Power to weight ratio or High performance

[JD]

Not necessarily. Suppose I have no knowledge of: the car in question,
what speed it's currently travelling, or what rev rate it's running at.
If you tell me the engine is outputting 150 HP at a given instant,
it's still trasferring energy to the car at the rate of 111.9
kilijoules/sec, regardless of engine speed. It could be at 2000/3000/
4000/10,000/100,000/etc RPM, and the rate at which energy is added to
increase speed and/or offset parasitic losses would be the same.
That's what power is - the rate that energy is made available in one
form or another.

I'll keep on saying that knowing torque at the crankshaft, devoid of
knowing other factors (rev rate), will not tell how much energy is
being transferred from the engine to the rest of the car. Knowing
the power output at the crankshaft, devoid of knowing other factors,
does indicate the rate of energy being transferred to the rest of the
car.

Who cares? My point, in the original post is that HP is derived from torque
at a specific RPM. The engine rotates. It applies force by rotating.
Therefore, everything from the engine is dervived from its torque at a
specific RPM. If I know the torque and the RPM, I can derive its energy
output; whether in HP or KW; but I can derive its power. If I know its
maximum power, I still need to know its RPM to dervive its torque, or its
potential. Either way, power, gives no indication as to the performance of
the car unless you know a whole lot of other things.

I have a 300 HP car with 300 lb-ft of torque at its peaks. It easily smokes
my friend's 73 Dodge producing 342 HP (at the wheels) with 320 some-odd
lb-ft at the peaks. Why? Because my car is lighter, geared better, and is
AWD. HP alone will not give an indication as to the car's performance.

 

[y_p_w]

Who cares? My point, in the original post is that HP is derived from torque
at a specific RPM. The engine rotates. It applies force by rotating.
Therefore, everything from the engine is dervived from its torque at a
specific RPM. If I know the torque and the RPM, I can derive its energy
output; whether in HP or KW; but I can derive its power. If I know its
maximum power, I still need to know its RPM to dervive its torque, or its
potential. Either way, power, gives no indication as to the performance of
the car unless you know a whole lot of other things.

That's not what I'm getting at. There are so many people who say,
"my car puts out X amount of torque" like it really means anything.
At what RPMs? How flat is the curve over a large range. Is the
gearing well suited to get the most out of that engine? The secret
to selecting gears is to find ones that keep the engine in the
fat part of the power curve the longest.

What I really don't care for is the addage that "torque accelerates
and power gives top speed", which is patently wrong. It's "power
under the curve" (i.e. along the rev range) and "proper gearing" that
accelerates.

I have a 300 HP car with 300 lb-ft of torque at its peaks. It easily smokes
my friend's 73 Dodge producing 342 HP (at the wheels) with 320 some-odd
lb-ft at the peaks. Why? Because my car is lighter, geared better, and is
AWD. HP alone will not give an indication as to the car's performance.

I thought the initial discussion was power to weight ratio. If
everything is the same between two cars (weight, current speed,
parasitic losses), and both engines are putting out the same
power at different revs, both car will be accelerating at the
same instantaneous rate (let's say in m/sec^2).

 

[BlueSTi]

I don't even know what the skid numbers were stock but I added Tien HA
coilovers and stickier tires and I get a consistent reading of 1.1G. I have
never launched hard and never will...too easy to break things with that much
traction but I do drive fast and hard on occasion.
I could only afford a 12 year old VR-4...but it only has 59,000 miles on
it and is real sweet, love the four wheel steering and limited slips.
I also added a K&N FIPK and Magnaflow catback with gutted cats and Stillen
downpipe...runs good even with stock boost, but with it cranked up a bit
...WOW! TG

I've been pretty satisfied with the STi in stock form. But, there are
a few folks out there running a little over 400 hp out of them with no
ill effects.
I done a few hard launches and it's exactly that, a lauch! I haven't
heard of many components breaking on the STi. The thing is pretty
sturdy right from the factory. There are a few folks who have burnt
up a clutch, but, that's about the only transmission component that
seems to get destroyed. The STi has a pretty unique transmission when
compared with other Subaru's. Doesn't have much in common with any of
them actually. The thing is set up for "Ricky Road Racer" right from
the factory. A fully syncronized transmission is handy for shifting.
Most of the time I feel really bad about just using it as a commuter
vehicle.

