A while back there was some dicussion on what was considered a high performance vehicle. I found a site with some interesting information requarding power to weight ratio's.
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That formula give my STi a .091 power to weight ratio. Not as high performance as I first thought. Funny thing is that the 0-60 is better in the STi than with cars with higher power to weight ratio's. I think that has something to do with AWD. But, with 400 hp the STi gets a power to weight ratio of .122. This is entirely possible in a realistic street driveable STi. Something to think about. But, then again, power is like that. Once you have it, you seem to always want more! BlueSTi "Scary-Fast"
In and of itself, a peak power # gives a good approximation of how well a car accelerates. That being said, you don't have the same power at WOT at 3000/4000/5000 RPM. The benefit of a turbo is an increased amount of power in the middle of the rev range. The "power under the curve" gives a better indication of acceleration.
Instantaneous power=acceleration. However, your engine isn't putting out the same power all the time. How fast the engine puts out enough energy to get the car up to 60 MPH isn't based on a single peak power figure.
I agree with most of that. However, horsepower is torque at some RPM. It is torque that equals acceleration. Horspeower equals speed. But you are correct in that both torque and HP vary with the RPM of the engine.
If torque = acceleration than HP = acceleration. If HP = speed, than torque = speed. (Just with different constants).
HP provides you with both acceleration and speed - through the use of gearing. If everything except torque and gearing is the same between 2 vehicles (same HP, weight, shape, etc.), then proper selection of gearing will yield identical acceleration and speed numbers.
Nobody ever really wants to talk about the impact of gearing on HP/torque curves. I guess the math is too hard or something.
My VR-4 has a .086 ratio and that site you referenced gave 0-60 at 5.8 but a magazine test got a 4.9....that's a big difference. The point is the numbers for empirical performance vary but on the other hand the ratio is constant. The factor you seem to have missed in your 0-60 comment was the gearing. My car has 5 gears and will go 160 MPH, I don't know about the SIT but I would think it would be geared lower which would provide better acceleration. The numbers I like are the skidpad forces...got you beat there. TG
I don't disagree. However, only torque is important to an engine. Since horsepower is simply measured by torque at a specific RPM (which is where we get force/time), and low-horsepower, high-torque engine is one where the torque curve is on the low end of the RPM range, while a high-horsepower, low-torque engine is one where you have to get the revs up high in order to get any power out of it. So, between the two, assuming the same final drive ratio, the high HP engine will have a higher top end, while the high torque engine will accelerate faster from a standing stop, but drop off more quickly. You're right in that you can compensate with proper gearing.
However, because we don't generally want to do the math (although the math is really not that hard to do; I just can't remember the RPM constant to use for HP), HP is generally concerned with the top end (ie. how fast you can go), while the torque curve is about the low end (ie. how fast you can get to the top end).
But the whole point is that two engines with different HP/torque curves WILL have a different final drive ratio. Assuming the same drive ratio makes the whole comparison rather trivial, and somewhat useless. Saying that only torque is important to an engine is one thing, but saying that only torque is important to acceleration is quite another.
If I told you I had an engine in my car that made a peak torque of 10 foot pounds, and asked if you wanted a drag race from a standing start over 1/8th mile for pink slips, would you take it?
Would it change your mind if you turned up to the race to find my engine wound up to 400,000 RPM?
Knowing instantaneous torque without power tells nothing unless the rev rate is known. Knowing the current power output without knowing anything else gives acceleration. Accelerating a mass means adding energy to the system. The rate at which energy is added to a system is power.
As you indicated, the main question with any car should be whether the gearing is designed to bring the engine up to that peak power quickly. A high revving engine will be piss poor if mated to a tranny designed for a larger lower revving engine. Honda DOHC VTEC engines are mated to a tranny with lower gearing and a lighter flywheel that helps bring the engine up to that 7-9000 RPM redline where the peak power is.
I remember someone did a calculation on a 50 ft high water wheel that he saw at a mall. The sucker had oodles of torque (somewhere over
2500 ft-lbs) but only output about 6 HP.
If all one has is a single gear, then yes, torque will be a good indicator of how fast a car can accelerate. However - we live in a world where one can shift gears.
It most definitely would change my mind because if you told me your peak torque was 10 lb-ft, then I would ask where is the peak? But it also depends on what the car is tuned for. If it is a race car, you can play with the final drive ratios. If you are talking about a street car, the final drive ratios vary from around 3.5:1 up to about 4.5:1. Consequently, it is not really a trivial comparison for a street car. For example, my car, and STi makes peak torque at 4000 RPM; pretty useable. I assume the WRX peak is around the same RPM. An Acura RSX has HP similar to a WRX, but its peak torque is quite low, and at relatively low RPM. Most of my buddies with WRXes have no trouble dispensing with an RSX; they have similar final drive ratios.
The trouble with your 400K engine with a peak torque of 10 lb-ft, is there is probably no clutch that could handle it and get the car going. I didn't say that only torque was important to acceleration. I said only torque is improtant to the engine; HP is derived from torque and RPM. I also said a high-torque engine will have better acceleration from a standing stop than a high-horsepower, low-torque engine.
