P90 vs. P79 Cylinder Head for NA 3.1L

[JMortensen]

With a lot of duration you are pushing the charge out the exhaust at low rpm, but it works better at higher rpm; it's a trade off. With a small cam, you are keeping it in the cylinder at low rpm, but it doesn't work very well at higher rpm. If you read online you'll find people that say that you need torque, and people that say you need hp. For my money with an L6, having run both, I'll take the hp. If you go for torque, there just isn't much hp to be had. If you're talking about a V8, it's a different story, but even there it's not a clear decision.

 

Basically you have my previous post right, but I'll rephrase for clarity: If you want to go down to the local gas station and fill up, don't get too zippy on the compression ratio. If you don't mind paying for race gas then go for the compression. If you gave me a choice of the same motor built with 9.5:1 compression that was known to run on pump gas with the timing optimized or the same motor built with 11:1 compression where I knew I had to at least mix race gas to make it run with the timing optimized, I'd go for the lower compression motor. E85 might be a solution, I don't know I haven't used it. I think if you used high octane gasoline you'd get more power than E85 just because it has more energy per gallon, but E85 might be a way to build an 11:1 motor and not detonate the crap out of it trying to get the timing to where it makes the most hp. How that compares to the 9.5:1 motor, I really don't know, but I would suspect that E85 makes a bigger difference on a forced induction motor than it does on an NA engine.

 

Heavier pp was referring to the ACT pressure plate. The point there was that I had a combo that everyone says would be absolutely horrible on the street: big(ger) cam, 44 triples, heavy clutch, big exhaust, and it really was easy to drive in traffic. I never dynoed my engine so I don't have any numbers to give you for comparison. I always thought the money that I could spend at the dyno would be better spent on some new suspension mod.

[Gollum]

Bah, the age old "hp vs torque" debate. I know Jon M probably knows all what I'm going to share, so nobody take this as me pitting him against the wall in a corner, just me sharing what I can while the topic's come up.

 

 

HP, is impossible to DIRECTLY measure, but is in actuality directly CALCULATED. What I mean is that HP is just an EQUATION, not a force.

 

HP is how much work can get done.

 

RPM is how fast work load moves.

 

Torque is the base multiple factor.

 

Here's an analogy to put into your mind before I really get going here. Think of two huge wood stacks that need to get moved. There's two guys that are going to move their stacks. They both have identical wood stacks that need to go identical distances. One guy can move 10 pieces at a time and takes 60 seconds to move those 10 pieces from one place to the next. The second guy can only take 8 at a time, but only takes 45 seconds to move those 8. This means that the first guy can move 10 a minute, or 600 an hour. The second guy can move 10.4 a minute, or 624 an hour.

 

In this analogy, the load that can be taken is like the torque abilities of an engine. This is what we use as the foundation of what work can be accomplished. With a measurement here of 0, no work would ever get done. It doesn't matter how fast a guy can run from one pile of wood to the next if he doesn't carry any wood with him.

 

How fast they move that wood is like the RPM of an engine. The first guy taking 10 would move a lot more if he could just squeeze his time down a few seconds, or increase his RPM. Notice that this measurement is 100% time based.

 

So in this analogy you can see that we can't physically see or measure how much work will get done, we have to CALCULATE it in order to figure out the outcome. This is what makes HP such an elusive object and so impossible to define with our senses. Our nerves/feelings will never be able to tell us what HP is. We can only feel force and perceive how fast something might be happening.

 

 

 

So onto the real meat of the matter. The cold HARD fact is that if we only ever used engines at ONE RPM all the time (meaning a 100% continually variable transmission) the only thing that would matter would be PEAK HP, how much torque and RPM we can extract for one small moment of the RPM RANGE. But the reality is that no engine really gets to see these circumstances except maybe in industrial application like generators. But in cars it's just not a reality. In our actual reality we find that there is no "magic engine" that's perfect for all application. We have huge torque monster diesels for 18 wheelers, and we have little 3 cylinder metros that get 50mpg. But when it comes to racing....

 

It's all about AVERAGE USABLE HORSEPOWER (at least that's what I call it). Some people might also think of this as "area under the curve" which is much more vague I think. Average usable HP is the average of horsepower that you get when you cut to top of the HP curve on the dynograph like a knife scrapping the top of the mound of butter. What's the average you get when you take a given range of RPM. A motor that is very "peaky" as they can be called, is one that usually climbs HP until redline and is very linear. It's usually called "peaky" because you need to be at the top of the RPM range for the motor to be worth anything to you. As a general rule of thumb, high RPM motors tend to be more peaky simply for the sake that it's harder to get a cylinder head and valvetrain to breathe extremely well at the bottom and middle of the RPM range. There's many more factors, but this is a big one. Then we have "torquey" motors. What I find most comical about motors described as torque monsters is that many times they actually have very little torque for their displacement, and have very miserable horsepower for their size as well. But they're known as "powerful" because of how they "feel" and sometimes it can be a tough call. But now we'll get into my favorite motor comparison which I think many will find enlightening and hits the core of why this was brought up to begin with.

