Dodgezilla

I can't quite grasp how it can measure two things at once.. Also, how do you figure the correction factor when setting up the dyno? Lastly, is there a formula for figuring what your numbers equate to at the crank?

Inquiring minds want to know..

jrs_dodge_diesel

I believe that HP and TQ are directly related to each other. And if I remember right (more knowledgable people will chime in here), HP is a product of torque and RPM.

Shovelhead

Torque is measured, HP is a mathematical equation based off the torque figure.

wannadiesel

Correction factor is based on the air pressure and temp vs. standard pressure and temp.

The inertia dynos figure HP based on the amount of power it takes to accelerate the drum. The computer program knows the weight of the drum, and it knows how quickly your truck accelerated it, so it just calculates the power the truck has based on that information.

Like Ed said, HP is an equation. HP = TQ x RPM / 5250
Knowing this, you can see why diesels have such huge torque numbers even if HP is relatively low, and why gassers can make lots of HP with very little torque.

Add 15% to your rear wheel numbers and you'll be pretty close at the flywheel.

Dodgezilla

Thank you gentlemen....... I have been enlightened.

Pop-Pop

You've got it right. My old Professor used the formula to calculate HP as: grunt, grunt grunt 19TS. (Horsepower = .00019XtorqueXspeed). Of course you've gotta get the units right.

04ctd

http://www.land-and-sea.com/eddy-cur...ynamometer.htm

has a lot of tech that is hard to understand.

this looks better:
http://www.powroll.com/tech_specs_dyno.htm

the % is different by platform, camaro's used to be 10% for stick,
15% for automatic,
but a mustang automatic was 20% due to the high loss thru the AOD.

an eddy current, vice a water brake, has a higher correctoin factor.
262hp x 1.31 = 343 hp
is the only dyno sheet i have laying around, that's an Eddy, for a manual.
it said 1.37 for an automatic.





5 ways to cheat on dyno:

Dyslexia. Manually enter in the air density and temperature. If you want a high horsepower number, enter in high temp and low air density.

Gearing. Most dyno runs are done in 3rd gear. Drop the bike into 2nd or 1st. The numbers can be nice and big.

Overcooling or overheating the engine. Making a dyno run when the engine is still cold will reduce the hp numbers, just as running an engine too hot will normally do the same. This trick is used on the 'before' runs, where the runs on the 'after' tests are done at regular operating temperature.

Gas. Filling up with an oxygenated fuel like Nutek can add 3%. Run pump gas on your first runs, then the oxygenated fuels for your 'modified' runs.

Build a big blower. Install the dyno in the center of a large building. Now you need to get clean air in, and exhaust air out -- so build a sealed dyno room, then an air intake system with large fans, and run a hose that clamps to the exhaust system on the bike with more fans for it. You can then add air conditioning units to the incoming air. Now, if you want, you can increase the fan speed of the incoming air -- this will basically force more air into the sealed room, then into the engine and increase power (basically a blown engine!). Expensive, but some shops think it's worth it.

Hannibal

The way I've been learned, an engine dyno measures torque and hp is calculated using torque X rpm / 5252. On a chassis dyno, hp is measured and torque is calculated using hp X 5252 / rpm. Horsepower is constant reguardless of gear ratios. There's too many variables to try to measure torque at the rear wheels such as torque converter, gear ratios and tire diameter. Horsepower is simply the amount of work done over time such as how fast we can climb a grade towing a load. Diesels make far more torque at lower rpm and therefore make far more horsepower at lower rpm. So yes, it's the torque. It's the torque that makes all that horsepower. And diesels make more of it. My Hemi makes more hp than a 610 Cummins. At 5400rpm. Totally useless for practical comparison. The 610 Cummins makes over 200hp at 2,000rpm. I have to be at 3,000rpm to make the same hp. I need to be in 2nd gear at 4200rpm to make the same 300hp the 610 Cummins makes at 2800rpm. Gear ratios multiply torque to the rear wheels to equal that of the Cummins at the rear wheels. But you can see I'll be using a lot more fuel to get the job done and won't be doing it for a million miles. The Cummins is a lot more practical and is the prefered method of producing the necessary horsepower to get the heavy stuff from here to there.

Sidewinder

Torque is measured directly at the rollers. That is why if you cannot get RPM information you can still get a TORQUE/MPH graph off of a dyno. To get the HP you need to know the RPM. Some dynos do not have the equipment to measure the RPM of a DIESEL (no spak plugs) so you only get a torque/MPH graph.

BigBlueDodge

Then how come some dyno's give HP without giving torque ? I want to a dyno event with a Mustang dyno, and the optical pickup wasn't working, so they could only give HP numbers. I couldn't understand why they couldn't derive torque.

