Power to weight ratio or High performance

[BlueSTi]

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.
http://auto.howstuffworks.com/horsepower4.htm

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"

 

[y_p_w]

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.
http://auto.howstuffworks.com/horsepower4.htm

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.

 

[JD]

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.

 

[Cam Penner]

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.

 

[TG]

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

 

[JD]

"Cam Penner"

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.

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).

 

[Cam Penner]

"Cam Penner"


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.

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?

 

[y_p_w]

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.

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.

<http://www.v8914.com/Horsepower-v-torque.htm>

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.

 

[JD]

"Cam Penner"

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?

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.

 

[JD]

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.

<http://www.v8914.com/Horsepower-v-torque.htm>

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.

I believe that is exactly what we have been saying; HP is simply calculated
as a function of torque and RPM.

 

[Cam Penner]

Cam.Penner.news1ATpleasedontspamgoldmedalsystems.com@hotmai
l.com says...

blah blah.

Sorry for the rant. Torque vs. HP is one of my pet peeves.

 

[Cam Penner]

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.

 

[y_p_w]

where we



I believe that is exactly what we have been saying; HP is simply calculated
as a function of torque and RPM.

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.

 

[BlueSTi]

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

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"

 

[Ross]

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.

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.

Ross

 

[y_p_w]

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.

 

[Mickey]

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.

While you fellows discuss this, take into consideration:
* Torque is a force around a rotating axis.
* HP is a mathematical calc taking force, time & distance into
consideration. 33,000 ft-pounds per min.

Additionally torque and kilowatts aren't related. One is force the
other is a quantity of power. One kw = 1.34 HP

Mickey

 

[Cam Penner]

While you fellows discuss this, take into consideration:
* Torque is a force around a rotating axis.
* HP is a mathematical calc taking force, time & distance into
consideration. 33,000 ft-pounds per min.

Additionally torque and kilowatts aren't related. One is force the
other is a quantity of power. One kw = 1.34 HP

.... but torque IS related to HP. It is calculated
mathematically by combining the RPM, the torque and a
constant.

 

[JD]

"Cam Penner"

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.

I totally agree. However, it also has to do with the type of driving that
is done in N. America vs the rest of the world as well. Here, most people
quote drag strip numbers when talking fast cars; acceleration from a
standing stop. But highway speed limits in N. America are in the 100-110
KPH range (roughly 60-65 MPH), where in Europe, they are well above that.
Also, we don't worry about gas prices like they do.

Because European speeds are higher on the highway, acceleration from a high
speed to a higher speed is more important than acceleration from a standing
stop and lighting up your tires. While it is still torque converted to
power, low-end torque is not as important as higher-end torque. So, KW at
higher RPM is great for what they do with their cars; in Imperial or US
measurements, that equates to HP. But my point was, in the original post,
that torque, not HP is what an engine operates on, and since HP is really
just a calculation of torque for a given RPM, we can't look solely at HP
numbers to determine how high a performer a car really is (discounting what
it actually means to be a high performer).

But, I agree that it does influence the type of engines that sell well. In
N. America (except for motorcycle enthusiasts who know the difference) a
big-assed displacement engine that revs low with gobs of low-end grunt and a
transmission the size of a cattle barn, seems to reign supreme, while
small-displacement, high-revving engines are king everywhere else.

 

[JD]

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).

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.

"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.

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.

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.

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.