Someone clarify something for me re: HP vs. Torque

[wheelman]

Bastaad525,

You probably aren't breaking the rear tire(s) loose when on a roll because the increase in torque at the onset of boost isn't enough at the rear wheels to overcome the traction at that point. When you look at your dyno graphs do you see a very large increase in torque over a very short RPM range? If not then the torque is applied to the rear wheels gradually enough that they don't accelerate fast enough to exceed their traction. I hope this helps.

 

I thought of another item you'll want to address if you make a cam change. You might need to upgrade your valve springs. If you use a high lift cam it might exceed to lift allowed by the stock ones and at higher RPMs you'll be dealing with valve float. In fact you might be experiencing float now at the higher RPMs. I know SBCs start seeing it around 5500-6000 RPM mostly because of lifter pump-up, but weak springs will contribute to it as well, not sure when a L28 starts to have problems with this.

 

Wheelman

[JMortensen]

The problem with L series valve springs is coil bind, not valve float. I had a theoretically impossible .490 lift cam with stock valve springs and it only floated the valves once and that was at 7500 rpms. BTW you can only get that combo to work with some REALLY crappy machine work. I just happened to find a really crappy machinist when I built my first engine.

 

If your new cam goes over .460 (I would hope that it does) then wheelman is right, you will need valve springs and retainers. If it goes over .480 then you need the shorter valve stem seal.

 

I know you've already seen this Bastaad, but for anyone else following this thread: http://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewtopic&t=31967

[JustAFantaZ]

... if it's 400ftlbs or whatever... it's 400ftlbs... why does it make it faster when it occurs in higher RPM? [/quote']Because a higher engine RPM means you can select a lower gear ratio for any particular vehicle speed to create even MORE torque at the rear wheels.

Would you clarify that a little bit?

higher engine rpm...compared to what?

lower gear ratio...compared to what?

more torque...compared to what?

 

Thanks.

[Guest bastaad525]

Wheelman - actually, torque does hit very fast, it hits it's peak at about 3000rpm, and comes on very rapidly, i.e.: it increases over 100ft lbs, from 200 to 300ft lbs, in the space of 500rpm, from 2500 to 3000rpm.

 

Conversely... with such low gearing, maybe my road speed is just too fast by that point for the tires to break loose. 1st gear at 3000rpm is going about 20mph... I thought that was slow enough though that'd they'd be able to break loose, especially considering the lack of LSD and crappy all season tires, and only 195-70-14's at that.

 

 

 

Guys I just wanted to sum up one thing here, and be sure I really DO have this all correct.

 

In a given instant during a race, if we have two exact same cars, with two different engines, running at different RPM's and putting down different amounts of torque, but both putting down the SAME hp at that exact instant... Let's say one spinning at 4000rpm, putting down 300ftlbs and 228rwhp, and the other, spinning at 7000rpm, putting down 171ftlbs and same 228hp, and assuming same gearing/rear end/torque multiplication at the wheels, that at that instant both vehicles would be accelerating at the same rate, correct?

 

If I understand it right, that means two motors that are rated at the same hp (if rated accurately ) but have different torque curves, have the potential to accelerate at the same rate, but they would do so at different points in their power band, i.e.: in the Z3 vs. S2000 example, the torquier Z3 would pull off the line faster but once they were both in their respective power bands acceleration would be just about equal? Just want to be sure I have all this right.

[GabeRoc]

let me try my hand at this

 

First. Torque has to do with acceleration. Basically torque is a force for something that is rotating. Torque at the wheels (read: taking into account all gearing) is converted into a linear force and hence an acceleration because f=ma (yay physics!) now, in all truth instantaneous acceleration has little to do with final speed. In order for speed to increase significantly torque (a force) must be applied over a period of time (making power). Energy cannot be created or destroyed according to the first law of thermo, so in order for a car to posses kinetic energy, that energy has to come from somewhere. Kinetic energy is 1/2*mass*velocity^2. So, if I want to obtain a certain velocity in a certain period of time, (say 100+ mph in under 12 seconds) there is going to be a minimum power per second I *have* to produce to obtain that velocity. So torque at the wheels is what you feel pushing you back into your seat but it is horsepower that allows you to reach a certain speed in a certain amount of time. gearing becomes very importiant at this point because good gearing allows the maximum energy transfer from the motor (HP) to the car (kinetic energy) in a given period of time.