 

Okay, two motors will be discussed here. The Ford pushrod 5.0, which was rated at 225hp, and the Honda F20C rated at 240HP. Going back to our wood stacker's analogy, the Honda motor here would be better, because it's getting more work done. It might have less torque, but it has RPM that makes up for it, meaning it has more peak HP. But in the real world it gets much more complicated than that. In reality those engines have to be behind transmissions, which transfer power to the ground. And a key in this to remember is that gears in transmissions are always RATIOS, so a gear change will never have a RPM difference, but a RATIO difference. This is why at redline going from one gear to the next might move your engine down 3,000 or more RPM, but yet you can make that same gear change at only 2,000 to begin with and it only drops a little bit. It's a ratio. Keep that in mind here.

 

So make this a drag race with two identical cars and transmissions. Both cars take off. The 5.0 gets off the line better thanks to it's much higher torque threshold meaning it has a lot more HP down in the lower RPM range where the cars launched at. But this lead doesn't last forever, the 5.0 has to shift much sooner from 1st to 2nd while the honda keeps climbing, not even into it's power band yet really. Now the Honda is really coming alive as it nears redline, but then it has it shift into 2nd, and the 5.0 just walks past it since it's already back near it's peak HP, while the honda is way bellow. This story continues but the 5.0 will increase it's lead consistently as the race goes on.

 

Now, why does the 5.0 win when it has less HP? Is it because it has more torque? No actually. Not at all. It's because it has a FLATTER HP curve that shows up at LOWER RPM. Let me explain this too, if both motors had JUST AS FLAT of HP curves, but one had double the peak RPM, meaning it's flat HP curve shows up twice as "late" into the powerband, it'd still be slower behind the same transmission. And here's a big key. That higher RPM motor will drop FARTHER down it's HP curve with every gear change, because it has more RPM to give up to the RATIO of the gear change. This mean it has more RPM to climb back up to it's HP curve again.

 

Now, going back to our two cars with different engines. If we do this race again, but give the Honda a transmission with 6 gears that are very close to each other, and give the 5.0 a wide 4 speed, everything changes. As they come off the line they might actually be neck and neck this time, but as both cars shift around the same time, the honda will start to walk away, as it's very near it's MAX HP still, while the 5.0 dropped well bellow.

 

So we start to see that the transmission you're going to use in the real world has JUST AS MUCH to do with everything as things like CAM selection, displacement choice, etc. You can choose to build a HIGH RPM MAX HP L series motor that might make 400hp, but if the transmission you're using drops you from 10k to 6.5k between shifts, you might very well be SLOWER than a L series with 300hp at 8k that will only drop to 6k at the same shift, because he'll have more HP at 6k than the higher RPM motor at 6.5k most likely. Why do you think the F20C in real life comes with a close ratio 6 speed? Look at the gear ratios in motorcycle engines. 6 or more gears and they're all ridiculously close. Shifting at lower RPM in those combos show almost now RPM drop, but then again those motors are barely moving at 3k. Those gear ratios would be a complete waste on a broad HP motor like the 5.0

 

But that's not all!!! Because let's not forget that you can take that 5.0 motor and completely build it for MAX HP and get it to 550 crank HP which takes loads of RPM and high end heads with detailed port work and such. This motor would no longer have a "flat" HP curve. The reason it had a "flat" hp curve to begin with was largely due to the fact it was making so little HP for it's displacement. This motor can easily make 200hp by 4k, so to peak it off at 225 with a redline of 6.5k gives you one fat HP curve. The new 550hp curve will probably only have 400hp at 5k. It's not like this motor will now magically make 500hp at 4k and then redline at 6.5k still. It's a completely different HP curve.

 

What it comes down to is how much power you're extracting out of the limitations of the heads in your application. At your head's MAX flow condition, you've probably maximized the conditions for peak HP which means you're trying to get it to flow BEST in higher RPM so you have torque AND RPM on your side, but by doing this you've given up flow potential at lower RPM. This is true for ANY cylinder head. There's no magic bullet for this, except maybe removal of the valve entirely like through rotational valves that were banned from F1 not long ago.

 

So I've said all this, just to get to the point of saying, choose wisely. Know what you want. A high strung L series might be a lot of fun, but it won't be much fun on the street without the right gears to go with it, which can be as expensive of an endeavor as the engine build itself. I believe this is why so many people just do a V8 conversion. It makes lots of torque with very little effort, and makes for a great street car with almost no work. And in the long run you're able to run a wider transmission with longer gears making the car easy to enjoy.

 

 

Hope that helps some of the "torque" versus "HP" debate some people have had through the years. Personally, I like both, and prefer certain motor types for certain application.

[Dan Baldwin]

I got to ask though, with duration of 280+ degrees, aren't you just pushing the intake fuel/air charge right out the exhaust port?

