Quote:
|
Originally Posted by boggtown
Just because its at its peak horsepower doesnt mean it wont go any faster, unless the guy was flooring it and it stayed at "150" for like 10 seconds without the rpms going up any more (aerodynamicaly limited). Ive topped out my bmw under "controlled and track regulated conditions" and I only took my foot off the gas because I didnt want to run in the red for too long, and its peak torque was at like 4 grand. I guess all im saying is that just because it reached its peak torque/horsepower doesnt mean it wont continue to accelerate, it just wont do it as fast.
|
Hi,
We disagree, which, when discussing Torque, Horsepower and Top Speed isn't unusual because there is a huge variance in the knowledge base, how it applies, and the fact that when making such all-inclusive statements, you're really discussing the theoretical, which is rarely present. The following is an except from a pretty good article I have on the subject. I don't have the source handy, but I'll look for it and edit this post later.
There's been a certain amount of discussion about the concepts of horsepower and torque, how they relate to each other, and how they apply in terms of a car’s performance.
Looking at top speed, horsepower wins, in the sense that making more
torque at high rpm means you can use a taller gear for any given car speed,
and thus have more effective torque at the drive wheels.
Finally, operating at the power peak means you are doing the absolute best you can at any given car speed, measuring torque at the drive wheels. I know I said that acceleration follows the torque curve in any given gear, but if you factor in gearing versus car speed, the power peak is it.
An example, of the LT1 Corvette will illustrate this. If you take it up to its torque peak (3600 rpm) in a gear, it will generate some level of torque (340 foot-pounds times whatever overall gearing) at the drive wheels, which is the best it will do in that gear (meaning, that's where it is pulling hardest in that gear).
But, if you gear the car so it is operating at the power peak (5000 rpm) at the same car speed, it will deliver more torque to the drive wheels, because you'll need to gear it up by nearly 39% (5000/3600), while engine torque has only dropped by a little over 7% (315/340). You'll net a 29% gain in drive wheel torque at the power peak versus the torque peak, at a given car speed.
At any other rpm (other than the power peak) at a given car speed will net you a lower torque value at the drive wheels. This would be true of any car on the planet, so, theoretical "best" top speed will always occur when a given vehicle is operating at its power peak [sic] Horsepower.
Now, again, it's important to note that the author uses the term
theoretical, because these are not the only factors. In fact, the variables are almost too uncontrolled to account for, or duplicate accurately every time such as; Air Density, Parasitic Losses, Frontal Area and Cd, etc. This is why two cars with the same Power/Torque values and curves can have very different performance.
We need an easy way to make some assumptions and so these are often not included, but make a huge difference in real life. Instead, we use HP/Torque curves from which to draw some conclusions.
But, even the value
Horsepower is an arbitrary figure. James Watt determined this to be 33,000 ft.lbs. of work per minute, but he did so after observing some Draught Horses pulling coal from a mine and actually determining that they produced 22,000 Ft. Lbs. of work in one minute. He then threw in an arbitrary 50%
Fudge Factor to come up with the 33,000 Ft. Lbs./min. which today we call 1 Horsepower
I did some rough calculating using a standard model (STP) and an estimate of the Boxster's Frontal Area, Cd, Parasitic Loss, Standard Gear Ratios, and such. The estimates are necessary because I lack such things as my own personal Wind Tunnel and Rolling Road.
Based on these reasonable estimates, the calculations showed that the Boxster will
hit the wall (aerodynamically limited) at 176.2MPH (Coincidentally, I have never heard of a Natrually Aspirated, Gasoline fueled Boxster reaching this speed). The car would need to be modified to produce 532 HP (my
theoretical limit of how much you can Bore and Stroke the motor and still use Natural Aspiration, Gasoline as a Fuel, and retain reasonable strengh).
I'm sure my calculation could be debated, but I'm equally sure that it has an accuracy within ±5MPH. Using these same factors, my calculations on the 2.5L Boxster show it's limit to be 148.8MPH (manual tranny) and 146.1MPH (Tiptronic S) which agrees pretty much with the published data.
Remember, it takes 4 times the power to go only twice as fast at a given speed. In the end, Drag wins over Torque, Horsepower, or Gearing everytime(again, except in theory)...
Happy Motoring!... Jim'99