But the torque that is important to acceleration is the torque at the wheels - not at the transmission input. It's post-transmission torque that supplies the force, not pre- transmission.
Granted, in any extreme example there are issues like clutches and numbers of gears, etc. But there are solutions to many of those things.
The torque vs. HP wars are largely culturally based. Torque numbers are king over in North America, while in other areas of the world, the number of kilowatts is more important. Not surprisingly, this tends to influence which type of motors sell well.
Sure. However - a torque reading at the crankshaft essentially tells one nothing in the absence of revs. Knowing the power at the crankshaft (in the absence of revs or a torque figure) will still indicate how much kinetic energy is being added to the system (i.e. your car).
"Only torque is important to an engine" is an incorrect statement. Torque is one of those things that sounds cool, but doesn't really say much without knowing more. Essentially all torque indicates is the amount of energy that can be produced per angular displacement (radians/degrees/revs). You **have to** know how fast the sucker is revving to get any useful idea of how much work/energy the engine is able to produce and transfer to the wheels.
Personally I don't particularly like the terms "high torque" or "low torque" engine because they don't really describe the true nature of the engine. I'd think "high grunt" and "high revving" are better terms. A "high grunt" (your typical large displacement [possibly diesel] engine) would produce more torque (and thus power) at lower revs. This would be better suited towards automatic transmissions and/or hauling large loads, since they can produce adequate (but not peak) power at lower revs.
The STi is suppose have been clocked at around 4.5 to 4.9 seconds for the 0-60 time depending a lot on the driver. I've never timed mine. I haven't had it out to the track yet. The STi has 6 gears and is speed limited at 155 mph. In sixth gear at 75 mph I'm running about
3000 rpms. I have yet to hit the speed limiter. But, I have been up to 140 mph and it was pretty smooth. The STi is definitely geared lower. But, the nice thing is that you can always stay in the "sweet-spot" of the power band at all times. The bad thing is that you end up shifting a lot! The last car mag I read had the STi at .95 to 1.01g's on the skid pad with the factory tires. All I can say is by the time the tires are slipping your are doing some serious driving! I drive like an old grandpa so I have rarely broken the tires loose.
I had considered the VR4 on several occasions. But, I really wanted 4 doors. And the few VR4's I came across were just too dag gone expensive. Heck, saw a '99 VR4 going for $25K the other day. That's not much less than I paid for my STi new. Nice cars and easy to get a lot of power out of them but my wife would never go for it. BlueSTi "Scary-Fast"
You bodys power output = Car HP The weight of the shovel lifted = Car's Torque Number of lifts per min = Cars engine RPM The work or effort require to accelarate the car = The amount of sand. The sand falling through the glass = Friction (Wind, gears etc..) The amount of sand in the top = Cars current speed
As a Human you can vary your shovel weight.
You need to shovel 1 metric tonne of sand from the bottom of an hourglass to the top, the more sand in the top the faster it falls through to the bottom, with one tonne of sand in the top the sand falls through at 30kgs per min. Your bodys power output is fixed. Using a small shovel you can shift 1 kg with each lift and you can do this fast say 30 lifts/min (30kgs/min). With the large shovel you can lift 10kgs but it takes you longer to do this (3lifts per min). When the hour glas is full you can only maintain the amount of sand in there NOT add to it. If you wanted to add more you need more power.
If you doubled the power output but kept the shovels the same you could lift
1 kg 60times/min or 10 kg 6 times/min, this getting to a full top of the hour glass (accelaration) quicker. youd also be able to cope with 60kgs of sand falling through per min.
The 1kg shovel is an example of low torque high RPM (Petrol), the 10kg shovel is like a diesel, high torque low RPM. Both cars with the same HP figure, geared correctly to take advantage of the engine characteristics and of the exact same friction and mass would have the same 0 - 60 time as previously stated.
Its the rotational element of torqe that confuses everyone. the above makes it linear and in my opinion easier to understand.
Sure. A lot of people have heard the expression, "Give me a lever long enough, and I can move the world." That's a case where a lever is doubled in length, the same force applied to the end will output twice as much energy for any angular displacement. However - it will also take twice as long.
Torque isn't some magical figure. In essence, all it describes is the amount of energy that can be output for however many revolutions of the crankshaft. It doesn't tell anything about how much work is done in time. Power alone, without knowing what gear or what rev speed tells a lot more than just knowing torque alone.
But you don't know the power unless you know the revs since HP varies with RPM the same as torque; because it is calculated as a function of torque and RPM.
It does not change the fact that the instantaneous force applied by a crankshaft, or the power measured over time, is rotational energy in a car; torque. Since the engine only produces torque as useful output (it produces heat, chemical emissions, etc. but these are not useful outputs per se), then everything else you measure about the engine is derived from torque.
OK. So you don't like the terms. What I mean by 'high-torque' is an engine which produces the majority of its torque at low RPM, while a high HP engine is one that either produces very little torque at low RPM and higher torque at high RPM, or maintains its low torque well past its peak; like an F1 engine.
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