Characteristics of high v. low torque given the same HP?

Here is my best shot.
Horsepower is a calculation. It in itself is not measured.
HP is torque x RPM Divided by 5252.

So without corrections for atmospheric conditions it should be.

400 Ft Lbs of torque at 5252 RPM is 400 HP.

It is save to say, I think, more displacement = more torque. HP falls where it will.

Less displacement gets less torque, but smaller displacement is easier to turn more RPM with, so more HP and in a higher RPM range.

Just remember, torque and RPM can be measured. HP is just a calculation.

 

Simplistic explanation: hp = torque X rpm

You want both (torque and hp) for best driving experience.

Great example is high-torque diesel vs Ferrari of same maximum torque output (without regard to rpm), which engine would you rather have in your racecar?

The Corvette engine "tune" makes that car easier to use all the power, low rpm, high rpm, just choose a gear and drive.

Best,
Rob

Sent from my Nexus 5 using Tapatalk

 

High rpm torque allows you to make the best use of gearing.

 

given the sheer amount of variables there is no hard and fast rule for displacement and TQ curves.

cam timing, head flow, rod/stroke ratio, number of valves, bore dia, and on and on etc..

Hp is calculated as noted above, we measure Tq. force over time.

given two engines of similar power the one with more hp farther up the rpm range will be faster. since hp is calc'd from tq over time the engine that develop more tq at higher rpms can develop more power and thus move the vehicle faster.

 

for the same HP, high torque=low rpm and low torque=high rpm. That's the difference.

In terms of outright speed torque by itself is meaningless, average HP will win (torque with rpm).

 

Good questions,
the low tq engine is going to have initial acceleration that is greater then one that doesn't have that low tq curve, but it will fall off and require a gear change to keep acceleration constant. the engine that develops it's peak tq farther up the rpm scale will not have to shift as soon and thus can wait to shift. utilizing the tq curve efficiently is key. less gear changes means less gears needed in a gear box thus less weight to carry.

Hp per fixed displacement is not a really useful metric either, the dynamic displacement is more useful as that is related to efficiency.

 

As mentioned previously, its really about application ie low RPM diesel in a truck that's towing a heavy load makes tons of torque and doesn't need crazy horsepower.

I've always thought the Mazda rotary engine was an interesting example of this trade off. It seems to me that a rotary is great for a race application where you keep the engine spinning most of the time and rarely accelerate from a dead stop. On the street, I think they're wanting for more torque since there is so much stop and go in a street car. I will say though, that I've never driven one so I could be wrong, this is just my understanding from reading.

 

Here is a simplistic, qualitative way to look at it.....

Torque (at any RPM) is a function of how much force is available to drive the piston (down on the power stroke) --- stroke length, compression ratio, combustion quality & quantity (size & duration), cylinder volume, piston diameter, etc. all influence the amount of force generated --- the better / bigger the "bang" on the piston, the more force produced. Also, for a given piston force, the longer the effective length of crank arm / connecting rod link geometry, the more torque produced on the crankshaft --- resulting from greater leverage.

Power is a function of the rate at which the torque is applied --- force / torque over time. For a given torque, the faster it is applied (RPM), the greater the power generated.

Also, it is important to note that with internal combustion engines, neither peak power nor peak torque, typically occur at maximum allowable RPM. And, peak power and peak torque generally do not occur at the same value of RPM, either. This is due to the many and complex variances in fluid flow and combustion characteristics that change dynamically throughout the RPM range. It is an inherent phenomenon in any piston / valve / inflow / outflow arrangement.

 

Torque is how big a bucket of water I can carry from the well to the house (Work or force)

HP is how many of those buckets I can carry from the well to the house in an hour (Work done in a given period of time)


Lots of torque makes a great car to drive on the street and a requirement for trucks, trailer towing etc.

Trading torque for a wider powerband is usually preferred for high performance and racing cars.

 

great analogy

 

There are a number of differences, but in general high TQ equal HP wins all racing situations. This was first proven at leMans by the Ford GT40.
Details:
1) a car accelerates according to the TQ curve, the Transmission and differential ratios, and the rolling radius of the driven wheels divided by the mass of the vehicle.
2) A high TQ equal HP motor has more area under the TQ and HP curves, and therefore acceleates faster.
3) it is easier to control a low TQ high HP motor in a turn beacues the acceleration is essentially constant (weight transfer is essentially constant).
4) if is harder to control a high TQ equal HP motor because the weight transfer is constantly changing.

 

You're close, and what you say about an engine with lots of torque being easy to drive is certainly correct. But your physics terms are a bit off the mark as is your comment on the power band of a racing engine.

At the risk of picking nits, saying "Work or Force" implies that they're two flavors of the same thing; they aren't. In simple terms, work happens when you apply a force on an object and that object moves some distance. When you take into account time, then you can calculate power which is the rate of doing work. In other words, producing Force means you're pushing (or pulling) on something, if it moves you're producing work, and the faster you move it, the more power you're producing.

If by "powerband" you mean the the range of operating speeds under which the engine or motor is able to operate efficiently (that's the definition of "power band", by the way), then racing engines typically have narrower power bands than street engines, not wider.

Consider this: When a racing class is restricted by displacement and nearly no other restrictions except perhaps "No turbo, runs on gasoline", then you get high RPM engines with narrow (sometimes incredibly narrow) power bands especially when money is no object. In other words, engine designers typically trade off torque in search of higher RPM in a quest to produce more power and this, in turn, leads to a narrow power band shifted toward the high end of the RPM envelope while also sacrificing driveability/rideability. But horse power at high RPM always rules in the end when there are few rules restrictions assuming you have plenty of money.

For example, in 1966 Honda built a 125cc, five cylinder (35mm bore), four stroke, four-valve-per-cyl, motorcycle engine. It produced 35 hp at 20,500 rpm but it was so peaky that it would hardly run below 17,000 rpm. It had a 9 speed gear box so that the rider could keep the rpm in the narrow power band as much as possible.

About the same time Suzuki made the even more extreme RP68 (prototype only) which was a 3 cylinder 50cc engine (28mm bore) producing an astounding 19 hp but so narrow was the power band that the bike was fitted with a 16 speed gear box! Keeping this engine "on the pipe" as they say, must have been a real struggle. Imagine keeping track of 16 gears.

Of course, that was half a century ago. Now there are plenty of rules, including RPM limits in most racing classes, and money is always an object these days. Nevertheless, racing engine designers are nearly always willing to accept a narrower power band, shoot for the highest allowed RPM, accept less peak torque, and live with less driveability. When rules makers start restricting the number of gears, then the engine guys shift their thinking the other way a little bit.

To answer the OP's question I'll say this. Engines with lots of torque are easy to drive and good for towing campers (think Dodge Ram Pickup, 800 lbf-ft torque). Engines producing the same hp but with less torque at higher RPM are a lot more fun to drive (think Ferrari 360, 275 lbf-ft torque).