 

In a given instant during a race, if we have two exact same cars, with two different engines, running at different RPM's and putting down different amounts of torque, but both putting down the SAME hp at that exact instant... Let's say one spinning at 4000rpm, putting down 300ftlbs and 228rwhp, and the other, spinning at 7000rpm, putting down 171ftlbs and same 228hp, and assuming same gearing/rear end/torque multiplication at the wheels, that at that instant both vehicles would be accelerating at the same rate, correct?

 

No, because one car (the one at a higher RPM) will be traveling almost twice as fast as the other. Now, if you were to fix the gearing so that in a certain gear both cars had the same speed with one car running 4000 RPM and the other 7000 RPM, then yes. They should be accelerating at the same rate. What is interesting is that if the gearing were set as I have outlined, the torque at the wheels for both cars would be the same. It has to be for them to accelerate at the same rate.

[Guest bastaad525]

No' date=' because one car (the one at a higher RPM) will be traveling almost twice as fast as the other. Now, if you were to fix the gearing so that in a certain gear both cars had the same speed with one car running 4000 RPM and the other 7000 RPM, then yes. They should be accelerating at the same rate. What is interesting is that if the gearing were set as I have outlined, the torque at the wheels for both cars would be the same. It has to be for them to accelerate at the same rate.[/quote']

 

 

Okay that just confused the heck outta me. Well, not the first part... I should have clarified that the cars would be moving at the same speed. The second part threw me though... so because of the multiplication, the torque from the higher rpm motor would be the same at the wheels? I'm trying to figure out how this all relates to my car... in which torque falls off relatively soon and pretty rapidly after 4000rpm, yet, horsepower is tabletop flat from 3500-6000rpm. So I'm trying to figure out at which point in the power curve my car is accelerating the fastest.

[Pop N Wood]

... if it's 400ftlbs or whatever... it's 400ftlbs... why does it make it faster when it occurs in higher RPM? [/quote']Because a higher engine RPM means you can select a lower gear ratio for any particular vehicle speed to create even MORE torque at the rear wheels.

Would you clarify that a little bit?

higher engine rpm...compared to what?

lower gear ratio...compared to what?

more torque...compared to what?

 

Thanks.

 

Think of it this way. You are cruising along at 30 mph in 4th gear when someone jumps you. You want to downshift and accelerate as quickly as possible. What gear will you choose?

 

At that exact instant in time your car is going a certain speed, so that means your rear wheels are turning at a certain speed. If you drop down to 3rd gear, your engine revs will jump to some speed. If you drop down to 2nd gear (i.e. a lower gear), your engine revs will jump to higher speed. All of this assumes you don’t lose traction.

 

Let's say you have one of the new Dodge hemis. Those things have torque curves that are almost completely flat from something like 2000 RPM to 5000 RPM (at least that is what I remember from an old post). Let's also say going to 3rd gear will put you at 2500 RPM, but going to 2nd puts you at 3500 RPM (I made this up just for an example). At both RPM's the engine is kicking out the same torque, so it shouldn't matter, right? But the torque at the rear wheels is the engine torque multiplied by the overall gear ratio. So going to a lower gear will give you more torque at the rear wheels and faster acceleration.

 

What is the difference between 2500 RPM and 3500 RPM? Horse power.