See Jmort's response. It's only at low rpm that you lose volumetric efficiency due to pushing intake charge back through the still-open intake valve. At higher rpm, "on the cam" as it were, the column of air has significant momentum and doesn't just stop and turn around past BDC, it continues filling the cylinder even as the piston is rising. You can actually get significant ram effect from this and with tuned intake tract lengths you can actually get over 100% volumetric efficiency over a small range of rpm.

Your last configuration for example uses a cam of 310 degrees duration and .550" of lift. I think the biggest cam (in terms of duration) I have run in a SBC has been around 230 degrees duration and .5" of lift (and that was 2.02"/~51mm intake valves, and 1.89"/~48mm exhaust values). How does the engine idle? Can you drive it on the street without constantly having the engine stall out? That thing must sound like a beast.

The specs I gave are at zero lash, I don't know what the specs are at .050". But yeah, it idles at 1000-1200rpm. Sounds pretty bad ass

 

I'm going to assume you went with such a long duration cam for high RPM power... is there any chance you have a dyno chart for any of your engine configurations? I would be really interested in seeing what the torque curve looks like, and where in the RPM you start building power, and what the average horsepower over an RPM range looks like compared to the peak horsepower.

This is the only dyno chart I have for it, unfortunately the electronic 280Z distributor crapped out the night before the dyno, and I had to throw in my 240Z distributor, complete with POINTS. Breakup above 6000 is points bounce. Pertronix is in there now, pulls hard to 7200rpm rev-limiter

[JMortensen]

So I've said all this, just to get to the point of saying, choose wisely. Know what you want. A high strung L series might be a lot of fun, but it won't be much fun on the street without the right gears to go with it, which can be as expensive of an endeavor as the engine build itself. I believe this is why so many people just do a V8 conversion. It makes lots of torque with very little effort, and makes for a great street car with almost no work. And in the long run you're able to run a wider transmission with longer gears making the car easy to enjoy.

OK, but look at Dan's dyno plot. In L series terms, that is a pretty big cam. Even so, from 2700-6500 he's got over 150 ft/lb of torque. If you can't keep it in that range, yeah, you need gears or transmission swap. It's a good point to make and you're right to make it, but I just thought some perspective would be helpful. The L6 is inherently torquey. Go for the hp. My $.02.

Edited December 30, 2010 by JMortensen

[Gollum]

And I think that in this case Jon, you're probably right. I think the common ZX 5 speed paired with one of the common 3.545 or higher rear diffs would be fine for almost any of the high strung street L engines out there. It's not until you get into competitive racing or have one of those 400+HP monster L engines that the stock tranny options just don't really cut it.

 

And looking at Dan's dyno you can see he probably has nearly 200 ft/lbs anywhere once he puts his foot into it. That bottom curve of the dyno is recording before WOT I think. But looking at the HP curve of that motor, at a race track you wouldn't want to be under 4500 after shifts as there's a decent difference in torque there. Using the stock later ZX 5 speed IF you take that engine to the 7200 redline you land 2nd at 4450, which is about perfect, 2nd to 3rd lands at 5k, 3rd to 4th at 5500. Overall pretty good match. But I'd like to see a clean pull to 8k and see what the curve does. I bet it levels off a bit, and though he might only gain 15-30hp a shift from 8k lands 2nd at 4900 which is a possible 40hp disparity. That's enough to make a considerable difference. Now, the later gears would be more like a 20-30hp difference, but that's more of a HP difference than he might gain peak. This is why "area under the curve" is such a touchy topic as it can be so conditional to how the car will be used.

 

If Dan's car had an ever so slightly milder cam he might loose as much as 10 peak hp, but really smooth the torque down low. Worth it? Totally depends. And the worst part is that in the end only the owner's opinion matters, because the customer is always right. People are allowed to want whatever they want. That's why they're free to have their own opinion in the matter.

 

But overall, I'm with Jon. Build an engine/head capable of higher RPM and go for the aggressive cam. It sounds wicked, still will have plenty of torque for a tiny Z car, and really put you in your seat up top. Just don't expect to do much passing on the freeway in 5th unless you've got super short gears, or enjoy down shifting like all the rest of us gear heads love anyways.

[johnc]

It's not until you get into competitive racing or have one of those 400+HP monster L engines that the stock tranny options just don't really cut it.

 

 

Not to digress too much, but in the road racing world you start to have issues with any of the -A or -B series transmissions at power levels around 300hp. The Vintage racing guys around here get, at most, two seasons out of these older Nissan transmissions. The -C is better but the trend is towards gForce T5s as the least expensive good upgrade.

[Gollum]

Actually that's a very good point I hadn't thought about John. And looking at the ratios they offer for the T5, they're a bit tighter than the nissan ratios anyways. And I know those things sure can take a beating.

[Dan Baldwin]

HP, is impossible to DIRECTLY measure, but is in actuality directly CALCULATED. What I mean is that HP is just an EQUATION, not a force.

Power is the rate of doing work, or force multiplied by speed.

