dyno measure torque/hp ?? - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 12 (permalink) Old 03-09-2008, 08:12 AM Thread Starter
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dyno measure torque/hp ??

i've always been under the impressions that dynos MEASURE torque, and that they CALCULATE horsepower from this measurement..
so TORQUE is actually MEASURED, and HP is determined w/mathematical equation ... have i been misinformed ??

there are two questions i'd like to get informed answers to..

1. i went to make some dyno pulls today to see the difference with my new tranny/converter setup since my last tranny was slipping badly on the dyno last time...
when i got it all strapped down, the guy asked me where he could hook up his "tach" lead to..(spark plug wire for instance haha).. i told them they needed to hook up an optical to the crank dampr..well their optical was broken and i had no idea where to get a rpm signal w/out tearing a bunch of **** open to get to an injector signal or crank position signal..
he told me """ no problem, we can still dyno it, but we just won't be able to get a torque reading, just HP"""...
how is it that they can dyno it, get a HP reading w/out a tach signal, but they can't get a torque reading w/out tach signal ?? i know that the formula to calculate HP from torque is based on an RPM constant... so question 1 is : why can i get a HP reading w/out knowing the torque, and why is rpm necessary for torque but not HP (isn't torque NEEDED for the formula to work) ??????

question 2 :: since HP is derived from a mathematical formula based on torque (assuming i'm correct in my above thinking) and the dyno does not know whether the vehicle being dyno'd is a gas or diesel (or does it) how come it can correctly give hp/torque relationship ??
let me explain my question.. let's say truck "D" is a turbo diesel that makes 500HP and truck "G" is a turbo gasser that makes 500HP also.

"G" pulls on dyno and makes his 500hp pull.. torque reading is ~500
"D" pulls on and makes his 500hp pull.. torque reading comes back at 1200

or even backward might makes more sense:
"G" makes a pull of 500 FTLBS... hp calculates out to 500
"D" makes a pull of 1200FTLBS... hp calculates ALSO to 500...

how is it possible that it KNOWS to have different HP figures when both trucks made DRASTICALLY DIFFERENT torque..
does the dyno have to be told that the vehicle is diesel vs. gas and does it use a different formulat to calculate the HP accordingly ??

does anyone know the answer to my questions, or am i just trippin ??

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post #2 of 12 (permalink) Old 03-09-2008, 09:04 AM
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HP is calculated from the following formula:

HP= (torque X RPM)/5252

Dynos that only measure hp are making a series of calculations.

Force = Mass x Acceleration, Work = Force x Distance and Power = Work / Time

The mass is the weight of the rollers.

The acceleration is obtained by measuring the speed the rollers.

Distance is obtained from the circumference of the rollers and the number of revolutions they made.

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post #3 of 12 (permalink) Old 03-09-2008, 11:23 AM Thread Starter
what's egt mean ??
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okay i knew there was a formula, and i've seen it before.. i assume that the just mathematically divide the formula for diesels ?? i say this because we don't generally see 5252rpm on our pulls .. this part i'm okay with, i follow you ..

but any idea as to the whole "D" vs. "G" thing ? is this because the dyno is actually measuring the HP and TQ separately instead of calculating one using the other ???
this is where i don't know why a dyno can give you drastically different tq reading w/vehicles of similar HP ..
i guess first i want to know: does the dyno have to be 'told' whether the vehicle is gas or diesel so that it uses a different set of 'parameters' to come up with the final readings.

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post #4 of 12 (permalink) Old 03-09-2008, 03:01 PM
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The simple summary is that torque is a measure of FORCE applied, with no conern for the time over which that force is applied; HP is force applied over a certain time. Because dynos measure both the force applied and the time, they technically access both torque and HP. I don't see how the two can be separated.

I found a decent wite up (credit where redit is due: http://www.procivic.com/pages-horsep...e/index.html):

Horsepower vs Torque

The concepts of horsepower and torque are probably the most misunderstood basic concepts in the auto world. The problem is a lot of people are very adamant about their opinions but haven't really done the research to back up what they're saying. I hear a lot of this, especially on the forums. In truth, though, horsepower and torque are NOT that intuitive of concepts. Remember that guy Isaac Newton? Well, it took him years to come up with this stuff and he was definitely no slouch when it came to mental capabilities.