Hannibal

Not adjusting for driveline losses and using advertised torque output, you 600ft/lb Cummins would be putting out 600ft/lbs at the flywheel, then multiplied by gear ratio in the tranny, most likely 1:1 in direct, then 4.10 if you have a 4.10 rear end, then reduced by the more than 12" radius from the center of the axle to the surface of the tire on the drum. You would come up with around 2400ft/lbs of torque at the rear axle. A chassis dyno measures HP as the drum is a rolling road, not a twisting force. It doesn't care about gear ratios or rpm. It only measures how fast you accelerate the drum. That's why with no attachments to the vehicle you can still measure HP. With the rpm of the motor monitored, the chassis dyno can calculate flywheel torque. An engine dyno measures torque and compares this torque to the rpm it's turning to calculate horsepower.

04ctd

as hinted above,
the dyno hooks up to a spark plug (never dyno'ed my diesel) and knows the RPM of the engine, the gear ratio of the tranny, and the rear end, and the tire size.

i dyno'ed at a one man shop, and IIRC, we did a "roll" thru each gear to calibrate the system to all those parameters, and then we dyno'ed in 3rd to maintain roller speed below some limit.

so, a dyno does know everything, like Big Blue said, if it don't know everything, it can't correctly calculate the data.

Hannibal

How right you are. All it takes is the rpm of the engine compared to the rpm of the rear wheels to figure overall gear ratio. In drive with a 4.10 ratio, the engine will be turning 4.1 times faster than the rear wheels. So the measured (horsepower X 5252/rpm)/overall ratio, calculates flywheel torque. The dyno does this throughout the run. Horsepower is constant while torque changes with every gear change and must be calculated to reflect flywheel torque. In first gear with the manual, you'll be putting over 12,000ft/lbs of torque to the rear axle if you nail the loud peddle. It'll either spin the tire, break the axle or, you'll snap your neck back and leap up to 15mph in an instant. But while the dyno sees 12,000 ft/lbs of torque, it's at a very low rpm at the drum. Even the Cummins doesn't put 12,000ft/lbs out at the flywheel. So the dyno compares rear wheel hp to engine rpm and calculates flywheel torque.

staarma

An inertia chassis dyno is much less complicated than that. HP is measured using time, distance and weight. The distance is the circumference of the drums, the weight is the weight of the drums and the time is provided by the vehicle which is why the dyno knows what gear ratio you have for a final. So, the dyno knows that after a run you just accelerated X=weight for X=distance in X=time and displays what HP it took to do all of that because of one (I believe it's the 2nd) of Newton's laws that said Force = Mass x Acceleration. Next, if you measured RPM then the dyno can calculate TQ based on the equation of HPX5252/RPM.

That's the reason you can get a HP figure without RPM. If there is no RPM then you can't get a TQ reading. However, if you know what RPM is approximately peak TQ and you know at which MPH you are turning at that RPM in the same gear you ran on the dyno then you can take the HP at that MPH on the graph and run it through the formula to get a torque number you just won't have the whole TQ curve on a pretty printout. BTW, the TQ on a chassis dyno is at the output shaft and HP is at the rear wheel. In order to measure TQ at the rear wheel you would have to measure tractive forces through a torque cell and then you could display that TQ number but it does us no good because it changes with each gear and many other factors. HP will change with gear when measured at the rear wheel due to acceleration rates and moment of inertia but that's a different discussion and it doesn't typically change by that much. The best gear for running on a chassis dyno is the gear which gives you a 1:1 ratio which on a NV5600 is 5th gear.

A load based chassis dyno measures TQ using a TQ cell and calculates HP by TQXRPM/5252. Basically you have a eddy current electrical, water, or hydraulic brake that provides resistance to the drums and a torque cell is mounted between the brake and the frame work so it can measure how much resistance is being put on the drum set. This resistance is measured from a torque cell and is measured as torque then HP is calculated using the above explained formula.

Either dyno principal will require RPM to get HP or TQ.

Correction factors are another part of the measurment. Absolute pressure, temperature and humidity are measured during runs and then compared to an SAE (society of automotive engineers) spec. The difference is calculated to a Xnumber that when multiplied to the measured HP and/or TQ would give you a "corrected" number meaning to say that if your vehicle was ran during the "best" weather conditions then this is what your numbers would be. So, if the correction factor is say...1.06 then for every 100 HP your number would get 6 HP added to it. If you made 300 hp then you would take 300X1.06= 318 Corrected HP. This is a little tricky on a forced aspiration vehicle such as our turbo'd engines because they compensate for air pressure by creating PSI with a turbo. That's why most dyno places will give you "actual" numbers on your graph. There is a correction factor used for forced aspiration but it is not published by SAE and most dyno companies will not use it in their math because they want to compare as close as possible with other dyno companies.

There is no way to "backwards" figure your crankshaft HP from your rear wheel numbers unless you dyno'd your whole truck on a chassis dyno and then pulled the engine and ran that engine on an engine dyno. Then and only then could you say for sure what the % difference is. However, the rule of thumb on average is that standards are about 18% loss to the rear wheels and auto are about 22% loss to the rear wheels from the crankshaft; obviously there are lots of variables that crush that rule of thumb. It doesn't really mean anything though because what counts is what is going to the ground and that is what's being measured by the chassis dynos.

Hopefully this clears a few things up. Next time you run your truck, run your numbers through some of these equations to see how they work and check your math. It's kind of cool.

Dodgezilla

Dang Staarma! That's deep......