With an eddy current dyno, you are directly measuring force via a load cell, and speed by an toothed wheel and magnetic pickup. Horsepower is simply the force at the rear tire/dyno drum contact in pounds multiplied by speed in ft/sec, divided by 550 (1 hp = 550 lb-ft/sec). (or dyno drum torque times dyno drum rpm /5252)

 

With an inertial dyno, you know the rotational inertia of the drum, and the rotational acceleration of the drum via rate of change in speed. From that you know the torque at the drum and hence force at the tire contact patch. So, horsepower again is force in pounds times ft/sec divided by 550 (or drum torque * drum rpm / 5252)

 

You need additional information in order to calculate engine torque (engine rpm, or gear ratios and tire working radius). So in that sense, it is power that is the more "direct" measure.

[Gollum]

I disagree, the FORCE the dyno sees IS the torque, you just can't know what the engine's torque is unless you also measure the RPM, thanks to the torque multiplication factor of the transmission, final drive ratio, and wheels.

 

As you've just said, power is a measurement of work done over TIME. Thus you HAVE to actually CALCULATE only AFTER you know the torque measurement involved.

 

 

It's the same as measuring a WATT. Most meters don't measure watts because it has to be calculated and cheaper meters aren't going to have that slightly more complicated ability built into it.

 

You CAN however convert HP directly into WATTS.

 

In this calculation AMPS is like the TORQUE, the FORCE involved that can be directly measured. The VOLTS is like the RPM which tells us how many cycles we have in a time frame. From having those two basic measurements we can then calculate the watt of a force over time.

 

 

That's what I'm trying to get at though. Any measurement is going to measure a FORCE, which can only be described as torque, not HP. When you drive a car you don't ever feel the horsepower, you feel the torque. When a Honda hits Vtec you don't all of the sudden feel the peak HP of the car, you feel the huge bulge of torque that shows up out of nowhere. You can kinda of "prove" this in a sense when you consider that the force of that Vtec engine after it hits Vtec is a very even and consistent pull that keeps you in your seat. Looking at the torque curve on the dyno you see that it's very flat after Vtec, varying usually less than 10%. Getting into a muscle car like a pushrod mustang, you feel the loads of torque off the line, and it seems to put you into your seat MOST before the 3k mark. Look at the dyno on most "stockish" V8's and you see that torque drop off gradually after the torque peak and be way low by redline, thus they give you a long smooth HP curve.

 

The only thing we can do in our mind to conceive the HP of a vehicle we're in, is to factor in our head how fast we're going, and then consider the variables of the weight of the car, road conditions, etc. We do this in our head so well we don't realize it. But in the end, SPEED is a direct relation to TIME. When you drive a Subie STI and you realize how amazingly easy you just got to 150mph on a freeway on ramp, you know in your mind there's a lot of power beneath that right pedal. Your mind has just done a calculation of the force involved in a matter of time.

 

So again, horsepower is not a force, it's a figure we use to figure out how much work will get done. This is the common misconception that people bring into the age old torque vs hp debate. Because in the end we all know that you can't move a car without torque, or HP. It's just a matter of where people prefer the HP to show up in the RPM band, low down, or up high, or evenly across.

[Dan Baldwin]

I disagree, the FORCE the dyno sees IS the torque, you just can't know what the engine's torque is unless you also measure the RPM, thanks to the torque multiplication factor of the transmission, final drive ratio, and wheels.

With an inertial dyno, the dyno doesn't "see" force at all, it only sees speed.

Either way, power is the more direct measurement, engine torque requires further calculation with additional information.

 

As you've just said, power is a measurement of work done over TIME. Thus you HAVE to actually CALCULATE only AFTER you know the torque measurement involved.

Power is the instantaneous rate of doing work. The "calculation" requires torque (or force) at the wheels and speed, yes.

 

It's the same as measuring a WATT. Most meters don't measure watts because it has to be calculated and cheaper meters aren't going to have that slightly more complicated ability built into it.

Slightly more complicated? It's only a scale factor, 1hp = 745.7 Watts. As far as the computer is concerned, it is not more complicated. The dyno is measuring the same thing, power. Whether you want it in units of Watts, kW, horsepower, ft-lb/sec, mile-oz/fortnight, whatever units, it's only a scale factor.

 

That's what I'm trying to get at though. Any measurement is going to measure a FORCE, which can only be described as torque, not HP.

Again, an inertial dyno doesn't measure force at all, only speed. It backs out the dyno drum torque, or force at the contact patch, based on rate of change of speed. The notion that "power" is some nebulous thing that you don't really feel or experience directly is misguided.

 

When you drive a car you don't ever feel the horsepower, you feel the torque.

In a sense, you *do* feel engine power more than you feel engine torque. If you feel a certain amount of acceleration in car of a given weight, with no info regarding rpm, you can say something about the engine's power, but nothing about its torque.

 

When a Honda hits Vtec you don't all of the sudden feel the peak HP of the car, you feel the huge bulge of torque that shows up out of nowhere.

The torque hit you feel when you go into VTEC is sort of a gimmick. Ideally, the engine would shift over to the high-rpm lobes at the rpm point where the low- and high-speed lobes make the same torque and power, and you wouldn't feel a big increase in torque. On the AP1 in particular, the VTEC rpm is set much higher than what would be ideal.

