Factory HP ratings

[derf]

One of the things a lot of people have trouble with is the difference between horsepower and torque.

Sure, our AMC engines were rated for pretty low horsepower numbers but they have a bunch of torque where it's useful. And peak numbers, especially horsepower, are about as useful as a Kardashian. They're only there for show and have no other use.

Why are peak HP numbers useless? Because horsepower has nothing to do with the ability of an engine to accelerate a vehicle. To determine that, you need to know the torque. And not just the peak torque but the torque available across the RPM range.

The way to convert between torque and horsepower is with a simple equation:

Torque × Engine RPM ÷ 5252 = Horsepower

So, at 1500 RPM an engine making 300 ft-lbs of torque would be making 300 × 1500 ÷ 5252 = 86 HP.

But a different engine that makes the same 300 ft-lbs at 6,000 RPM would be making 300 × 6000 ÷ 5252 = 343 HP.

So you can see that as you get an engine to make power at higher RPMs (usually by sacrificing lower RPM torque) you can raise the horsepower number while still sacrificing real power.

Take the V8 engines in the 60's and 70's. Look at the torque curve of those engines. They make most of their torque from off idle through about 4,000 to 4,5000 RPM and they falling off before that. But one of those engines making 300 ft-lbs at 3,500 RPM gives you 300 × 3500 ÷ 5252 = 200 HP. And that looks like a pretty small number. But it's pulling like a train with that 300 ft-lbs all the way until it drops off.

Now look at the Pentastar V6 that's in my 2012 Wrangler. Sure, it makes 285 HP. But it doesn't make that horsepower until it reaches 6,400 RPM. And at that point 285 HP × 5252 ÷ 6400 RPM = 234 ft-lbs. That's a stupid high RPM to be running no matter what conditions you're driving in. Peak torque is only 260 and it has to get all the way to 4,800 RPM to make that. Who pushes their engine to nearly 5 grand when driving around town? I sure as heck don't. And while you're still pushing around 250 ft-lbs between 2,000 and 4,800 peak, the performance gets really bad the lower you go. There's a huge drop off in torque as you get below 2,000 RPM. You're well below 200 ft-lbs and falling fast when you're as low as 1,200 RPM. But that's exactly the RPM range you need to be making torque if you want an engine to push a heavy vehicle down the road.

Sure, you can make up for it with an 8 speed transmission with a really low first gear and a high stall torque converter to get your RPMS up at low speed, but you're still compromising. My manual transmission has a 4.46:1 first gear and it struggles to get moving from a stop. Once it reaches a few miles an hour and the engine RPM gets up to about 2,000 it finally starts acting like it wants to move. But I have to deal with that sluggish start every time I get moving. Beyond that, I have to push 2,500-3,000 RPM on the highway just to maintain highway speeds on a mild grade. I have to downshift from 6th to 4th or even 3rd, to maintain 75 up I-70 to the Eisenhower tunnel. That puts me well above 4,000 RPM just to maintain speed. A torquey V8 can do the same thing at a much lower RPM even though it has 1/2 the peak horsepower.

With that 285 hp/260 lb-ft of torque, I still find it nearly impossible to rock crawl in my Jeep. Even in first gear/low range I often times either stall or go bouncing over an obstacle because the off-idle torque in the engine isn't sufficient to get my Jeep moving. A 4.0 straight 6 with the same transmission and similar transfer case as mine with the same axle gears and tires has a lot less trouble with stalling because the 4.0 makes a ton of torque right off idle. But it's horsepower numbers are way lower than my Pentastar. I find myself in the position of installing a 4 speed Atlas transfer case just so I can get to stupid low gearing to make up for the lack of low end torque in my engine.


So what is the practical upshot of all this?

Simple. Horsepower is a meaningless measurement of an engine, especially peak horsepower. And even peak torque is meaningless. To really evaluate an engine you need to look at the torque curve over the entire RPM range to really evaluate the worth of an engine for its intended task.

[Nikkormat]

So what is the practical upshot of all this?

Simple. Horsepower is a meaningless measurement of an engine, especially peak horsepower. And even peak torque is meaningless. To really evaluate an engine you need to look at the torque curve over the entire RPM range to really evaluate the worth of an engine for its intended task.

This man speaks the truth.

[Nikkormat]

The trick I've found to Eisenhower in the vintage automobile is to hit the bottom of the grade as fast as you dare, and keep speed as far as you can.

Last time I had to climb it I was doing great, within shouting distance of the top, and the guy in the lane next to me decides he wants to move over into my lane. He's doing 40, I had the Jeep wound up to 70 foot on the floor. I couldn't get around him and was forced to slow down and downshift to hold 40. The Jeep did not like that on the first batch of fall gas.