 

Take this example one step further. Say going to 1st gear would put the engine at just under 5000 RPM. Same assumed torque, yet lower gear, so should accelerate even better, right? Yes BUT dropping down to first and accelerating will quickly put the engine over 5000 RPM, torque will start going down and depending on the power curve the engine might go past it’s HP peak. At any rate you will run out of top end very quickly all the while running the risk of lighting up the tires with all the rear wheel torque. So in this case you are probably better going to 2nd and winding the engine out.

[Pop N Wood]

The second part threw me though... so because of the multiplication, the torque from the higher rpm motor would be the same at the wheels? I'm trying to figure out how this all relates to my car... in which torque falls off relatively soon and pretty rapidly after 4000rpm, yet, horsepower is tabletop flat from 3500-6000rpm. So I'm trying to figure out at which point in the power curve my car is accelerating the fastest.

 

Because of the multiplication, both motors should be producing the same torque at the wheels. You would have to go through the math and take the gear ratios for both cars times the engine torque they are actually producing. But if you are saying that both vehicles are going the same speed and both engines are kicking out exactly 228 HP, I claim that tells you all you need to know. Mathematically torque times RPM yields HP. HP is the constant and can neither be “created nor destroyedâ€. So in your example, both cars have the rear wheels spinning at the same RPM so the higher reving engine must use a lower gear. This will multiply the torque, but you know that when all the math is done both cars will be producing the same torque at the rear wheels. Why? Because they are producing the same HP and HP = torque X RPM. Changing gearing to affect RPM means the torque will go the opposite direction. Same HP at the same RPM = same torque.

 

When you include gearing acceleration is directly related to engine HP. So the two cars should be able to accelerate equally.

 

For your engine, torque is dropping as engine speed increases. But if HP stays constant, then that means torque is dropping at the same rate that the engine RPM increases (HP = torque X RPM). So if you believe what I say that the available acceleration at any vehicle speed is directly proportional to engine HP, then it really doesn’t matter if your engine is spinning 3500 or 5500 RPM. The potential acceleration should be the same. This is especially true if your HP curve was measured on a rear wheel dyno where drivetrain losses are accounted for. Go with whatever gear you can hold the longest at that portion of the track.

 

BTW. That is an unusual engine to have such a flat HP curve. I personally have seen lots of dyno curves with flat torque curves, but not many with super flat HP curves.

[Guest bastaad525]

heheh well here it is :

 

 

It is the flatest HP curve I have personally ever seen. Maybe 'table top flat' was an exaggeration *shrug*. I was initially dissapointed to see torque falling off so early, and always (until reading this thread) figured that the torque was what determined rate of acceleration. I was left wondering... should I be shifting at like 4k rpm or what? To take advantage of all the torque, that seemed to make sense. The huge difference between the torque peak and hp peak numbers is what got me wondering and caused me to post this thread.

 

So now that you see the curve, what would you say is the ideal shift point? Given a standard nissan 5spd trans that drops about 1000rpm when you shift to the next highest gear, so what 1000rpm 'chunk' of the power curve should I stay in?

[Dan Baldwin]

I say this because 500 lb/ft of torque means you can move[/color'] 500 lbs for a distance of 1 foot.

 

500 lb-ft of torque is a twisting force equivalent to a 500 lb force acting on a 1ft moment arm or a 1 lb force acting over a 500ft moment arm.

Note that it doesn't necessarily imply any movement, or lack thereof. What you've described (if the 1ft you mention is VERTICAL, that is) is WORK, which is in the same units as torque, but is a DIFFERENT quantity. You can have torque with zero work being done.

 

Power is the rate of work. You can't have power with no work being done.

Power isn't exactly "torque over time", but rather the instantaneous rate of work.

 

Power is the more important value, as it tells you the whole story. Power is the force applied at the ground to accelerate the car multiplied by the speed. Torque is incomplete, it doesn't tell you the whole story.

 

Rear wheel dynos actually measure POWER (force at drum*speed at drum) and calculate the torque out of it by measuring the rpm. If you don't have rpm data, you can still get the power-to-the-wheels curve, but no torque data.