So, why does any of this matter anyways? For starters, understand the truth behind horsepower and torque will give you a better understanding of engines and why certain types of engines are good for some things and not others. Second, you won't shift at the wrong point just because your friend says: "shift now, you'll get more torque in the next gear." It's also just satisfying to know exactly what makes your car move.

All right then, I've blabbered too much so now on to the good stuff. I'll try to keep it as brief and simple as possible but, remember, these are tough concepts to REALLY grasp.

Definitions

Force: Simply put, a push or a pull. Typically measured in pounds (lb) or newtons (N).

Work: Force over a certain distance, expressed as force * distance. Typically measured in foot-pounds (ft-lb), newton-meters (Nm), or joules (J). Note that a force only counts as towards work done if it is in the same direction as the distance being traveled.

Power: Work done over a certain amount of time, expressed as force * distance / time. Typically measured in watts (W).

Newton's Second Law of Motion: Force = Mass * Acceleration, or F = ma. This law says that when a force is applied to an object, the object's acceleration will be equal to the applied force divided by the object's mass.

Torque: A twisting or turning force. Typically measured in pound-feet (lb-ft) or newton-meters (Nm). Note that torque is NOT work, even though it is measured in the same units. The difference is that the force in torque is not in the same direction as the distance (it is perpendicular), so it does not count as work. Notice also that lb-ft is used instead of ft-lb; engineers do this as a convenient way to quickly tell the difference between a torque and work measurement.

Horsepower: A measurement for power that is commonly used in the U.S., abbreviated as "hp". James Watt invented this unit after observing that a horse could lift at a rate of about 550 foot-pounds per second for about an 8-hour shift.

Deriving the Relationship

To be clear, torque is a force that causes something to rotate. That something is usually circular, but it doesn't have to be. The distance part of torque is also called a lever-arm (or moment arm). The lever-arm acts to multiply the force being exerted. Think of opening a door, if you try pushing near the hinge, it takes a lot more force than pushing form near the handle. The distance from where you push to the hinge is the length of the lever-arm. Again, don't confuse torque with work. Think of torque as an instantaneous push on the door, where as work would be pushing the door over a distance.

Horsepower is how quickly you can apply torque over a given distance. Watt's definition says that 1 hp = 550 ft-lb/s. Now, how do we convert horsepower into something that relates to torque and engine rpm? Well, keep in mind that horsepower is force times distance divided by time, and our distance is going to be in a circle (i.e. in revolutions), so we can do the following conversions (I'll explain everything after the result):

1 hp = 550 lb * 1 ft / 1 sec

1 hp = 550 lb-ft * 1 rad / 1 sec

1 hp = (550 lb-ft * 1 rad / 1 sec) * (1 rev / 2pi rad) * (60 sec / 1 min)

1 hp = 5252 lb-ft * 1 rev / 1 min

1 hp = 5252 lb-ft * 1 rpm

-The first line is the original equation written out to better show the units and values.

-In the second line we want to convert the straight-line work to its rotational equivalent. This involves using radians instead of feet, and torque (lb-ft) instead of just pounds. Radians are actually a unitless measurement of a circular distance, and are abbreviated as "rad".

-The third line converts radians to revolutions and seconds to minutes. There are 2pi, or about 6.28, radians in one revolution and 60 seconds in 1 minute.

-The fourth line multiples the constants and cancels out some of the units, leaving us with a simpler equation.

-The last line just substitutes in the common abbreviation "rpm" (revolutions per minute).

So one horsepower is a force (a torque to be exact) of 5252 lb-ft @ 1 rpm. Now it is easy to come up with a formula for horsepower given engine rpm and torque. Say we have X lb-ft of torque @ Y rpm and we want to convert that amount of power to horsepower. The last equation tells us we can convert from (lb-ft * rpm) units to horsepower units by multiplying torque and rpm and then dividing by 5252:

power = X lb-ft * Y rpm
power = X lb-ft * Y rpm * (1 hp / (5252 lb-ft * 1 rpm))
power = (X lb-ft * Y rpm / 5252) hp

So,

horsepower = torque * rpm / 5252

Now you know exactly how horsepower and torque are related. There are no exceptions to this rule; they will always be related by this formula. In fact, it might be better to see them as just flip sides of the same coin. One result of this formula is that below 5252 rpm, torque will always be less than horsepower, at 5252 rpm they will be equal, and above 5252 rpm torque will be less. Note that a dyno never measures horsepower; it can only measure torque and then use the above formula to get horsepower.

