't a good one for comparing maximum values. So what is a good metric?
Then again, you might simply be best off looking at the 0-60 mph and horsepower figures if you want to have a fast car, while torque is best for smooth and fluid driving with less shifting.
Could you agree with that, mjtdevries?
In a car with a (turbo-) diesel engine on the other hand, the above is never a problem. You have plenty of torque at low rpm. But to accelerate fast from 0 to 100 km/h (60 mph), you need to choose between falling back or shifting two times, instead of only once for a gasoline car.
Then again, for normal driving around as ShootMyMonkey said, you generally never rev your engine above 4000 rpm. And you might want to drive around the city in third gear, for good fuel efficiency. And in that case, lots of torque at low rpm make the car feel faster and much more responsive. You press down the gas pedal and it still accelerates. Which would require me to shift back to second gear.
So, while a gasoline car is better for things like 0-60 mph and 1/4 mile, a (turbo-) diesel engine is better for driving in the city. Or a V8 with lots of cubic inches, of course.
And on the highway at constant speed, it doesn't matter very much in either case. But then again: if you drive in fifth or sixth gear on the higway and want to accelerate to pass a car, you're still better off with a diesel engine, as with the gasoline one you would want to shift down to third gear.
So, yes, simply looking at high horsepower and using a gearbox that tries to use the full rpm range possible, you win the 0-60 mph and 1/4 mile contests. But you have to work harder when you're simply driving around and miss the low-end torque. Unless you like to keep the engine revved as high as possible whenever you want to be fast.
So, horsepower isn't a good metric for fluid driving, 0-60 and 1/4 mile aren't for the same reason, and while torque is a good metric for that, it isn't a good one for comparing maximum values. So what is a good metric?
Then again, you might simply be best off looking at the 0-60 mph and horsepower figures if you want to have a fast car, while torque is best for smooth and fluid driving with less shifting.
Could you agree with that, mjtdevries?
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ShootMyMonkey
Veteran
Most cars of a decent size are geared such that even going from 55 to 75, you probably won't ever even cross 3000 rpm. Those with smaller engines are generally going to be optimized to stay at around 2500 rpm when making 100 km/hr (~63 mph). Proper gearing for any vehicle means making sure you can stay in the power band, so you can achieve that just as easily on one drive train as another. Of course, this is what's nice about TDI in that it will pretty much give you that kind of response all the time.
I'm not saying power is meaningless, I'm saying that it's when you get your power that's meaningful. I do wish TDI were possible, but there's practically no such thing in the state of California (to say nothing of finding a diesel filling station).
It's still true actually, because making the higher compression rates and injecting under higher pressure means that all the components need to be much beefier than a gasoline counterpart. It does means that diesels are heavier and more expensive to produce than gas engines, but in the EU, there is some level of subsidization on this. The US will never have such a thing.
Well, Wankel rotaries, you can certainly make more power than any piston engine of the same size, but they're hardly torque-y. They can rev very high very smoothly (since the shaft does 3 rotations for every one full rotation of the rotor). The combustion is very inefficient (dynamic compression ratio is lousy) so fuel mixtures are very rich in order to make power, and it also inherently burns oil as part of the process, so the emissions are pretty crappy. However, this does mean they respond well to turbos and the power curve is pretty flat.
Veselovsky rotaries combat a lot of the problems with Wankels, but they aren't really proven in any automotive applications as of yet. They can probably be constructed to achieve compression ratios good enough to run on diesel fuel.
Blazkowicz
Legend
what often matters is overtaking Grandpa or some heavy slow truck on a short distance on normal/country road, i.e. get quickly from 70 km/h at low rev to say 110 (in theory you shouldn't be speeding for overtake but do you care ). turbodiesel is nice, you don't even need to downshift. though downshitfting to accelerate on a gas engine is fun.
well, let's say I'd take gas for a fun < 1 ton car, and turbodiesel for a boring bigger family car. and in any case, manual gearbox (I never drove a automatic, I guess I would be frustrated and bored, as if I had to use a computer with a one mouse button )
). turbodiesel is nice, you don't even need to downshift. though downshitfting to accelerate on a gas engine is fun.well, let's say I'd take gas for a fun < 1 ton car, and turbodiesel for a boring bigger family car. and in any case, manual gearbox (I never drove a automatic, I guess I would be frustrated and bored, as if I had to use a computer with a one mouse button
)I wouldn't say so.nthd said:
They make good power for their weight and size but they get poor fuel economy and you have to check the oil level every time you fill up because it injects oil into the combustion chamber to lubricate the apex seals
They're fragile engines, so a bit of perdetonation (usally turbo applications with a bad tune) can kill an rotary, whereas a piston engine can take a lot more predetonation (knocking).
They do have nice torque curves though- quite linear, they're comparible an electric motor in that respect, but t hey don't make a lot of torque.
The Renesis in the rx8 is the best n/a rotary in terms of fuel milage and power, but compared to the old rx7 it doesn't make nearly as much power, however it should be a lot more reliable.
The major problem it has is that while it's rated at 238hp, it makes closer to 215-220HP at the flywheel so people who dyno them get pissed off when they see they're only making 170-180hp when they should be making 210-220 to the wheels.
http://en.wikipedia.org/wiki/Wankel_engine
mjtdevries
Newcomer
As I said before: Torque on the wheels is a good metric. And preferably displayed in a graph with the speed. (rpm of the wheels)
Because that takes into account all the important factors instead of just one: Torque of the engine, HP of the engine, AND the gearbox, which is equally important.
The problem then because educating people on the use of that graph.
Then again, you might simply be best off looking at the 0-60 mph and horsepower figures if you want to have a fast car, while torque is best for smooth and fluid driving with less shifting.
Could you agree with that, mjtdevries?
Oh yes, When you suddenly start talking about things like less shifting, than the situation dramatically changes and for sure Torque also becomes important.
But if you want smooth and fluid driving with less shifting, you should consider an automatic gearbox, instead of choosing between gasoline or diesel.
You will also launch from traffic lights faster than anybody else...
(They might keep up or be faster, right untill the moment they have to shift gears. And then you'll seem them in your rearview mirror)
Agreed.mjtdevries said:
Ah, ok. But it's hard (for me) to say the right thing all in one go.
Yes, good point.But if you want smooth and fluid driving with less shifting, you should consider an automatic gearbox, instead of choosing between gasoline or diesel.
In theory, but an automatic will have worse ratios, and is normally quite a bit slower at the 0-100 km/h sprint as when using manual shifting. But that might be because they're build for average driving, not to milk the maximum amount of torque from the engine.
