Axle ratios vs dyno numbers?

[Steve M]

2900rpm on the tire or on the motor? 2900rpm on the motor is devided by 4. 1 in my truck before it reaches the tires. 2900rpm motor rpm is almost 70mph in my truck in direct drive. That amounts to alittle over 700 rpm at the tires. 2900rpm at the tires would be almost 300mph.

Horsepower can't be measured anyhow. It's calculated. Torque is measured at the rear wheels. Gear reduction is the same principle as 2:1 roping or simply putting a longer handle on a wrench. Imagine a 4ft handle on a crank to a rock tumbler. You could turn it very easily but not very fast. You apply alot of torque throught the leverage of the long handle. Now picture a 6" handle on the rock tumbler. You could turn it very fast but only if it turns very easily as you don't have much leverage at all. Same with the motor. My 460 advertised ft/lbs of torque is multiplied 4. 1 times at the differential. That with 2. 5:1 reduction in 1st gear gives me a 10. 25 total gear reduction/torque multiplication not including the 2:1 ratio of the torque converter at stall speed. If you could put a torque wrench on my rear axle and hold it while I stepped on the loud peddle in first gear, you would read real close to 4715ft/lbs of torque or higher at stall speed. If you don't believe this, and you believe it's only going to be less than the 460ft/lbs produced at the flywheel, get yourself a good 500ft/lb torque wrench, brace it against something solid, stand back and watch it shatter when the power is applied. Probably not a good idea. Someone would get hurt real bad...

The terminalogy of a chassis dyno is misleading. HP and torque at the rear wheels is calculated, not actual. Actual HP/Torque at the rear wheels would be very different. 460ft/lbs X 700tire rpm /5252=61HP. Pretty depressing eh? But with the torque multiplied through the 4. 10 ratio rear end you'll get 1886ft/lbs X 700tire rpm /5252= 251HP. Makes more sense doesn't it? That's what the dyno sees. Then it uses engine speed to calculate gear ratios. Engine rpm is 2900 while tire rpm is 700~. That means there's a 4. 1 ratio in there somewhere. So the dyno devides 1886ft/lbs by 4. 1 to give you your calculated flywheel torque. 1886 / 4. 1 is 460ft/lbs. 460 X 2900 / 5252 =253~HP.

Personally, I'd like to see the printout read actual tire rpm/mph with actual torque at the rear wheel and then on a seperate line, calculate flywheel hp/torque. Then you'd start seeing the effects of gear reduction on torque and top speed and the trade offs involved.

 

[klenger]

Steve, your explanation make sense. It seems that all engines produce torque, HP is just a side product.

 

[Gary - K7GLD]

Keith sez:



"Steve, what kind of dyno are you talking about? A lot measure tq, a Dynojet INERTIAL style dyno measure HP at the wheels. "



My question further up asks what values the dyno PRINTOUT is typically showing - REAR WHEEL numbers - or an "estimate" of FLYWHEEL HP? I'm not as interested in whether the dyno itself is an inertial type, or some other type, only what they typically provide the customer in a printout - flywheel estimate, or actual rear wheel numbers?



Seems most fellas assume as I have, that those numbers are showing as accurately as the dyno is capable, the actual REAR WHEEL numbers - are we wrong in that assumption?



I specifically asked Ken Imler at Imler performance diesel in Sacramento this question a few years back, and was told the printout was REAR WHEEL numbers (Mustang dyno), and that if I wanted to estimate FLYWHEEL numbers, I should add about 15% to what the printout displayed (Manual transmission).



Now, maybe a Mustang dyno is different in that regard than a Dynojet - or maybe that's just the way he sets HIS up - but it certainly doesn't seem to make much sense to READ HP and torque at the rear wheels, THEN "convert" those numbers to flywheel readings - THEN, if what you are REALLY interested in is rear wheel HP, have to convert BACK again to GET those RW numbrs you really wanted to begin with!



SO - I *still* am unsure if my sig is correct or not, when I claim 311 HP and 725 ft. Lbs. of torque *at the rear wheels*, as indicated on the printout last time I dynoed my truck...