 

The only thing we can do in our mind to conceive the HP of a vehicle we're in, is to factor in our head how fast we're going, and then consider the variables of the weight of the car, road conditions, etc. We do this in our head so well we don't realize it. But in the end, SPEED is a direct relation to TIME. When you drive a Subie STI and you realize how amazingly easy you just got to 150mph on a freeway on ramp, you know in your mind there's a lot of power beneath that right pedal. Your mind has just done a calculation of the force involved in a matter of time.

A given amount of acceleration at a given speed is power, which you can feel instantaneously, you don't have to feel it over a period of time.

 

So again, horsepower is not a force, it's a figure we use to figure out how much work will get done.

It is also a figure that will tell you how quickly you can accelerate from a given speed. If you feel 0.5g of acceleration at 60mph in a 3000 lb. car, you're FEELING 240hp, whether the engine is making 300 lb-ft at 4200rpm or 150 lb-ft at 8400rpm. Integrating power over time will tell you how much work is done, but power *is* something that you can and do feel instantaneously as well.

 

This is the common misconception that people bring into the age old torque vs hp debate. Because in the end we all know that you can't move a car without torque, or HP. It's just a matter of where people prefer the HP to show up in the RPM band, low down, or up high, or evenly across.

Preferring hp to show up down low in the rpm range is equivalent to preferring less power period!

 

It seems we may be arguing semantics here, I just want to get across that power is not "torque over time", but rather the instantaneous rate of doing work. Power is what you feel when you accelerate out of a corner as well as what you feel at the end of the straightaway.

[Gollum]

I probable am just arguing semantics, so hopefully we can keep this lighthearted as we have thus far.

 

But I do realize that the calculation from HP to watt is simple, but I was talking about the calculation to such from amps and volts in a circuit, hence why I mention cheaper meters don't measure watts. Measuring an amp or volt is easy, but most sub $20 meters won't have that slightly more complicated calculation to measure both at once and form the conversion.

 

I think my point still stands, which is that HP isn't truly a power we see or feel, but simply a number we use to define something. You can very easily create a mechanical device to measure torque, as seen as how simple it is to make a torque wrench, and it's very easy to make a simple mechanical device that can keep time. But measuring horsepower or watts has always been more complicated and requires more complex calculations. You can make a torque wrench that measures it's own scale that's not accurate to foot pounds, but at least it's accurate to itself. You can make a time piece that might not follow exact seconds but will be extremely consistent in it's own right. You're not going to make a "simple" device that measures the watts used by a light bulb that has any bearing on reality, even by it's own scale, unless you can measure the force (torque/amp) and the time (rpm,volt) to some level of accuracy of relative scale from each other.

 

To some extent I do agree that when you feel a car accelerate you feel the "power" of the engine, but what I'm getting at is that much of what we actually experience is a calculation that happens in our own heads without us even realizing. Instantaneous HP rating of an engine at a given RPM will be felt at corner exit, but I'm still not certain it's "HP" we feel, but rather TORQUE that's given momentum with time. Again, this is just based on the fact that torque is a much more real force in the world we live in.

[Dan Baldwin]

I probable am just arguing semantics, so hopefully we can keep this lighthearted as we have thus far.

Of course

 

I think my point still stands, which is that HP isn't truly a power we see or feel, but simply a number we use to define something.

Power is very real, you can certainly feel it.

 

You can very easily create a mechanical device to measure torque, as seen as how simple it is to make a torque wrench, and it's very easy to make a simple mechanical device that can keep time. But measuring horsepower or watts has always been more complicated and requires more complex calculations.

I mentioned above exactly how eddy current and inertial dynamometers measure power. It's not complicated at all, the calculations are quite straightforward. Not complex at all in layman's terms or in the mathematical sense (no imaginary numbers involved!).

 

To some extent I do agree that when you feel a car accelerate you feel the "power" of the engine, but what I'm getting at is that much of what we actually experience is a calculation that happens in our own heads without us even realizing. Instantaneous HP rating of an engine at a given RPM will be felt at corner exit, but I'm still not certain it's "HP" we feel, but rather TORQUE that's given momentum with time.

This is a common misconception, power *IS* an instantaneous value, it's nothing to do with "momentum over time" (bit of a misnomer there anyway). Think of it this way: Torque at the wheels at a given speed is a function of engine POWER. Knowing engine torque doesn't give you the whole story, but knowing engine POWER does.

 

Again, this is just based on the fact that torque is a much more real force in the world we live in.

Torque is no more "real" than power, in fact, power IS the torque you're making at the wheels at a given speed.

 

If you're making 240hp at 60mph, this is EXACTLY, PRECISELY the same thing as saying you're applying 1500 lb of thrust at the rear tire contact patches at that speed, or 1500 lb * tire working radius in rear wheel torque (or 1500 lb-ft at the wheels for tire working radius of 12").