Just as I crested the top the Jeep vapor locked hard, I coasted half way through the tunnel in neutral attempting to restart it. I was down to 30 before she finally came back to life. That was a tense ride!

[eggman918]

I second that torque down low makes all the difference in the world I replaced the 390 4v an engine known for it's torque in my '68 F-250 that was built as a tow motor it was ~325hp/435ft-lbs with a 4BT ~250 cubic inch it is ~200/450 so just by the numbers should be no comparison yet the 4BT will out work the 390 in every way the difference is the 4BT makes it's 450ft/lbs @ ~1800 rpm and well over 200 right off idle to get the 4BT to work well took totally different gearing in both the transmission and the final drive ratio as it's happiest between 1500 rpm and 2000 rpm.
The 4BT also does this using half the fuel the 390 did!
Carroll Shelby said it best "horse power sells cars but torque wins races"

[SJTD]

So if I tell you I have this motor that makes 500 ft-lb what's the first thing you're going to ask me about it?

[eggman918]

At what RPM does it make it at.
The new OTR trucks are cruising at 1,100/1,200 rpm and are netting 8-9 mpg compared to 5 or 6 years ago where they were they were turning 1,500/1,600 and getting 5-6 mpg.

[derf]

So if I tell you I have this motor that makes 500 ft-lb what's the first thing you're going to ask me about it?

The three most important questions are:

What does the torque curve look like?

What type of vehicle is it going in?

How is the vehicle going to be used?

[FSJunkie]

For 1/2 ton trucks the F150 EcoBoost does amazing. You aren't right about any vehicle being down on power at that altitude. A properly done turbo will still reach max boost meaning you lose no power at all.

That's interesting. I always had it in the back of my mind that turbo engines might have a way of doing that, I just didn't know for sure. They probably have some way of controlling turbine RPM and crank it up at higher altitude to compensate for less atmospheric pressure, because otherwise boost and power would drop, with a corresponding drop in exhaust flow and turbine RPM.

Probably variable pitch control on the turbine.

[Nikkormat]

Ecoboost 401 anyone?

[tgreese]

Well, torque is as meaningful as horsepower. They are simply different quantities.

Torque is measure of force around an axis. It has nothing to do with speed, but instead with acceleration, or equivalently, change in momentum.

Power (of which horsepower is a unit) is simply work done per unit time. You can have tons of torque and make no power. A waterwheel is a good example of lots of torque with little power (because it's moving so slowly).

Torque is what gets you moving, but power is what keeps you moving. You need a certain amount of power to overcome drag (hills, wind resistance, static load, whatever) at a particular speed. Your engine can have lots of torque down low, but unless it makes enough power, you won't be able to top that hill, pull that load, whatever.

So don't discount how meaningful power is, just because you can calculate it using torque. Power includes the rate at which work is being done, or equivalently the energy expended per unit time. Torque is just the force applied at any given instant, and if that torque is completely opposed (ie the axis is not allowed to turn), no work is done. Different.

[Stuka]

For 1/2 ton trucks the F150 EcoBoost does amazing. You aren't right about any vehicle being down on power at that altitude. A properly done turbo will still reach max boost meaning you lose no power at all.

That's interesting. I always had it in the back of my mind that turbo engines might have a way of doing that, I just didn't know for sure. They probably have some way of controlling turbine RPM and crank it up at higher altitude to compensate for less atmospheric pressure, because otherwise boost and power would drop, with a corresponding drop in exhaust flow and turbine RPM.

Probably variable pitch control on the turbine.

Its all based off of manifold pressure. The turbine speed is only controlled by the amount of exhaust gasses going through it. There are turbos that are variable vain so they can adjust how much exhaust gasses are going through the impeller.

A properly done turbo is capable of making more boost that required at lower altitudes, so that when they reach higher altitudes, they can still reach peak boost, or at least get close to it.

Forced induction (be it turbo or super charger) is a major game changer at altitude. This is why the invention of the super charger really changed fighter aircraft that used piston driven propellers. It allowed them to travel at much higher altitudes without having power issues.

[SJTD]

Well, eggman bit outright and asked the horsepower (indirectly), derf nibbled around the edges and asked for a curve, torque vs rpm which is still power.

The point is you need the rpm for a meaningful understanding of the engine's capability. With the torque and rpm you have hp which has some meaning. 350 hp is 350 hp. You know you'll need gearing to do anything with it but you know what you have. like Tim said, 500 ft-lbs is simply a torque.

Of course, in a non stationary application a single hp rating doesn't tell you much. Like derf said, you need the torque curve. I will add OR the hp curve. Doesn't really matter which since either one can be derived from the other.