Acceleration

So we now know that horsepower and torque are basically two different views of the same thing - change one and the other most also change. What we are really interested in is how they work together to affect a car's overall acceleration. Using Newton's Second Law, we know that F = ma. So we just rearrange the formula to solve for acceleration:

a = F / m

So acceleration is just the force at the wheels pushing the car forward divided by the mass (measured in kilograms) of the car. The force at the wheels is directly proportional to the torque exerted on the wheels by the driveshaft. Of course, the wheels don't actually move the car forward, the friction from the tires does, and we have to assume in all of this that we're not peeling out like in the picture.

So, our acceleration at any given moment is only dependent on the torque at the wheels (again, assuming that traction is not broken). That means the acceleration of the car is constantly changing to match the torque curve, and the car accelerates hardest at the torque peak. The name of the game is to keep your car producing as much torque as possible, for the longest amount of time possible. That will give you the best acceleration and fastest quarter mile times.

The Transmission Effect

Now comes the tricky part: the transmission. The fact that our engines cannot rev indefinitely means we must have transmissions to allow our wheels to keep spinning faster while keeping the revs under redline. A transmission essentially makes engine torque meaningless because it can multiple your flywheel torque to any amount it pleases, and is only limited by physical gear size. This is because gears can be arranged to increase or decrease their lever-arms and thus change the final torque output.

However, don't go out and buy yourself a giant gear set just yet: transmissions cannot amplify horsepower. This would violate the law of conservation of energy and change the world as we know it. Instead, as we increase the torque output for a certain gear, we decrease the maximum speed that we can have in that gear. Would you want your Civic to have 400 lb-ft of torque in 1st gear but only be able to go 8 mph in that gear? Probably not. The opposite is also true; as you decrease the torque output of a certain gear, you increase maximum speed, assuming you can reach it. That is the main reason why you accelerate slower in higher gears.

The Verdict

So what advantage does a higher horsepower engine have compared to a lower one, assuming they both have similar torque curves? In short, the higher horsepower one will perform much better. The car with the high-reving, high-horsepower engine can run a more aggressive set of gears while still being able to spend the same amount of time in each gear as the slower car. This will give it better overall acceleration because of increased torque to the wheels. If both cars run the same gears, then the high-power one will be able to spend a longer time in each gear, still giving it better overall acceleration. Either way, it is an advantage. If you take away anything from this article, let it be that gearing is extremely important in determining a car's overall performance.

As far as shifting goes, always shift to maximize transmission output torque. It turns out this is exactly the same as saying shift to maximize engine power. Never shift at torque peak, even if your best friend tells you he swears it's faster that way. You will lose the overall higher torque of the current gear and it will also put you in a worse spot in the next gear. Most cars (Civics included) will obtain their best quarter mile times by shifting at redline, but it is not true for every car. For some cars, it is necessary to look at a dyno graph to really see where the best point is.

And that's it. Now if someone claims your Civic can't go fast because it doesn't have enough torque, you can prove them wrong!

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post #5 of 12 (permalink) Old 03-09-2008, 03:20 PM
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We went to the dyno yesterday. Something was wrong and it would read the HP correct, but wouldn't read the torque right...sometimes they got HP readings and no torque so I don't think you need a torque read out to determine HP.

Maybe it just depends on the dyno??

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post #6 of 12 (permalink) Old 03-09-2008, 03:25 PM
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There are a few different types of dynos, but the DynoJet is the most common. This type uses a fixed mass rolling drum, and measures how quickly you accelerate the mass. In this way, you get a measure of work (HP). In truth, the way the equation works in the DynoJet software, Torque is measured, and HP is calculated for the VEHICLE, not the dyno.

Other dyno types employ a method called "eddy current", and they are more accurate and repeatable, but do not offer the same bragging numbers, and are WAY more expensive than the more basic "rolling mass" types.

The dyno software needs a tach signal because HP is a calculation based on actual leasured Tq output of a power source. As others have correctly discussed, HP = (Tq x RPM) / 5252. Without knowing RPM, you cannot calculate either HP or Tq at any given moment. If the dyno software automatically calculates Tq (from the work done accelerating the drum for example), then it needs RPM to calculate vehicle HP. It already knows the Tq, but not HP.

I should add that no two dynos are the same, and you will almost never measure (actually calculate) the same HP on the same dyno on two different days. Hence the saying: "Different dyno, different day".