 

Power tells you exactly what the force is where the rubber meets the road. It's not some nebulous "torque over time" or "momentum over time" concept, it's instantaneously tangible physical reality!

Edited January 3, 2011 by Dan Baldwin

[Gollum]

Heehee, we're spinning in circles here and in my eyes you're almost arguing my point.

 

HorsePOWER is a misleading name in my opinion. But to some degree I can agree that knowing the POWER is a more concrete way of knowing the FORCE involved, but it has NOTHING to do with understanding the engine force in question. We humans are just using the term horsePOWER to bypass knowing all the facts and just knowing again "how much work will get done". But very sadly, people have taken this "how much work will get done" figure and assigned it to a FORCE, a law of physics.

 

And the dyno meter example is not a true "mechanical" device like a torque wrench or a mechanical watch. It's an mechanical device with an electrical counterpart which requires at least a basic logic controller to do the equations.

 

We might be able to "feel" what you call "power" but you can't actually feel work load. Going back to my wood stacker analogy, the weight, or stress, the worker will feel will be the weight of the wood in his arms and the stress this places on his body. This would be a TORQUE measurement. You hold out your arms, you put weight in your hands, it has a leverage on your arms. That's torque. The POWER, or WORK, in this equation is how much they'll move in the day. That has nothing to do with the FORCES involved, which are what are directly felt.

 

I can agree though, that if I was going to be given only ONE figure in an equation, I'd want the workload figure. If you're given two of the three you can always calculate the third, so it doesn't matter which two you receive. And there lies why we always rely on the horsepower figure, because it's the COMBINATION of the FORCE over TIME, thus it's more meaningful to us. That' doesn't make it more meaningful to the laws of physics.

 

And I disagree about 240hp at 60mph meaning you have 1500lbs of thrust at the wheels. By the very definition, FOOT POUNDS is directly related to TORQUE applied. Your transmission and rear gears MULTIPLE torque at the expense of SPEED. If you're in 4th gear (1:1 ratio) and you have a 4.11 rear end, being at 60mph and at a RPM that would yield 240HP at say, 4000rpm. That means the engine would have 315 FOOT POUNDS of torque. You then multiply that by 4.11 and you get 1294.65 pound feet of torque working at the rear wheel centerline on a 12" radius. If that 240hp engine had that power at, now lets say 6000rpm that means it's only applying 210 pound feet, multiplied by 4.11 would be 863.1 pound feet. Both cars would accelerate at the same rate if the given HP curves where the same. Why? Because the one that's applying less pound feet of torque is doing so at a HIGHER rate of speed. The one with more torque at the wheels wouldn't be faster because it's gaining it's torque through the sacrifice of speed. Thus the equations of work done would come out to be the same. Both cars would tie at the finish line.

[Dan Baldwin]

HorsePOWER is a misleading name in my opinion. But to some degree I can agree that knowing the POWER is a more concrete way of knowing the FORCE involved, but it has NOTHING to do with understanding the engine force in question. We humans are just using the term horsePOWER to bypass knowing all the facts and just knowing again "how much work will get done". But very sadly, people have taken this "how much work will get done" figure and assigned it to a FORCE, a law of physics.

The term POWER *is* the correct term for the rate of doing work (which if integrated over time would tell you "how much work will get done", but that's not what we're talking about here, we're talking about the instantaneous accelerative thrust or rear wheel torque available at a given speed).

1500 lb. applied at the rear tire contact patches at 60mph (88 ft/s) = 132,000 ft-lb/sec = 240 horsepower (1hp = 550 ft-lb/sec).

If you're making 240rwhp at 60mph, you're generating 1500 lb. of FORCE where the rubber meets the road.

PERIOD. This is physics 101.

 

POWER is not "sadly" or otherwise inappropriately applied here, it has a STRICTLY DEFINED RELATIONSHIP WITH FORCE AND SPEED (or torque and rpm).

 

And the dyno meter example is not a true "mechanical" device like a torque wrench or a mechanical watch. It's an mechanical device with an electrical counterpart which requires at least a basic logic controller to do the equations.

I told you what they were a few posts up, here they are again:

With an eddy current dyno, the FORCE required to hold the eddy current brake assembly against rotating is measured with a load cell. From this you know the torque at the dyno drum, and can calculate the force at the tire/drum contact. You know the speed via a toothed wheel and magnetic pickup. POWER in units of horsepower is simply dyno drum TORQUE in lb-ft multiplied by dyno drum RPM divided by 5252 (or tire/drum FORCE in lb. multiplied by tire/drum SPEED in ft/sec divided by 550).

 

This is nothing fancy! Nothing particularly difficult to grasp here (I hope!).

 

We might be able to "feel" what you call "power" but you can't actually feel work load. Going back to my wood stacker analogy, the weight, or stress, the worker will feel will be the weight of the wood in his arms and the stress this places on his body. This would be a TORQUE measurement.

This is what the load cell in an eddy current dynamometer is measuring.

