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WHy does the torque on a gas engine go DOWN--as the HP goes UP?

R

rca2000

Super Member
I know there are a lot of car guys here (and probably a few women too--but I'll include that in my term "guys" ) who know a LOT about engines and such.

I am again in possession of a car that has what I consider about the BEST engine EVER produced in Detroit--at least--the best V-6 engine. A 1989 Olds Trofeo--with the 3800V-6. I REALLY do think this is the best V-6 engine-EVER from ANY company. Not the most powerful--(but one CAN get a LOT of power from a 3800 with a bit of tweaking)-- and not the most Hi-tech--but STILL a relatively modern--computer-managed engine with PFI and no distributor. But the 3800 may just be about the most RELIABLE, DURABLE, and LOWEST MAINTENANCE engine out there!! Smooth--competently powerful and VERY thrifty--I get OVER 30MPG on the highway--and about 22-24 around town. And with about 112k--this engine does NOT use more than about 1/2 quart of oil--between changes--MUCH better than my 2001 Chrysler minivan.

ONE thing about this engine puzzles me--and this goes for most others too. In this engine--the "Torque' seems to peak about 2000-3000RPM--according to reports--and the HP peaks a a LOT higher RPMS. Most gas engines seems to be like this. And the ONE somewhat disappointing thing about the 1989 3800 is--that above about 3k or so--there is NOT much "left". And so...above about 80 or so...acceleration is NOT too good...but i do NOT go that fast much anyway....so it is not an issue for me. passing and take-off on this engine are VERY pleasing.

NOW--It would seem to me...that the torque would peak at the HIGHEST RPM--as would the HP-- since the pistons would be moving faster and hence--more power---it would SEEM to be that way anyway. But they do NOT "follow each other", much at all.

WHY is this the case??
 
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It's all in the definitions.

Horsepower = (rpm x torque)/5252

Torque = (5252 x horsepower)/rpm

So, as rpm goes up, horsepower goes up (rpm in the numerator of the first formula), and torque goes down (rpm in the denominator of the second formula).

The formulas also show why horsepower always equals torque (in ft/lbs) at 5252 rpm. ;)

Read here: http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=467

Good book for ya: http://www.amazon.com/Auto-Math-Handbook-John-Lawlor/dp/1557880204
 
rca2000 said:
I know there are a lot of car guys here (and probably a few women too--but I'll include that in my term "guys" ) who know a LOT about engines and such.

I am again in possession of a car that has what I consider about the BEST engine EVER produced in Detroit--at least--the best V-6 engine. A 1989 Olds Trofeo--with the 3800V-6. I REALLY do think this is the best V-6 engine-EVER from ANY company. Not the most powerful--(but one CAN get a LOT of power from a 3800 with a bit of tweaking)-- and not the most Hi-tech--but STILL a relatively modern--computer-managed engine with PFI and no distributor. But the 3800 may just be about the most RELIABLE, DURABLE, and LOWEST MAINTENANCE engine out there!! Smooth--competently powerful and VERY thrifty--I get OVER 30MPG on the highway--and about 22-24 around town. And with about 112k--this engine does NOT use more than about 1/2 quart of oil--between changes--MUCH better than my 2001 Chrysler minivan.

ONE thing about this engine puzzles me--and this goes for most others too. In this engine--the "Torque' seems to peak about 2000-3000RPM--according to reports--and the HP peaks a a LOT higher RPMS. Most gas engines seems to be like this. And the ONE somewhat disappointing thing about the 1989 3800 is--that above about 3k or so--there is NOT much "left". And so...above about 80 or so...acceleration is NOT too good...but i do NOT go that fast much anyway....so it is not an issue for me. passing and take-off on this engine are VERY pleasing.

NOW--It would seem to me...that the torque would peak at the HIGHEST RPM--as would the HP-- since the pistons would be moving faster and hence--more power---it would SEEM to be that way anyway. But they do NOT "follow each other", much at all.

WHY is this the case??
Click to expand...

Horsepower is a function of torque, horsepower is measured over time. Think of torque as the power required to turn something, such as loosen a lug nut, and hp as the rate at which you can spin it over time.

Torque gets you off the line, hp gets you to the end of the straight.

Best V6 ever goes dead over 3k rpm? That would disqualify it in my book. ;) My Alfa V6 motors come alive at 3k rpm!

Best V6 ever, imo, is the original production V6, found in the Lancia Aurelia, early/mid 50's. I've never driven anything as smooth and vibration free as one of those. Unfortunately, investors have realized the significance of that car, and nobody can afford them anymore.

My .02,

bs
 
1HP = 550ft/lbs per second of torque. This can be measured on the dyno, and horsepower calculated from that using the formula I posted above.
 