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post #7 of 12 (permalink) Old 03-09-2008, 06:53 PM Thread Starter
what's egt mean ??
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Quote:
Originally Posted by ralphinnj

Now you know exactly how horsepower and torque are related. There are no exceptions to this rule; they will always be related by this formula. In fact, it might be better to see them as just flip sides of the same coin. One result of this formula is that below 5252 rpm, torque will always be less than horsepower, at 5252 rpm they will be equal, and above 5252 rpm torque will be less. Note that a dyno never measures horsepower; it can only measure torque and then use the above formula to get horsepower.
!
that's what i thought !!!

then how come w/out a tach signal yesterday, the dyno gave me a HP readout but "couldn't give me a torque reading w/out rpm" ???

i would have thought i could have gotten a torque reading but no HP reading ??

either way i did a MPH in my dyno graph, and seeing as how my dyno file always uses the same gear, i compared old dyno sheets that had HP, TQ, MPH, and RPM so that i could tell at what rpm i was at what mph..

then i used the formula (i had the 5252 one) to figure out my torque based on my HP and my RPM

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post #8 of 12 (permalink) Old 03-09-2008, 07:04 PM Thread Starter
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Quote:
Originally Posted by John_G
There are a few different types of dynos, but the DynoJet is the most common. This type uses a fixed mass rolling drum, and measures how quickly you accelerate the mass. In this way, you get a measure of work (HP). In truth, the way the equation works in the DynoJet software, Torque is measured, and HP is calculated for the VEHICLE, not the dyno.

Other dyno types employ a method called "eddy current", and they are more accurate and repeatable, but do not offer the same bragging numbers, and are WAY more expensive than the more basic "rolling mass" types.

The dyno software needs a tach signal because HP is a calculation based on actual leasured Tq output of a power source. As others have correctly discussed, HP = (Tq x RPM) / 5252. Without knowing RPM, you cannot calculate either HP or Tq at any given moment. If the dyno software automatically calculates Tq (from the work done accelerating the drum for example), then it needs RPM to calculate vehicle HP. It already knows the Tq, but not HP.

I should add that no two dynos are the same, and you will almost never measure (actually calculate) the same HP on the same dyno on two different days. Hence the saying: "Different dyno, different day".
again, same thing i've always been told.. but yet somehow with my own eyes i see it: dyno my truck yesterday, no RPM signal.. i got a HORSEPOWER graph printout, but NO TORQUE can someone explain why this is, because every answer i get says "no hp w/out rpm, only torque"..yet i clearly see this is not the case....

2004 6oh...
relatively speaking, it's pretty stock.
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post #9 of 12 (permalink) Old 03-09-2008, 07:14 PM Thread Starter
what's egt mean ??
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Quote:
Originally Posted by HD_F250
We went to the dyno yesterday. Something was wrong and it would read the HP correct, but wouldn't read the torque right...sometimes they got HP readings and no torque so I don't think you need a torque read out to determine HP.

Maybe it just depends on the dyno??
i had that problem last year, i had painted my harmonic balancer w/metalized paint, and the reflective nature of the paint was interefering w/the optical pickup on the RPM sender (reflective tape on balancer)..

i would always get an accurate HORSEPOWER reading, but the torque figures were thru the roof, even at 400HP i could make over 1250FTLBS.. and we all know that's just not what happened...

i see some crazy numbers for some of these dodges and now i'm starting to make more sense of it.. since 5252rpm is the "constant" in the equation, TQ will always be MORE than HP under 5252rpm, and HP will always be MORE than TQ above 5252..

which explains why i can make more HP than the newer PCM trucks, but i will most likely never make the TQ figures they do::
my power peaks at 3000RPM and the newer ones seem to peak around 2000-2200 rpm... the closer you get to 5252 the more the torque drops as the HP rises....
and since the cummins have a much lower redline, and their peak power is usually down around 1600-1800RPM, they will have a stupidly high TQ to HP ratio.. whereas my TQ to HP ratio is only like 1.6 : 1 and a dodge with comparable HP (but at 1900rpm) will have a 2.7 : 1 ratio. torque more than 2.5 times the HP figure ! ! ! ! !

2004 6oh...
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post #10 of 12 (permalink) Old 03-09-2008, 07:25 PM
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Like I said I don't know how it works and don't claim too.

All I know is that they were getting good HP readings, but the torque was way off or wouldn't read.

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