 

You hold out your arms, you put weight in your hands, it has a leverage on your arms. That's torque. The POWER, or WORK, in this equation is how much they'll move in the day. That has nothing to do with the FORCES involved, which are what are directly felt.

It has EVERYTHING to do with the forces involved.

And again I have to restate for the umpteenth time, POWER is not how much work is done, POWER is the RATE OF DOING WORK.

POWER is NOT WORK.

POWER is to WORK as SPEED is to DISTANCE.

 

If you push a block with 10 lb. of force at 10 ft/second, that's 100 ft-lb/sec, or 0.18 horsepower. If you do that for 10 seconds, you've done 1000 ft-lb of WORK. If you do it for a tenth of a second, you've done 10 ft-lb. of WORK. The power is the same whether you do it over 10 years or a millisecond.

 

And PLEASE note that "ft-lb" here is NOT TORQUE (which you should always put in units of "lb-ft" to avoid confusion), it is WORK.

 

Don't confuse POWER with WORK, and don't confuse TORQUE with WORK, they are fundamentally different.

 

And I disagree about 240hp at 60mph meaning you have 1500lbs of thrust at the wheels.

Then you do not understand the physics of power, work, torque, and how they are all intimately related.

If you are making 240rwhp at 60mph, you are BY DEFINITION making 1500 lb. of thrust at the tire contact patches.

240 hp * 550 ft-lb/sec = 132,000 ft-lb/sec.

132,000 ft-lb/sec divided by 88 ft/sec (60mph) = 1500 lb.

 

PERIOD.

 

 

By the very definition, FOOT POUNDS is directly related to TORQUE applied. Your transmission and rear gears MULTIPLE torque at the expense of SPEED. If you're in 4th gear (1:1 ratio) and you have a 4.11 rear end, being at 60mph and at a RPM that would yield 240HP at say, 4000rpm. That means the engine would have 315 FOOT POUNDS of torque.

POUND-FEET is the preferred unit for torque, see above!

 

You then multiply that by 4.11 and you get 1294.65 pound feet of torque working at the rear wheel centerline on a 12" radius.

You screwed up here.

If you're spinning the engine at 4000rpm through a 1:1 4th gear and have a 4.11 diff, then the rear wheels are spinning at 973rpm. If the rear tire working radius is 12", circumference = 75.4" = 6.283'. Speed is then 973rpm * 6.283' = 6113 feet per minute = 102 ft/sec = 69.5 mph, NOT 60mph.

 

Back-calculating power we take the thrust force of 1295 lb. and multiply by the speed of 102 ft/sec and get 132,090 ft-lb/sec (+90 from round-off error, I wasn't carrying decimals through), dividing by 550 ft-lb/sec gives 240hp.

 

THIS IS THE LAW.

 

 

If that 240hp engine had that power at, now lets say 6000rpm that means it's only applying 210 pound feet, multiplied by 4.11 would be 863.1 pound feet.

Now in this situation you're going significantly faster, 153 ft/sec or 104mph. Power is the SAME, though:

863.1 lb * 153 ft/sec = 132,054 ft-lb/sec = 240hp.

 

If you're making 240hp at a lower speed, you're putting more torque to the wheels than if you're making 240hp at a higher speed.

Car 2 is going 1.5 times as fast as car 1, so it's only making 2/3 the rear wheel torque, though both are making 240rwhp.

 

Both cars would accelerate at the same rate if the given HP curves where the same.

The second car is going to be accelerating a lot slower due to less torque to the wheels (meaning less thrust). And aero drag will of course be greater as well...

 

Why? Because the one that's applying less pound feet of torque is doing so at a HIGHER rate of speed. The one with more torque at the wheels wouldn't be faster because it's gaining it's torque through the sacrifice of speed. Thus the equations of work done would come out to be the same. Both cars would tie at the finish line.

Um, if the first car starts at ~70mph, and the second car starts at 104mph, the second car will win of course...

[Gollum]

Aww, sorry for the disconnection of terms between pound feet and foot pounds. My mistake!

 

But in regards to straight line thrust, your math is exactly accurate. But let me take it from this route (I must admit everything you say is the truth, just the way we think about it seems different), as per this site here (I know, quoting the internet can be doddgey at times) http://www.car-videos.net/articles/horsepower_torque.asp We see it explained that WORK is simply torque causing movement, or torque applied. Torque without speed, I agree, will do nothing cannot be felt. This is probably where the difference of our semantics comes from.

 

But as even stated there on that site, rotational movement power cannot be calculated as horsepower, but must be calculated from torque over time.

 

An engine is a rotational device, as well is the transmission, rear gears, and wheels. These are all torque over time forces, and that's how they're calculated, even if it's an eddy current dyno with resistance that calculates the torque over time required to move it at a certain rate. But a car goes in a straight line, and as it goes in the straight line we can calculate the horsepower required to do so, such as calculating the HP of an engine from 1/4 mile times.

[Leon]

Interesting discussion, but I want to add that when, for example, accelerating out of a corner, you feel something (a "force") pulling you back into your seat. Whether you describe the force as something that comes from engine torque or engine power is irrelevant, both are meters by which engine performance can be measured. You can describe engines with meters like BMEP, specific power, power density, mean piston speed, VE, blah blah blah ad infinitum.

