s13lover

I am having friendly argument with one of my professors about calculating horsepower at the wheels. I dyno'd my 240sx last semester and got peak reading of 102 hp and 163 lb/ft at the wheels. Do these numbers seem odd? I think so. A factory 240sx makes approximately 125 hp and 135 lb/ft to the wheels. It seemed funny to me that my car makes 28 lb/ft more than stock, but 23 hp less.

I know all about how a dyno works because I've been trained to use them. I figured there was an error in the software for the measured torque to respective horsepower calculation. To test my theory, I took several of my data points and calculated the horsepower by hand using: HP= (torque x rpm) / 5252. Using this equation, I got results that average about 35% higher than the dyno calculations. My new peak wheel horsepower was 137, which seems to make a lot more sense.

Now comes the argument. My professor feels that the equation I used is not applicable because it was designed for converting measured torque at the flywheel. I agree that the equation is meant for that purpose, but think that it shouldn't matter. If that equation finds a car to have 20% more hp than torque at the flywheel, would that car not still have the same 20% more ratio at the wheels? He believes that the dyno is correct and that I would need a horsepower equation which contained the rpm signal from the roller sensor on the dyno (not the engine rpm), my final drive ratio, and my wheel diameter. I do not know of any such equation.

carbonRX8

iTrader: (1)

I think your prof needs a new job. Perferably in the janitorial services area.

zoom44

the dyno measures the hp at the wheels and the calculates the T. where did your new hp number come from?

what kind of dyno?

carbonRX8

iTrader: (1)

I hate getting into these discussions cause they are endless, but drum type dynos dont measure ANYTHING except for the RPM of the drum. EVERYthing else is a calc. Most engine dynos you could argue measure HP directly, esp impellor type.

zoom44

yes you are correct- it measures how fast the drum is spun. but it is still calculating HP from that measurement AND THEN calculating the T based on the HP. how did he get a new HP number?

carbonRX8

iTrader: (1)

If I understand. He took the graph ang guessed at # then applied the equation and saw that his # did not match the HP graph. Again, I could be wrong here. If that is what he did, the equation applies as long as you say "to the wheels" Torque to the wheels, HP to the wheels.

EDIT: actually it reads as if he had the raw # and did the calc from those.

s13lover

It was a Mustang Chassis Dynamometer, which is an electric brake dyno. The dyno puts a specific resistance on the rollers based off of the hp required to keep the vehicle moving at 50 mph and the vehicle's weight. This is done via the PAU, which acts like a generator to create resistance on the rollers. There are two sensors which monitor the rollers. One is a magnetic pickup which reports roller speed (used for ¼ mile runs or test that require a vehicle to maintain a specific speed…like a 50/15 emissions test). The other is a load sensor which reports how much force is absorbed through the PAU. Since the PAU is magnetically pushing in one direction, but is magnetically pushed back on when the rollers are in motion, the PAU case physically rotates. The degree of rotation is converted into a respective torque value. This number is then calculated into a horsepower figure. No dyno measures hp, it is simply a calculated value.

Basically, what I'm asking is if a torque to hp equation at the motor will work for at the wheels?


Edit: becasue torque is measued and hp is calculated, I took the measured torque value of the dyno and calculated hp by hand to see if it match what the dyno calculated. My hp # where 35% higher.

carbonRX8

iTrader: (1)

That sounds like a different question. Are you asking if you can convert torque "at the wheels" to HP "at the wheels" and use HP=(tq*rpm)/5252?

zoom44

the T in this case is the T applied to the PAU by the roller. the calculation is done to find how much power the car has to applyto the roller in order to get that T reading from the PAU. then the T of the car is calculated from the HP of the car.

is that correct?^

r0tor

iTrader: (1)

your professor is right

s13lover

Yes.

s13lover

Torque, like you know, is rotational force. The force exerted on the rollers by the car's wheels is much greater than the force exerted in the opposite direction by the PAU. The PAU deflects by rotating. That rotational force is the car's measured torque. Horsepower is simply a unit of measure. Therefore, horsepower is a product of torque; it cannot be measured.


As a side note, this is how you can "cheat" a dyno. If you tell the PAU not to put any resistance on the rollers, it will not rotate because it is not connected to the rollers via magnetism. This will yield very high hp #'s but super low torque #'s (to a point). The opposite is also true. If excess resistance is exerted by the PAU, then you get lower hp #'s but higher torque #'s (to a point).

r0tor

iTrader: (1)

power is conserved through gearing while torque is multiplied through gearing...

so if you have 200 hp at the crank at 5252 rpms at the crank, then you have 200 lbft of torque at the flywheel...

go back to the wheels and go through a 2.5:1 gear and a 4:1 rear gear at the same engine rpms... you now will show 2000 lbft of torque at the wheels (assuming no losses for ease of calculation), but your speed is reduced by a factor of 10 so the rpms of the wheels is only 525.2 rpms - so you still have 200 hp

as you can see you can't take wheel torque and multiply for engine rpm

zoom44

wouldnt this

be the cause of your issue in the first place? why bother with the numbers at all? they are suspect until the dyno is calibrated and so shouldnt be used to derive anything with any of them

s13lover

There are two calibrations for the dyno. One involves hanging a specified weight on the end of a specified length of poll on the side of the roller to create leverage. This is done for base calibration and is done frequently. The other calibration comes from the data entered into the system (vehicle weight and hp at 50 mentioned above). Those numbers came from a federal guidebook so I hope they are right! To cheat the dyno you manipulated those numbers. For example, if I put my car weight in 200 lbs less that is should be, the PAU won't make as much resistance. Torque will go down some, but hp will go up some. If I only show the hp graph, no one will suspect a thing.

zoom44

but you are suggesting the software is not correct. so the numbers you got cannot be used for anything until that is corrected, thats what i meant by calibrated. the numbers are bogus so forget calculating with them.

s13lover

I think I understand what you are saying, but how would I enter that into an equation. I have a measured torque at the wheels of 143.75 at 3000 rpm and am running through 1:1 and 4.083:1 gears.

s13lover

What is important for my testing is the comparative values before and after a modification - did the product statistically make a difference; I don't need to know how much. So it really doesn't matter if the figures are off, since they are consistently off. I just want to find the true hp for my own desire.

Asmoran

Looks like you answered your own question to me. The Dyno wasn't calibrated correctly.

zoom44

So the Dyno is measuring the force applied by the drum against the strain guage and the speed of the drum.

so force(T the drum applies against the strain guage) x speed(rpms of the drum) is work over time. Thats Horspower. the dyno is not measuring the car at all. but the first thing it can calculate is the Horsepower the car is applying to the drum.

Then if you know the RPM of the car you can calculate the Torque of the car using

HP=(tq*rpm)/5252


you cannot calculate to find your "true hp" because the Torque number you want to plug in to the equation is not true.

s13lover

Funny, I emailed Mustang and got a reply from the same guy. However, he gave me a different answer. I ask him if my numbers looked odd and if the math I was using was correct. His response was simply that he needed more information about the edition dyno software I was using before he could make any conclusion.

It seems that if I could get the torque of the drum (not the torque of the car which is what the software outputs) and the speed of the drum then I could use my original HP formula. Now I just have to figure out how to pull those out of the software, or if I even can.

zoom44

hehe

i furthur asked

to which he replied

so yes if you had the T measured by the deflection of the pau and the rpm of the drum (actually the shaft is measured) then you could calculate the HP of your car and then with the rpm of your car calculate your T

s13lover

Thanks Zoom44!