People like to talk about a motor that has good low end as having torque but the fact is that if it makes good low end torque it makes good low end power; but in derf's example 300 ft-lbs sounds a lot better than 86 hp.


And that 500 ft-lb motor? That was me. I can even put out 1000 ft-lb.

[derf]

Well, eggman bit outright and asked the horsepower (indirectly), derf nibbled around the edges and asked for a curve, torque vs rpm which is still power.

The point is you need the rpm for a meaningful understanding of the engine's capability. With the torque and rpm you have hp which has some meaning. 350 hp is 350 hp. You know you'll need gearing to do anything with it but you know what you have. like Tim said, 500 ft-lbs is simply a torque.

Of course, in a non stationary application a single hp rating doesn't tell you much. Like derf said, you need the torque curve. I will add OR the hp curve. Doesn't really matter which since either one can be derived from the other.

People like to talk about a motor that has good low end as having torque but the fact is that if it makes good low end torque it makes good low end power; but in derf's example 300 ft-lbs sounds a lot better than 86 hp.


And that 500 ft-lb motor? That was me. I can even put out 1000 ft-lb.

By asking for the torque curve, I was asking for the RPM where it makes 500 ft-lbs. Because that number shows up on the curve. But by asking for the whole torque curve, I'm asking for more than just the peak number, I'm asking for the relevant performance information of the engine across the whole RPM range, which is far more important than asking about just one point on the graph.

And while a horsepower curve is just as useful as a torque curve, the torque curve (in my not so humble opinion) is a more obvious graph that gives you the information you need without having to extrapolate it. But to be perfectly honest, it's nearly impossible to get someone to show you a graph that doesn't have both overlaid on top of each other.

Either way, a single point of reference on the graph, be it horsepower or torque, really doesn't tell you anything, which is why I didn't bother asking about that one point of reference.

[Tatsadasayago]

Years ago one of my co-workers built up a Gremlin X with a nasty fire-breathing 401.
He did many strange things to the engine that I felt were counter-intuitive for a street/strip car.
We took the car to the chassis dyno at Yuell Brothers and strapped it in.

The HP peaked at 3800 rpm and astonishingly enough so did the torque. The best measurements at the rear tires after some tuning were 405Hp with 383 Lbs Feet of torque.
I admit I'd never seen an engine do that.
I'd built and raced many fast drag cars and THOUGHT I knew what 400HP to the ground felt like. That Gremlin was a handful at the launch and kept aiming for the sky until he put wheelie bars on it.

It was a lesson about how HP and torque work together.

[FSJunkie]

Torque and horsepower are only the beginning steps to knowing what your engine can accomplish. To really know what your engine can do you need to have both the torque curve and horsepower curve for your engine plus a reasonable background in calculus.
My favorite little measurement that I came up with is something I call "power excursion". This measurement is the integral of the horsepower curve. In pre-calculus terms: the area under the horsepower curve through a certain RPM range. It tells you the total power produced by the engine as it revs once through that RPM range. This is akin to how long it would take you to accelerate from 30 to 60 mph and is useful for comparing engines.

Example:

You are trying to decide on an engine for your Jeep and would like to know which engine will accelerate quicker from 0-40 mph (2000-5000 RPM). Here are the engine choices:

A. A stock AMC 360. 140 peak horsepower.
B. A modified AMC 360. 250 peak horsepower.

Your instant reaction is to think the 250 horsepower engine would be better, but keep in mind the engine won't always be at the RPM it produces that power!
Knowing the horsepower curves for each engine, you could calculate the integral bounded by 2000 and 4000 RPM to find the power excursion through that range. The results may surprise you, for engines with high peak horsepower often drop appreciably at low RPM and the wide change in RPM for this situation will kill the total power excursion for that engine, along with your acceleration. The lower-peaking engine may turn out to be the better choice if it's horsepower curve is flatter.

Graphs, engineering, and calculus

[SemperFIGuy]

This is the best science, math, physics-based class I have ever attended, where were you guys in high school, probably would have been a lot more attentive. Thank you.

Sent from my SM-T230NU using Tapatalk

[SJTD]

I was never good at math. Took all my Calculus classes at least twice but from Thermodynamics (which I also took twice) I think I recall that the integral of power is work?

So yeah, the more area under the power curve the more work as in accelerating a car, climbing a hill, towing a trailer...

[Tatsadasayago]

Great info guys!

[az chip]

If the engine moves the Jeep I am happy. I just pray it doesn't overheat.


1981 Cherokee Chief

[SJTD]

Good point!

Let's talk about high flow vs. low flow coolant pumps now.

Then we can do back pressure.