i think this caveat is valid and is frequently "achieved" with bad tuning. my counter example is more extreme... like when you have all kinds of pressure build up in the cylinder... but you're not releasing it fast due to some reason, torque would be crapped

 

this is the best explanation and most comprehensible i've ever heard, this is why F1 cars can make almost no torque and accelerate fast. Everything engine related is summarized in the statement above, good job i want to put this in my sig!

 

^^^ trinydex, this makes sense from a drag racing perspective.

However, I have always associated torque and horsepower with two entirely different feelings.

IMHO, horsepower feels like something is pulling you faster and faster forward (this is evidenced in a Stage 1 modded Evo by going WOT at like 6000rpm in third - where torque has already dropped off considerably); while torque feels like you just got rear-ended by another car (WOT at like 4000rpm).

 

But it is only one example why exhaust gas velocity is not a reliable indicator of torque.

Think about this: The greater the hp being produced, the greater the air mass being processed with respect to time. The greater the air mass being processed with respect to time, the greater the exhaust gas velocity must be to evacuate the same swept volume in the given time period. This being the case, exhaust gas velocity would more so be loosely linked to hp, not torque, simply because the hp curve (air mass vs. time) will still climb when the torque curve begins to drop.

 

i think torque has much to do with amount of air going inside the cylinders, i read in a tech book that this is directly related. The reason why torque always drops at top is because the intake valves close to soon at high rmps and not enough air enters the combustion chamber. That is why most stock cars torque peaks at low rpm

 

Torque mirrors VE (volumetric efficiency), which is the amount of air going into each cylinder per stroke. Generally speaking, VE is highest at the torque peak, which means the engine is ingesting the most air mass per stroke at the torque peak.

 

Maybe I'm way off the mark here but given the formula posted ( torque = change in momentum / change in time). Doesn't the length of the rod come into play here? A shorter rod would lessen the time it take to change the momentum, No?.

 

Ok, I had to go back and digest things a bit, and at the risk of causing confusion (or even confusing myself), let me give myself a bit of clarification and do it so that everyone can see what's happening . . .

The rate of acceleration does indeed mirror the torque curve (why one feels like getting rear ended at the torque peak). However, when considering acceleration, torque is meaningless without being able to attach the time factor (rpm) to it, which is where hp enters the picture (and after all, hp is a function of torque and rpm). In short, the higher in the rpm range we make peak torque, the greater hp we make, and we can take better advantage of gearing to increase the rate of acceleration.

If we measure the rate of acceleration of a normally aspirated car (eliminates the turbo fudge factor) in 2nd gear, then again in 3rd gear, we find the rate of acceleration in 3rd gear is less. Why? Because in 3rd gear, there is a greater change in speed for each change in rpm. This means we have to accelerate the car by a greater amount of mph with the same amount of power, so the rate of acceleration slows.

This being the case, generating the same amount of torque at higher rpm (more hp) allows us to better take advantage of gearing and accelerate the car faster. F1 cars take this to extreme because being capable of such high rpm, the gearing is very low. This means that the rate of acceleration is very high as compared to a car generating the same power, but at lower rpm (and therefore with much higher gearing).

Ok, I feel better now!

 

^ the more brief and generalizing view of this is that you still need to produce the WORK to make car go. low torque per rev means you need lots of revs to produce a large movement in car.

big torque per rev means you can do the same movement with less rev.

as it works out more torque usually means less revs anyway and higher revs usually means more power anyway.

 

I'll try to make an analogy.

Torque is like how much weight you can lift at one time. Power is the total amount of weight you've lifted over a set period of time.

So say you have this huge body builder that can bench press 500lbs at once and a 'normal' guy can only do 200lbs. So the body builder has a lot more torque. But that body builder can only bench press the 500lbs once every 5 seconds. The normal guy can bench press the 200lbs three times in 5 seconds. So, the normal guy has produced more power/done more work by lifting a total of 600lbs in 5 seconds vs. the big guy doing 500lbs.

torque, hp, rpm, same difference. big displacement, long stroke motors make a lot of torque but can't rev that high. Little motor with short stroke has low torque but can rev like crazy. And then you add the turbo which is like steriods