BlueSTi
"Scary-Fast"

 

[T]

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"

Do you just want to compare Dragsters or Automobiles in real world
conditions?

There are these bends in the road called curves and if all you have is
acceleration you will not be able to navigate them.


TBerk

 

[BlueSTi]

Do you just want to compare Dragsters or Automobiles in real world
conditions?

There are these bends in the road called curves and if all you have is
acceleration you will not be able to navigate them.


TBerk

Power to weight ratios are incredible important in AutoX too. I agree
that a purpose built dragster will probably not be your best AutoX
car. The STi was not designed to be a straight-line racer. But in
general, a Rally car will have pretty dag gone good straight line
performance too.
BlueSTi
"Scary-Fast"

 

[y_p_w]

Power to weight ratios are incredible important in AutoX too. I agree
that a purpose built dragster will probably not be your best AutoX
car. The STi was not designed to be a straight-line racer. But in
general, a Rally car will have pretty dag gone good straight line
performance too.

Frankly, a simple "power to weight" ratio makes for a very blunt
tool to determine acceleration. Power varies over the rev range.
Different cars have different efficiencies due to tires, wind
resistance, etc. For something as simple as a 0 to X time, the
"average" power delivered to the wheels should be a decent first-
order approximation of how much power is needed.

I ran a little exercise to figure out the average power needed
to get a 1500 kg car (about the weight of a Honda Accord) to 100
km/hr (62.5 mph) in 7 seconds. I assumed no parasitic losses or
energy stored in rotating masses (wheels/shafts/etc) which also
use power. I used metric units for ease of calculation.

E=0.5*m*v^2
=0.5*1500kg*(100km/hr)^2
=0.5*1500kg*(27.78m/sec)^2
=5.788*10^5 joules
=578 kilojoules

Average power (over 7 seconds)
=5.788*10^5 joules/7 sec
=8.269*10^4 joules/sec
=82.69kW
=112.4 HP

112 HP doesn't seem like much, but this is **average** power.
Power delivered to the wheels is severrely reduced or drops to
zero when shifting. Any engine that can really get a car this
weight to 62 MPH in 7 seconds is going to need a power peak much
greater than 112 HP.

 

[BlueSTi]

Frankly, a simple "power to weight" ratio makes for a very blunt
tool to determine acceleration. Power varies over the rev range.
Different cars have different efficiencies due to tires, wind
resistance, etc. For something as simple as a 0 to X time, the
"average" power delivered to the wheels should be a decent first-
order approximation of how much power is needed.

I ran a little exercise to figure out the average power needed
to get a 1500 kg car (about the weight of a Honda Accord) to 100
km/hr (62.5 mph) in 7 seconds. I assumed no parasitic losses or
energy stored in rotating masses (wheels/shafts/etc) which also
use power. I used metric units for ease of calculation.

E=0.5*m*v^2
=0.5*1500kg*(100km/hr)^2
=0.5*1500kg*(27.78m/sec)^2
=5.788*10^5 joules
=578 kilojoules

Average power (over 7 seconds)
=5.788*10^5 joules/7 sec
=8.269*10^4 joules/sec
=82.69kW
=112.4 HP

112 HP doesn't seem like much, but this is **average** power.
Power delivered to the wheels is severrely reduced or drops to
zero when shifting. Any engine that can really get a car this
weight to 62 MPH in 7 seconds is going to need a power peak much
greater than 112 HP.

I would say you are not too far off. I think a factory Honda Civic
has about 90 HP maybe a little less. The civic averages right at 10
seconds to go from 0-60. Not that impressive when you figure my STi
can cover the distance in half the time!

http://www.albeedigital.com/supercoupe/articles/0-60times.html

It's interesting to check out some of the 0-60 and 1/4 mile times for
production cars. Some are not as fast as you might think. And some
are a lot slower than I thought.
BlueSTi
"Scary-Fast"

 

[y_p_w]

I would say you are not too far off. I think a factory Honda Civic
has about 90 HP maybe a little less. The civic averages right at 10
seconds to go from 0-60. Not that impressive when you figure my STi
can cover the distance in half the time!

Distance or speed?

1/4 mile times and 0-60 times don't necessarily correlate with each
other. My simplified example was purely about transferring enough
energy to get 1500 kg of car to 100 km/hr in 7 seconds; that's
relatively easy. Time to distance is WAY more complicated.