Here's why.
First, what is torque? Torque is a twisting moment, defined as a force acting through a moment arm... lbs x feet (lb-ft). So when you turn a wrench, your arm provides a force and the length of the wrench provides the moment arm. Need more torque to break that bolt loose? You're only so strong, so get a longer wrench. Same force, longer wrench = more torque.

Note that torque has nothing to do with actually accomplishing anything. When you apply a torque to a fastener, you aren't accomplishing anything until the fastener starts to move. When it starts to move, time comes into the picture. If you move the fastener slowly, you are using little power. Move it fast, you are using more power. This is where the equation for power and torque comes into play... HP = Torque x RPM / 5252.

In an engine, torque is created by the force of the expanding combustion gasses pushing down on the piston, and that force is transmitted to the crankshaft through the offset of the crank pin to create torque. Long stroke engines have longer crank throws and are usually therefore "torquier" engines.

Engines can't do anything unless they are turning at some RPM. This is because in order to combust the fuel and get that force going on the piston, they first need to draw in the air and fuel, compress it, ignite it, get the power out of it, then exhaust the spent gas and start the process all over again. This requires the engine to turn - thus, RPM and that brings time into the picture. Also, as it turns there are losses involved with drawing the air in, pushing the exhaust out (together called "breathing,") as well as the friction of the pistons going up and down, the bearings, etc.

So, since engines need to spin to operate, they really can't produce "torque" independently. They produce power because the amount of fuel they can burn is dependent upon the speed they are turning.

Looking back at the equation for torque and power, we can see that if an engine could produce constant torque from idle to maximum RPM, the torque would stay the same and the power would just continue to increase with speed. Why can't an engine produce constant torque? Two main reasons.

1. As the engine's speed increases, so does the friction of the sliding parts. The friction increases roughly as the square of the speed, so the faster the engine goes, the more friction the engine has to overcome and it gets worse and worse as the engine goes faster. Even if the expanding gas on the piston was the same across the speed range, the friction as the engine goes faster subtracts from the torque the gas produces.
2. As the engine speed increases, the intake and exhaust systems (and valves) need to flow more gas. The size of the intake and exhaust piping and valves will become more and more restrictive as the speed increases. As the restriction begins to limit the amount of air the engine breaths, it will also limit the amount of fuel that can be burned efficiently. Less burning = less force on the piston = less torque. Cam timing and manifold design can take advantage of acoustic phenomena to make the engine breathe better over a certain speed range, and this is part of why one engine might have a torque peak at 2500 RPM while a different engine might peak at 3000 RPM. Nonetheless, at some speed all engines begin to run out of breath and this is where the torque really starts to decline rapidly.

Here's the crux of the answer to the question: As the engine's speed increases, its torque starts to decline but the increasing speed is still increasing the power faster than the decreasing torque decreases the power. At some point the torque decline matches the speed increase and the power curve flattens out. Sometimes this can result in an engine that produces peak power over a pretty wide speed range... this can be very desirable in some applications like over the road trucks. Finally, when the breathing and friction effects drag down the torque so fast that increasing engine speed can't compensate, the power starts to drop as well.

Why did I say that stuff about engines not really producing torque? Since they can only produce torque by operating at some RPM, they can only truly produce power. At any given power you can have whatever torque you want by using a gearbox. Just like you can use a longer wrench to get more torque with the same exertion from your arm, you can use a gearbox to get more torque from the same engine power. If all you want to do is move a weight but you don't care how fast, just get some tiny little engine, gear it way, way down to get gobs of torque and you can move anything. The motor in a wristwatch could theoretically be geared down enough to lift the Empire State Building. It would lift it very slowly, but it would lift it. If you want to move a weight fast (like getting that Chevelle down the track really fast) you need a lot of POWER. You use the gearing to keep the engine in its power band, and when the car is just getting started all that power is geared down so low the torque at the rear wheels is huge. As the car goes faster and faster, the gears are shifted and less and less torque goes to the wheels even though the power is the same... because there is less mechanical advantage being generated by the gearing (analogous to the length of the wrench). The best performance will be obtained by a system that can keep the engine operating close to peak power at all times. This can be accomplished by either an engine with a very flat power curve and few gears or a peakier engine with more gears. That's pretty much the difference in philosophy between a big American V-8 with a three speed automatic and a 2 liter European engine with a six speed gearbox. Both do the job, but they do it differently.
 
Well said. Engineering terms get slaughtered all the time.

Now, let's have a discussion on the difference between weight and mass in the US measurement system...
 