[Guest bastaad525]

Dan - now that is in direct conflict of everything I've heard up until now. EVERY thing I've heard regarding typical chassis dyno's says that actually they directly measure the torque and then figure the hp from there.

 

 

sometimes, you guys can be really confusing

[GabeRoc]

The force on the drum is the torque being applied. So the dyno multiplies rear wheel torque by rear wheel speed and gets rear wheel HP. then you divide by the engine speed (aka RPM data) and vola'! engine torque at the wheels. so the dyno does measure torque, but not engine torque. (I guess that you could measure front wheel torque and speed if your car was wrong wheel drive )

 

Dan, you are completely correct. power is work/time. I was trying to avoid unnecessary complexity by assuming the wheels were turning and not diving into instantaneous power. I felt that my explanation got the point across without clouding the issue with technicalities. It is very possible that I did not succeed in my attempt to keep it simple while maintaining an adequate representation of the facts. Regardless, I probably should have included a disclaimer that I was trying to use layman’s terms. And you get an amen to torque not telling the whole story. 8)

 

Bastaad525, you will want to shift such that the most power is sent to the wheels. Assuming your gears are spaced 1000 rpm apart, stay between about 3700 and 4700. You want the most power going to the wheels, let the gears worry about torque. Notice you are better off shifting a bit late than a bit early because the HP stays higher a little past 4700 than it does a little before 3700.

 

hope that is less confusing and more correct.

 

gabe

[crazy280]

This is funny, because I think Dan was commenting on my quote, but Gabe responded and said most of what I would have said. Thanks Gabe.

Dan, I think its just semantics, because I agree with you, but I think you are contradicting yourself when you say power is the rate of work, but not torque over time. Because "rate" pretty much means "over time" and:

 

horsepower = torque * RPM / 5252

 

.....and RPM = revolutions / minute

 

.....so horsepower = (torque * revolutions) / (minutes * 5252)

 

.....and minute = a unit of time

 

so to make it simple: power = (torque * revolutions) / time

 

which would mean: (torque * revolutions) = work

 

You said you can have torque with zero work being done, so I guess that means there would have to be zero revolutions involved. But when we talk about engines, revolutions are always involved, which means work is being done, and I didn't want to bring up work for simplicity's sake.

My goal in my first post was to keep it in lamens so that anyone could follow, so I just said "torque over time". I said that because that is the raw concept - torque alone (or work, I suppose) does not fully explain accelleration; you have to factor in rate, and that is what horsepower does.

 

Please correct me if I am wrong -- I am not a physicist, this is just my understanding of the concept.

[Dan Baldwin]

Crazy280, it is not just semantics. Torque is not work. They do have the same units, but they are not the same. 1 lb-ft of WORK is the work required to raise one pound one foot. 1 lb-ft of TORQUE is the torque from 1 pound of force on a one foot moment arm (no motion required). Consider an electric motor, where you CAN have zero rpm and a large torque value with no work being done.

 

IMO, power defined as "torque over time" confuses the issue and perpetuates the mistaken notion that it is engine TORQUE gives a car acceleration and that engine POWER, since it is "torque over time", gives a car top speed. This is incorrect, it is POWER (same at engine and at road) that gives both immediate acceleration and top speed. "Torque over time" gives the false impression that you have to WAIT for it, that time has to elapse to experience it, when in fact power is an instantaneous rate, and acts instantaneously.

 

IMO it is more instructive to describe power as the instantaneous accelerative force available at a given speed. Think of rw power as road speed multiplied by accelerative force at the tires. This puts the "rate" into something tangible, road speed, and allows you to think of acceleration as being proportional to POWER (same at engine and road), and not *engine* torque (very different from torque at the wheels).