 

[BlueSTi]

BlueSTi wrote:

Distance or speed?

1/4 mile times and 0-60 times don't necessarily correlate with each
other. My simplified example was purely about transferring enough
energy to get 1500 kg of car to 100 km/hr in 7 seconds; that's
relatively easy. Time to distance is WAY more complicated.

I agree, but a car with a fast 0-60 will without exception have a very
good 1/4 mile time. Any car with a 0-60 under 5 seconds will have a
1/4 mile time in the low 13's. Here are a few examples from
http://www.albeedigital.com/supercoupe/articles/0-60times.html

The first number is 0-60 time the second is 1/4 mile time:

2004 Subaru WRX Sti 4.9, 13.2
2002 Acura NSX 4.8 13.4
2002 Aston Martin V-12 Vanquish 4.4 12.9
2002 BMW M3 4.7 13.4
2001 BMW M5 4.7 13.2
2002 BMW Z8 4.5 12.8
1987 Buick Regal GNX 4.7 13.5
1990 Chevrolet Corvette ZR-1 4.4 12.8

And this is just A-C on the list. You won't find one car with a sub 5
second 0-60 time running a high time in the 1/4 mile. Seems that if
you have the power coming off the line you will finish well. So
mathmatics aside, in the real world, a fast 0-60 nets you a fast 1/4
mile time.

I will note that there are a few cars that have good 1/4 mile times
but fairly poor 0-60 times. In general these are 2WD vehicles with
large displacment motors. So, traction and vehicle weight do come
into play.
Something else you might notice is that the STi runs nose to nose with
Exotic Super Sports cars but carries a less than "Exotic" price tag.


BlueSTi
"Scary-Fast"

 

[y_p_w]

I agree, but a car with a fast 0-60 will without exception have a very
good 1/4 mile time. Any car with a 0-60 under 5 seconds will have a
1/4 mile time in the low 13's. Here are a few examples from
http://www.albeedigital.com/supercoupe/articles/0-60times.html

The first number is 0-60 time the second is 1/4 mile time:

2004 Subaru WRX Sti 4.9, 13.2
2002 Acura NSX 4.8 13.4
2002 Aston Martin V-12 Vanquish 4.4 12.9
2002 BMW M3 4.7 13.4
2001 BMW M5 4.7 13.2
2002 BMW Z8 4.5 12.8
1987 Buick Regal GNX 4.7 13.5
1990 Chevrolet Corvette ZR-1 4.4 12.8

And this is just A-C on the list. You won't find one car with a sub 5
second 0-60 time running a high time in the 1/4 mile. Seems that if
you have the power coming off the line you will finish well. So
mathmatics aside, in the real world, a fast 0-60 nets you a fast 1/4
mile time.

I was just getting that the math for an approximation of 0-60 (or
whatever target) is easy. Figure out the energy and the time
target, and average the energy over the time. One doesn't need
to know anything else.

Now the math for time to distance is really nasty. Distance can
be covered when power isn't applied. Just to do a basic approximation
would require calculus and a formula for power over time.

OTOH - in the real world there's a pretty close correlation.

I will note that there are a few cars that have good 1/4 mile times
but fairly poor 0-60 times. In general these are 2WD vehicles with
large displacment motors. So, traction and vehicle weight do come
into play.

The "screamer" engines might not cover as much ground on the
windup to redline in first gear.

Something else you might notice is that the STi runs nose to nose with
Exotic Super Sports cars but carries a less than "Exotic" price tag.

It's basically a basic compact chassis with an engine on steroids.
Nothing wrong with that, although one salesguy told me that one of
his customers bought one for her teenage son. He didn't do anything
wrong, but the mom tried taking it home, didn't adjust for the power,
and crashed it before getting there.

 

[JD]

I was just getting that the math for an approximation of 0-60 (or
whatever target) is easy. Figure out the energy and the time
target, and average the energy over the time. One doesn't need
to know anything else.

Now the math for time to distance is really nasty. Distance can
be covered when power isn't applied. Just to do a basic approximation
would require calculus and a formula for power over time.

OTOH - in the real world there's a pretty close correlation.


The "screamer" engines might not cover as much ground on the
windup to redline in first gear.



It's basically a basic compact chassis with an engine on steroids.
Nothing wrong with that, although one salesguy told me that one of
his customers bought one for her teenage son. He didn't do anything
wrong, but the mom tried taking it home, didn't adjust for the power,
and crashed it before getting there.