 

[Steve M]

Your sig is correct in the popular meaning of "read at the rear wheels". The goal of most people who run their beast on the dyno is "engine" hp/torque. As said before, it's not practical to remove the engine and put it on a stand so the torque readings are taken from the rear wheels, devided by ratios, compared to engine rpm, and your flywheel hp/torque is calculated. Reading torque at the rear wheels includes driveline losses so the operator is correct in the estimated adjustment.

I simply want to know the power output of the intire package, not just flywheel hp/torque.

 

[Will24]

HP is not a "side product"





Hp is PROPOTIONAL to Tq. Hp is a calculation made based on torque and rotational speed



It's not possible for torque to be higher than HP above 5252 RPM either





HP = (Tq. * RPM)/5252











SO let's say we have (535 ft lbs * 2800 rpm)/5252 = 285. 22 HP

 

[Gary - K7GLD]

For a bit of clarification to a few who might not have considered it...



An individual steps on a bathroom scale - the scale says they weigh 150 lbs. - is that person overweight or underweight?



AH - you say, it depends on how tall that person is, are they fully clothed? Large or small boned? Male or female?



The scale is merely an INSTRUMENT - it reports downward force, and is totally unmindful of WHAT is placed upon it...



A DYNO is exactly the same - an INSTRUMENT that is designed and built purely to measure rotational force and speed applied to a known load - it is TOTALLY indifferent to vehicle weight, engine RPM, tire sizes or gear ratios - THOSE various parameters are ONLY required in order to plot the force applied to the dyno rollers, and then graph them at various engine speeds - and actually, the only one of them really needed for rear wheel power developed vs engine speed, is engine RPM.



The dyno doesn't care in the LEAST what the engine speed is, or gear ratio - or tire size - all it knows is how long it takes to accellerate those rollers to a given RPM, and how much rotational force is applied to those rollers!



Two separate vehicles MIGHT accellerate the rollers equally fast and at identical speed - but differences in engine RPM, and the other factors indicated above will probably give 2 entirely DIFFERENT dyno graphs!



It's pretty much impossible to absolutely KNOW for a fact all the various elements of driveline losses and drag. even if we DO know the gear ratios and other factors - and that's why for MY purpose, and for my money, a pure reading of force and speed of accelleration of the dyno rollers plotted graphically against engine RPM *alone* are most reliable, meaningful and repeatable - *rear wheel horsepower. *



Attempting to plug in a "one size fits all" calculation to then provide an ESTIMATE of power at the flywheel MIGHT be pretty far off, and vary significanly from one time, vehicle or dyno to another, depending on formulas used and driveline changes occuring or made that have NOTHING to do with performance mods!



Do dyno operators change calculation depending on whether the truck is 2 or 4 wheel drive - with differing drivetrain losses? What about hot or cold temperatures, and resulting lubricant drag differences? An air conditioner accidentally left on - or an alternator that happens to be in full charge mode might make noticeable differences in power output - how do you create an equation for those... ?



Maybe MY brakes are dragging, and the newness of my truck creates enough parasitic drivetrain losses to absorb 75 HP by the time my power reaches the rear wheels - maybe YOUR truck, better setup and with more break in miles, will only waste 25 HP - but the power at the rear wheels, plotted against engine RPM, *will* provide a meaningful rear wheel graph you can take to the bank, REGARDLESS of other unseen or uncalculated variables, or differences between our seemingly identical 2 trucks!



WHICH is most important to YOU - the somewhat imaginary power you THINK your engine is making at the flywheel - OR the power you actually KNOW and can SHOW is being applied to the pavement?



Just food for thought...

 

[Steve M]

"THOSE various parameters are ONLY required in order to plot the force applied to the dyno rollers, and the graph them at various engine speeds - and actually, the only one of them really needed for rear wheel power developed vs engine speed, is engine RPM"



Interesting article. It states what I've been trying to say. They take actual torque at the rear wheels and actual engine rpm to "calculate" engine HP/Torque. And what I would like to see is an uncalculated print-out of actual torque at actual road speed at the rollers. It happens right there on the dyno every time someone runs but doesn't make the print.