 

"Feelings" are very hard things to analyze since they are subjective and very complex in their mechanisms. You can say that you're feeling torque or that you're feeling power and you'd both be right.

[Dan Baldwin]

Aww, sorry for the disconnection of terms between pound feet and foot pounds. My mistake!

I see now that you only did it that one time, so sorry for ragging you for what was more of a typo!

 

But in regards to straight line thrust, your math is exactly accurate. But let me take it from this route (I must admit everything you say is the truth, just the way we think about it seems different), as per this site here (I know, quoting the internet can be doddgey at times) http://www.car-videos.net/articles/horsepower_torque.asp We see it explained that WORK is simply torque causing movement, or torque applied.

It's the same thing. Work is force multiplied by distance, or torque multiplied by radians of rotation.

1500 lb. thrust at the contact patch, acting over 1 foot of distance => 1500 ft-lb of work done.

With a 12" working radius tire, torque is 1500 lb-ft, and one foot linear distance equates to one radian of rotation.

So 1500 lb-ft * 1 radian = 1500 ft-lb of work.

 

 

Torque without speed, I agree, will do nothing cannot be felt. This is probably where the difference of our semantics comes from.

Torque without speed *can* be felt. You feel it when you accelerate from a stop. Or if you are on a slight grade and let your car roll backwards before launching, you feel the same forward acceleration with negative velocity, as you pass through zero speed, and with forward velocity.

 

But as even stated there on that site, rotational movement power cannot be calculated as horsepower,

Say what?

 

but must be calculated from torque over time.

It is calculated as torque multiplied by rotational speed, or force multiplied by linear speed. It's the same thing. The whole "torque over time" notion of power is a misconception. You can hang a 1 lb. weight off of a fixed 1 ft. moment arm, which gives 1 lb-ft of torque, and there is ZERO POWER whether you hang it there for a second, an hour, a millenium. Power is emphatically NOT "torque over time"! It is torque multiplied by rotational speed. Or force multiplied by linear speed.

 

People see the units of power in ft-lb/sec and think "torque over time", an understandable mistake.

It's really WORK per unit time, or rate of doing work. The "ft-lb" in the numerator is NOT TORQUE, it's WORK.

 

To get WORK from TORQUE and ROTATIONAL DISPLACEMENT, you multiply the torque by the rotational displacement in radians.

1 lb-ft of torque applied about an axis going through one full rotation gives 1 lb-ft * 2pi radians = 6.283 ft-lb of work.

 

To get POWER from TORQUE and ROTATIONAL SPEED, you multiply the torque by the rotational speed in radians/sec.

1 lb-ft of torque applied at a rotational speed of 5252rpm (550 rad/sec) => 1 lb-ft * 550 rad/sec = 550 ft-lb/sec = 1 horsepower

 

An engine is a rotational device, as well is the transmission, rear gears, and wheels. These are all torque over time forces,

The expression "torque over time forces" is meaningless. You can go with engine torque and rpm and keep up with the changes at every gearing interface to calculate the rear wheel torque and thrust at the ground, or you can go with engine power and immediately know what the thrust is at the ground simply dividing by vehicle road speed.

 

and that's how they're calculated, even if it's an eddy current dyno with resistance that calculates the torque over time required to move it at a certain rate.

Eddy current dyno measures force via a load cell and calculates torque from that, or it converts drum rpm to linear velocity to calculate power, either way it's the same thing. Power = torque*rotational speed, power = force*linear speed.

 

But a car goes in a straight line, and as it goes in the straight line we can calculate the horsepower required to do so, such as calculating the HP of an engine from 1/4 mile times.

Converting from rotational to straight-line coordinates is dirt simple, of course, but the simple equation estimating hp from 1/4-mile time (or speed, which gives better results) is a fit from experimental data, not a conversion from rotational to linear.

 

The power you get where the rubber meets the road is the same as the power you're making at the engine, minus losses (usually ~15%). If you just want thrust at the ground, it's that easy to calculate. If you're using rear wheel horsepower numbers, you don't have to knock it down by 15% as losses are already accounted for.

[NewZed]

This thread has been jacked, tortured and murdered...

[Dan Baldwin]

This thread has been jacked, tortured and murdered...

 

Let it be known that any time I happen to see references to how power is "torque over time", or how torque is what you feel accelerating out of a corner and power is the speed you get at the end of the straightaway, or any other such nonsense, that I will divert a thread in a vain attempt to edumacate the unwashed masses.

 

Mods can feel free to delete posts or split threads as deemed necessary.

 

I can handle it on most forums, but I hold HybridZ to a higher standard!

[NewZed]

I was just trying to help kce out a little bit (and make a little joke), re the original P90 v P79 topic, plus I'm still interested in the "causes of detonation" discussion that was going on.

 

Yours is an interesting discussion and might be worth extracting as its own thread, as you suggest.