Power to weight ratio or High performance

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.

Reply to
JD
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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 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).

Reply to
y_p_w

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"

Reply to
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

Reply to
T

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"

Reply to
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.
Reply to
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!

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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"

Reply to
BlueSTi

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.
Reply to
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
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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"

Reply to
BlueSTi

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.

Reply to
y_p_w

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.

Reply to
JD

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"

Reply to
BlueSTi

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?)

Reply to
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.

Reply to
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.

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?

Reply to
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.

At about 225

Reply to
JD

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.

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

Reply to
k. ote.

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