Originally Posted by
BHCevo
I agree with everything trinyDex says. His stated correlations between exhaust gas velocity, mass flow, torque and horsepower more or less capture what happens in the real world and correctly highlight compromises needed between optimzing peak torque or optimizing peak horsepower.
That being said, I'll offer an equivalent but alternate explanation that better highlights the fundamental relationship between torque and horsepower.
First, I want you to think of your engine as something that makes torque. Forget about horsepower, think torque. Torque is force multiplied by a lever arm. In this case, force is the force supplied by the power stroke and lever arm is the geometry of the crank/rod/piston. In this case force is actually the time averaged force over all the power strokes of all the cylinders.
Okay, two things conecern us here
1) How do we increase torque? Well, most of the time we increase the power stroke force. The time averaged force goes up as
the total amount of fuel and oxygen we burn in all our cylinders increases. In general, we can inject arbitrary fuel (yeah EFI), so we are really limited by the amount of oxygen we can stuff into the cylinders. There are two ways we increase the total mass oxygen we burn
a) Increase combustion volume (displacement)
b) Increase combustion oxygen pressure (forced induction ie turbos or superchargers)
Or both hehe.
2) The other thing that concerns us is ...why does the torque output vary with engine speed? Generally speaking.
Simple. Torque is the product of (oxygen mass being burnt) times lever arm. Your lever arm isn't changing so clearly the oxygen mass being burnt is changing. as a function of engine speed. Your displacement isn't changing, so your combustion oxygen pressure MUST be changing, regardless of the presence of forced induction.
Why does the combustion chamber oxygen partial pressure change as a function of engine speed? This is because in the real world the fluid mechanics of gas (oxygen) flow matter greatly for establishing the actual amount of oxygen that got into the cylinders during the brief time the intake valves were open. Look at your torque curve....it is actually the curve of the efficiency with which your cylinders fill with air, as a function of engine speed. This is also called volumetric efficiency.
Volumetric efficiency depends greatly on intake geometry, exhaust geometry, intake and exhuast PORT geometry, intake manifold geometry and exhuast manifold geometry. It also depends on cam shaft timing and valve timing (hence will change with MIVEC or different cam shafts or cam gear settings).
How volumetric efficiency varies with port geometry is what Triny was saying above. In his language, a "torquey engine" means an engine with the torque (volumetric efficiency) peak at low engine speeds. Because the torque peak occurs at low engine speeds, there is no point in a high redline, so you better have big displacement to give you torque at the crank since you cant depend on short gears to give you that torque boost at the wheels.
A "high horsepower engine" means an engine with a torque peak (volumetric efficiency peak) ocurring at high(er) engine speeds. Thus you can get away with smaller displacement, and a small value of the peak torque, because you can use short gears to boost your smaller crank torque. You can use shorter gears because your useful redline is higher. Why a high engine speed torque peak is called high horsepower i'll explain below.
I'll summarize the important point one more time: to increase HORSEPOWER you sacrifice your peak torque height (at midrange) in order to improve your redline torque. Your peak hasn't moved to redline, although it has moved up some, but your torque near redline is better. Why does this increase horsepower? I will explain below.
Okay, torque torque torque. What about horsepower. Here ya go.
Horsepower is torque times engine speed. Thats it. Thats all you need to know. HP = Torque X RPMS.
From this simple fact follows many things:
1) Peak Horsepower occurs closer to redline than peak torque because as engine speed goes up horsepower goes up proportionally, independent of torque. Hence, in order to optimize horsepower usually you sacrifice your midrange torque peak in order to bolster your near redline torque. It usually turns out that Peak Torque X 4000 RPMS < Redline Torque X 7000 RPMS. You can think about this.
3) A high horsepower engine, everything else being equal, has a higher redline. The max engine speed is high.
2) A high horsepower engine, everything else being equal, pulls hard because you can get away with SHORTER GEARING at HIGHER SPEEDS. Thats it. Thats why a high horsepower engine provides good acceleration. No other good reason.
3) A high horsepower engine with tall gears is slow(er).
5) Maximum velocity goes up with horsepower (not torque), everything else being equal. Of course, it might take you forever to get there because your torque (acceleration) sucks but...
By the way, I'm assuming you understand how your gear ratios act as a torque multiplier at the wheels.
My last point is: what makes a car pull hard is torque. Thats it. The only reason high horsepower makes your car pull hard is because it lets you stay in lower gear for a longer time (low gears provide more torque at the wheels). Thats it. EVERTHING ELSE BEING EQUAL.