No surprising. It is not a particularly easy car to drive. It has a very
short first gear, and all the rest are all matched. Mine matches at a 700
RPM difference in all but 5th to 6th. Also the turbo boost comes on at very
low RPM (you start to feel its affects at around 2500). So, if you are used
to driving a car that has little low-end torque, this one will surprise you.
Heavy traffic is not the best place to take an STi for a first drive.

 

[BlueSTi]

BlueSTi wrote:
Big Snip<

It's basically a basic compact chassis with an engine on steroids.
Nothing wrong with that, although one salesguy told me that one of
his customers bought one for her teenage son. He didn't do anything
wrong, but the mom tried taking it home, didn't adjust for the power,
and crashed it before getting there.

The STi is a dangerous vehicle if under estimated. The biggest danger
IMO, is heavy accleration in a turn. Say, you are at a stop light and
getting ready to make a left-hand turn. If you stomp the gas and
crank the wheel hard left, the power comes on so strong you are libel
to end up in a ditch if you are not on top of things.

I would NEVER allow a teenager to driver an STi. I know there are
plenty of responsible young kids out there. But, the STi is not for
young kids to practice their driving skills. The turbo-crack monster
will get the best of them.
BlueSTi
"Scary-Fast"

 

[k. ote.]

No speed limiter on the Canadian STi. On a runway, we had mine turning a
little less that 6900 in sixth gear. It was limited by drag, and the
window seals were screetching from the wind coming off of the side
mirrors. My buddy is a highway cop and his static radar recorded a speed
of 269 KPH. We then calculated the speed based on the tire diameter, the
final drive ratio and 6900 RPMs. It works out to 271.24 KPH or 163.23
MPH. So we got it up to a little less than 163 MPH.

What year is your model? 2004? Was there anything special about yours that
your dealer's told you about? (Like, it's a press model, or low-mileage
but still a trade-in?)

 

[JD]

What year is your model? 2004? Was there anything special about yours that
your dealer's told you about? (Like, it's a press model, or low-mileage
but still a trade-in?)

Nope. Nothing special. It is an 04. It was delivered fully loaded (Stereo,
climate control, anti-theft, etc) with 63 KM on it. I know of two others
who have had the cars above 250KPH as well; also Canadian 04 STi.

 

[k. ote.]

Nope. Nothing special. It is an 04. It was delivered fully loaded
(Stereo, climate control, anti-theft, etc) with 63 KM on it. I know of
two others who have had the cars above 250KPH as well; also Canadian 04
STi.

Well, that's damn interesting! Now you've made me want to see what mine'll
do.

What tires were you running at the time? Also, where do you get your
wheels aligned? I went to a local shop here that has laser wheel alignment
and I'm not quite happy with the results, even though my former-M3 friend
recommended it to me.

Another question: when does that screeching sound start for you? I've
had it here (with side mirrors out) at its lowest around 145kph. With the
mirrors tucked in I haven't heard it yet. How about you?

 

[JD]

Well, that's damn interesting! Now you've made me want to see what mine'll
do.

What tires were you running at the time? Also, where do you get your
wheels aligned? I went to a local shop here that has laser wheel alignment
and I'm not quite happy with the results, even though my former-M3 friend
recommended it to me.

I had it done at the local tire shop. I was using the stock RE 70s and they
will rail like hell at low speed (giveing the impression of bad alignment).
However, it does get better at higher speeds.

Another question: when does that screeching sound start for you? I've
had it here (with side mirrors out) at its lowest around 145kph. With the
mirrors tucked in I haven't heard it yet. How about you?

At about 225

 

[k. ote.]

I had it done at the local tire shop. I was using the stock RE 70s and
they will rail like hell at low speed (giveing the impression of bad
alignment). However, it does get better at higher speeds.

My alignment felt great with the old RE070s. I put PZero Nero M+S on and
suddenly I had a slight left pull when I tromped on the accelerator.
Stupid me I forgot to get the alignment fixed immediately after installing
the new tires, but things are better now since the laser alignment.

Tracking I had no problem with. It was consistent with road conditions, so
I never felt like the car needed fixing.

At about 225

Grr.. I'll have to forward on an mp3 of the noise I hear around 140-180
(depending on crosswind.)