BlackBelt2025 said:
Since I am in the engineering field, I figure I should probably throw in my 2 cents to this debate.
I'm all ears!
BlackBelt2025 said:
Torque is measured in lb-ft (or N-m for metric). So picture this: if an engine produced 100 lb-ft of torque and had an outer wheel diameter of 12 inches, then the force that the wheels apply to the pavement would be 100 lbs. If the outer wheel diameter is 24 inches, then the force would be 50 lbs, and so on. Torque is a measure of how much force the engine can apply to the tires.
more torque = more force = higher acceleration.
Not bad so far, but no mention of the car's gearing which multiplies the engine's torque up to 20x or more depending on the car.
BlackBelt2025 said:
Horsepower is a measure of the power of the engine. Power is work over time. So an engine with more horsepower can produce more work in the same amount of time than an engine with less HP. This does not really affect acceleration too much, but it has everything to do with speed. At highers speeds, aerodynamic drag becomes a big factor in performance. An engine with high brake horsepower (bhp) has the ability to produce more work in order to counter these increased aerodynamic forces, meaning the car can go faster.
more BHP = more work = higher top speed.
WHAT? Did you just say power doesn't affect acceleration "too much", but has everything to do with speed? ARE YOU SERIOUS? Sorry dude... you post here claiming to be some sore of Subject Matter Expert and then spew this stuff? I hope you brought a flame suit 'cause you're about to get BURNED.
Power has EVERYTHING to do with acceleration AND speed. Torque has EVERYTHING to do with acceleration AND speed. Power and torque are intertwined, relative and dependant on each other in a car's engine. YOU DON"T GET ONE WITHOUT THE OTHER. If you want more power output you either need to make more torque, make the same torque at a higher engine speed, or some combination of the two.
BTW, torque is measured, horsepower is not. Horsepower is calculated based on the measured torque output and the known engine speed. A horsepower rating is nothing more than a convenient number allowing people to quickly compare the MAXIMUM OUTPUT of two different engines.
WRT turbodiesel engines, I don't care if it makes ONE MILLION ft-lbs of torque, if it's a 200 Hp engine it's a 200 Hp engine and will only do 200 Hp worth of work, whether it's accelerating a vehicle, turning a generator, pumping water out of a mine or turning the soft serve mixer at DQ.
What nobody here is talking about is the SHAPE of the TORQUE CURVE. The shape of the curve can and will have an affect on the AVERAGE POWER OUTPUT when the VEHICLE GEARING is taken into account. When we accelerate (through the 1/4 mile for example) we need to change gears if we want a good launch and to still be accelerating through the traps at the end. Cars usually redline (cut off) after their peak power has been reached. Why? So that when you shift gears the engine speed doesn't drop too far away from the POWER PEAK. Operating the engine in the meatiest part of the power curve (just before and after the power peak) keeps the engine operating at it's highest AVERAGE POWER OUTPUT.
A small power dense engine (read: small, high reving, variable cam timing, etc.) tends to have limited torque but caries that torque high up into the RPM range. This results in a relatively flat torque curve and a corespondingly straight line power curve with a fairly sharp "peak". The sharper this peak the greater the power drop when shifting into the next gear for a given % of RPM loss (gear spacing). This is why extremely tight gear spacing is used on many race cars; it helps them keep their average power up during gear changes.
A larger less power dense engine of the same peak power has a more traditional torque curve with a peak in the middle and a gentle roll of on either side. This results in a much broader power curve around its peak and keeps average power output higher during gear changes. For a given overall engine/vehicle speed ratio (both engines redline at the same vehicle speed in the same gear), it will also feel quicker due to the "hump" in the middle of the torque curve. If the vehicle weight is the same between both cars, the one with the larger engine will accelerate faster. If they both have the same aero drag and are optimally geared to redline at top speed, top speed will be the same (at terminal velocity there is no acceleration and engine RPM is held steady at the power peak, which is the same for both cars).
Another thing hurting the small high reving engine is inertia. To help overcome the lower torque output of the smaller engine, super short gearing is used in the lower gears. This means that the rate of engine acceleration is quite high and the inertia of the crankshaft, flywheel and cluth assemblies will limit the car's acceleration in lower gears. Sometimes this inertial effect is so high that, by the seat of the pants anyway, the car will appear to accelerate just as hard in 2nd gear as it does in 1st. Lower inertia parts will help this situation, but less inertia can only go so far before it starts costing big $$$.
I've only just scratched the surface here... The reason most forum posters can't get this information right is they A) usually don't fully understand the topic and B) don't have the time or patience to go through it in sufficient detail. Both of which are reasons that most people shouldn't bother "trying" to explain it in the first place!!!
It's too bad you're getting the brunt of this frustration, but of all the garbage posted in this thread (there's a lot of it) you're the only one who claimed to be some sort of "professional" before misleading and confusing people.
If anything I've written here doesn't look right to anyone, it's either because I have not explained it clearly or you are reading it wrong. This is all based on physics, not my opinions.
BlackBelt2025 said:
My easy way of remembering this is:
Torque overcomes inertia, horsepower overcomes drag.
Icing on the cake! LOL!
