Quote:
Originally Posted by Casull
Yeah, but look at those torque numbers on the BMW... Horsepower is deceiving. All it is is a function of RPM and torque. Torque is where you "feel" the car, so i would take an engine with massive torque over an engine with far less torque but more HP any day of the week.
As for the weight, that is not at all suprising. You are comparing a V-10 to a V-8.... Displacement only means bigger holes, more cylinders means more parts and thus more weight.
|
Weeeelll not so fast there... Let's look at this in greater detail:
Your correct, horsepower is (rpm*torque)/5252, so if you increase either engine speed or engine torque at a given speed, you have an increase in power... BUT all things are not equal.
Generating lots of torque requires strong engine internals... remember, torque comes from the force of the expanding gas acting on the piston head, connecting rod, and then acting on the crankshaft. Each of these must be stronger (read: heavier) to support this force... Additionally, the cylinder walls would need to be thicker to support the higher pressures in high displacement engine.
Given this fact, high-torque engines tend to be heavy, and slow (because the heavy engine internals can't spin as fast). This is why I was suprised the LS7 was lighter than the S85... given its large bore I would have expected the internal components to be heavy (making the engine heavy).
Engine torque is also less important than engine power.
Wait... let me explain (this is a pretty detailed look, forgive me if I ramble):
So lets say you have an engine that generates 1000ftlbs of torque, sounds like a lot doesn't it? Now, what if this engine could only spin to 2000rpm? (OK, its steam engine, just laugh and follow along) You would only be making 380 horsepower!
Now, if you have an engine that only makes 100 ftlbs, but can spin to 20,000rpm, you'll make the same power! That's right, 380hp. Some motorcycle engines can do this (I think a turbo Hayabusa has been dynoed at these levels). So you have the same power, but at much lower torque and much higher speed.
So you say you want 10,000 lbft of torque at the rear wheels?
Engine #1 [10,000 (at wheels)]/[1000 (at engine)] = 10:1 Gearbox Ratio
Engine #2 [10,000 (at wheels)]/[100 (at engine)] = 100:1 Gearbox Ratio
Either one of these gearboxes is easily designed and built with modern materials and technology...
Now for the punchline:
The difference in weight in the engines would be HUNDREDs of pounds (since a engine that can generate LOTS of torque would inherently have high cylinder pressures and large forces acting in it)
The difference in gearbox weight? MAYBE a hundred pounds... probably much less. Gears are VERY good at torque multiplication, and so can increase effective torque for relatively little weight.
So what's the downside? Acceleration. If you wanted to accelerate quickly with the first engine, you only have to get the engine up to speed 10 times faster than the wheels are going... which probably is only a couple hundred rpm/sec.
With the second engine, it would be accelerating at 100times the wheel speed, which means perhaps thousands of rpm/second! This means it has to be light, and quick to spin up.
In any case, the summary is that engine power is all that really matters (from a purely academic perspective) since engine torque can be multiplied as needed.
~LSx