It's like spinning a merry go round. It takes a lot more "push" or torque when you first start spinning it. Once it starts spinning its harder to apply more "push" because it's already moving. The faster it spins the less "push" or torque you can apply becomes harder and harder.

This is the same as an engine that is increasing in rpm.

Now if you had an engine that had a longer stroke (which would also include longer rods), the easier it would be to keep the "push" or torque going at a higher rpm.
 
jocko_nc said:
Now, let's have a discussion on the difference between weight and mass in the US measurement system...
Click to expand...

Oh boy... I went to engineering school when it seemed the US was going to switch to metric, so most of my education was in the metric system. I still get totally confused by the English system's use of lbf, lbm, slugs, and when to use the gravitational constant to match units. Kg, m, s, N, Pa... all make much more sense to me. Often I'll work a problem in metric and covert back to english at the end to avoid making mistakes.
 
When you get up in the morning to go to the bathroom and have to push your pecker down to take a leak, when the heels of your feet come off the floor - THAT'S torque.
 
DENNYDOG said:
It's like spinning a merry go round. It takes a lot more "push" or torque when you first start spinning it. Once it starts spinning its harder to apply more "push" because it's already moving. The faster it spins the less "push" or torque you can apply becomes harder and harder.

This is the same as an engine that is increasing in rpm.

Now if you had an engine that had a longer stroke (which would also include longer rods), the easier it would be to keep the "push" or torque going at a higher rpm.
Click to expand...

There are two effects. One is the force needed to overcome inertia... that relates to the power required to accelerate the merry go round. The second is the force required to overcome friction... that relates to the power required to keep it spinning at a given RPM.

Coming from rest, the RPM is increasing from zero, so there is very little power actually required to accelerate the merry go round. There may be a lot of static friction to overcome to get it turning, but once it's moving even a little bit if constant power is applied the acceleration will be highest at lowest merry go round speed. In real life, the motor is probably attached to the merry go round in such a way that the motor cannot provide full power until the merry go round is near its maximum speed. This is a good thing as a huge initial acceleration would be uncomfortable for the kiddies!

Once it's spinning it will take more power to continue to accelerate it. If the merry go round has constant friction in its bearings, it will take twice as much horsepower to push it at 10 rpm than it does at 5 rpm. And actually, a lot of the friction effects increase with speed. So that's why at some point you will not be able to push it any faster... your power will equal the power required to overcome friction at that RPM and that will be the top speed possible.
 
I was baiting someone to use the word "slug", one of my favorites...

I find it easier when discussing in the English system to not even mention mass. Assume they mean weight and divide by 386 in/sec^2.
 
DENNYDOG said:
It's like spinning a merry go round. It takes a lot more "push" or torque when you first start spinning it. Once it starts spinning its harder to apply more "push" because it's already moving. The faster it spins the less "push" or torque you can apply becomes harder and harder.
Click to expand...

The effort required to accelerate something spinning depends upon the mass and how the mass is distributed around the axis of rotation. Mass moment of inertia is constant for a given structure. Friction, drag, etc. at velocity will determine the top speed, as long as the structure holds together. Top speed is where the sum of losses equals the torque being applied. In the case of an engine, the torque varies with velocity. There is some point where the engine or vehicle will do no more.

In Dynamics all movement is broken down into two components, linear and rotational displacement. Most people inherently understand the linear relationships, the rotational are more abstract but analogous.

Force correlates to Torque.
Velocity correlates to Angular Velocity
Acceleration correlates to Angular Acceleration
Mass correlates to Mass Moment of Intertia

After that, all the usual relationships are similar.
 
I kinda guess it is like audio power then. and BTW THANKS for the very thorough , detailed explanations.

To get to the 80DB SPL (with say, efficient 94DB/W speakers), will NOT take NEARLY as much power as to get to 90DB SPL--and to get to the 110DB level will take MUCH more power...and so on. As they say--that "first watt" makes ALL of the difference". So...with engines--I guess it is the first 30--50MPH or so--that could be equated in the same way. To that point...it does NOT take NEARLY as much energy--as it THEN takes to get from 50-80 and then from 80-120MPH.

I guess the old law of "diminishing returns" comes into play--JUST like with Audio power!!
 
Tarl Of Gor said:
When you get up in the morning to go to the bathroom and have to push your pecker down to take a leak, when the heels of your feet come off the floor - THAT'S torque.
Click to expand...

That's why a longer stroke usually results in more torque.

Detonation can be a problem, though.
 
You need to try out a Dodge 3.9L V6 the cut down 318 V8. The one in my Dakota pickup pulls like a Mofo through 4500+ RPM (I try not to go above 4500rpm) A shift in second to third at 4500rpm will demonstrate plenty of raw torque.,
 
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