 

GabeRoc,

you're right about the dyno measuring the TORQUE (actually a FORCE acting at a DISTANCE) at the wheels and multiplying by rear wheel SPEED to get rear wheel power. The dyno doesn't have to know anything about the engine rpm or gearing to get engine hp applied to the wheels. It does have to know rpm to get *engine* torque, as you say.

Right on about maximizing engine POWER being the key to maximum acceleration. You should shift far enough beyond the POWER peak that you don't lose or gain power when you shift to the next gear. This method can have you shifting such that rpm is always above the engine's torque peak.

 

Looking at Bastaad's torque/power, the power being that flat gives a VERY broad rpm range, which will make it a lot easier to either stay in a lower gear or higher gear more of the time. Less shifting required to maintain performance. This is good. But there's a TON more power potential if it can be made to breathe better above 4000 rpm. If the power plateau could be bumped up to 5-6k vs. 4-5k, it would make something like 20% more power! That's gotta be a killer street motor, though.

[Guest bastaad525]

Dan - I'm working on it! Probably not until after X-mas, but soon, I'll throw a full 3" mandrel bent exhaust on there... hopefully that will broaden the torque curve. Well actually I've been doing a bit of research again, and it doesnt' look like exhaust will stretch the torque plateau out any, it will still start falling off shortly after 4000rpm, but the exhaust should cause it to fall off a bit slower and still give me more hp. I was looking at Jeff's page (the 450rwhp 280zx) when he did the before an after dyno tests on a bone stock ZXT that they then stuck his 3" system on, and torque still started falling off at the same point, just slower, after going full 3".

 

So it's probably either in the head, cam, or a different turbo that more real top end is probably going to be found. If exhaust doesn't do it I'll try going with a cam but this will surely be faaaaar in the future. A bigger turbo, the required EFI upgrades, or any kind of head work are pretty much out of the question.

[BrandonsZ]

Wow we really beat that horse to death and then kept beating till dogfood came out his butt!

[Guest bastaad525]

as every good thread should be!

[crazy280]

Yeah, I like my horses beaten thoroughly before I feed my dog

 

Battle of the science geeks and I'm in it! But seriously, Dan, we are on the same page. I said from the beginning that power is what accellerates a car, not torque. I just showed you how torque*revolutions is work (please correct my math if wrong) and engines must be rotating to make power (ENGINES not MOTORS). One lb/ft of torque is a one pound force on a one foot moment arm, right? If that arm is moved, it has done work. Again, I'm no physicist, but maybe 1 lb/ft torque * 1 revolution = 1 lb/ft work, I really don't know what the actual equation would be. But engines are doing work when making power. So I guess torque is the potential to do work? I don't know, now I'm all mixed up, It's late. Anyhow I stand by my stupid merry-go-round example

I'm gonna get some sleep

[Pop N Wood]

The units of torque have an angular component too. Mathematicians don't like it but you could specify it as lbs-ft per radian of revolution. RPM can be converted to radians per second. So when you multiply it all together the radians cancel and you get lbs-ft/sec, and one horsepower is defined as 550 ft-lbs/sec. That is where the "5252" comes from in all the car magazines. It just makes all the units work. Torque and work are not the same thing and do not have the same “unitsâ€.

 

Torque is the rotational equivalent of force. Push on the side of refrigerator and you are exerting force. If it doesn't move, then technically you have done no "work", even though you are expending energy at some rate. But all of that “if it didn’t move you didn’t do work†just confuses the issue so don’t worry about it. Also keep in mind that power can exist in different forms than just work. An engine being reved against a trans brake is still producing power. The power is just being dissipated as heat in the trans instead of work moving the car. Light bulbs could be rated in HP. A 75 watt light bulb is about a tenth of a HP.

[crazy280]

Ok, that makes sense, thanks Pop.

Hey, imagine if light bulbs were rated in HP. That would be pretty funny: "Yeah, I got about half a horsepower in that baby!"

Kinda reminds me of Tim Allen on "Home Improvement"