The bathroom scale is a good example. I want to know how much I weigh. So I step on the scale and out comes a print out of body fat content. Ok great but how much do I weigh? Oh it's right there see, you're at 40% fat content. You weigh too much. Ok but how much? 40% too much. So I throw the bathroom electronic scale out the window and buy an old fashioned balanced weight scale so I can see how much I weigh.



The torque reading on the dyno print out is calculated using engine rpm to compared to roller rpm to determine gear ratio and calculte the calculated torque.



What's the torque value of my lugnuts? Well, that depends... How fast will you be spinning the wrench?

 

[ronco]

I will throw my two cents in here. From my limited experience, the trucks, being equal except for axle ratio, will show more power/tq on the 3:55 geared truck. Why? you ask? Because the taller gears will enable the dyno to load the engine more so than the lower gear ratio! For those that have been on a dyno, remember the boost being lower on the dyno than that long hill near your house? The dyno is just not capable of loading these diesel engines to the point of completely accurate power measurement. The only way a turbo-charged engine can make max power is thru drivetrain load. Not in the drive way with your foot on the firewall!



Now with that said, the dyno can compare vehicles to determine who makes more power. But, most important to me, the dyno can tell you if your last box, injector swap, oil change, sticker application or bedliner is making more power or not.



Don't spend too much time worrying about what numbers somebody else made last week compared to what you made last month on a different dyno. Dynos are man made and are calibrated differently by different men. THEY VARY! If you need to compare yourself to somebody, use the same dyno. If you are tuning your truck, use the same dyno. All trucks strapped to the same dyno on the same day and loaded the same will produce accurate representations of power. PERIOD! This is why baseline numbers are so important before making changes!



I understand why performance manufacturers may be hesitant to publish dyno numbers. If you dyno two identically prepared trucks on two different dynos, you will get different numbers. You cannot call the manufacturer and complain about that.



Ok, I am off my soapbox now.



Ronco

 

[jarsong]

Originally posted by ronco





Now with that said, the dyno can compare vehicles to determine who makes more power. But, most important to me, the dyno can tell you if your last box, injector swap, oil change, sticker application or bedliner is making more power or not.



Don't spend too much time worrying about what numbers somebody else made last week compared to what you made last month on a different dyno. Dynos are man made and are calibrated differently by different men. THEY VARY!

Ronco

Good Post Ronco. Now I have always understood that Rear wheel dyno numbers are 10-15% less than flywheel. So a ETH/DEE at 245 would be somewhere in the 220's on the typical dyno. So in that sense I would think it's reading rear wheel HP but using the engine RPM for reference as to what RPM is was made at. Would the manufactures be false advertising if this wasn't the case? This question has run through my bean for some time...

Jarsong

 

[KLockliear]

Originally posted by Steve M

2900rpm on the tire or on the motor?



Horsepower can't be measured anyhow. It's calculated. Torque is measured at the rear wheels.

On a chassis dyno, RPM is measured at the engine. Sorry if I wasn't clear here. 2900 engine RPM.



Horsepower is measured on a Dynojet style inertia dyno, not torque. If you are talking about some other kind of dyno, OK, but not on a Dynojet style. I will assume that's what we are talking about here since this is what most of us use?

Originally posted by Gary - KJ6Q

Keith sez:



"Steve, what kind of dyno are you talking about? A lot measure tq, a Dynojet INERTIAL style dyno measure HP at the wheels. "



My question further up asks what values the dyno PRINTOUT is typically showing - REAR WHEEL numbers - or an "estimate" of FLYWHEEL HP? I'm not as interested in whether the dyno itself is an inertial type, or some other type, only what they typically provide the customer in a printout - flywheel estimate, or actual rear wheel numbers?

Gary, I am going to clarify this - my comments only apply to a Dynojet style inertia dyno. May not be the same on other brands.



The printout shows rear wheel horsepower. If you also measure engine RPM, it can calculate the flywheel torque. It's flywheel because that's where the RPM measurement is being taken.



As Steve has suggested, you could hypothetically measure wheel RPM, then do the math to convert it back to engine based on the gear ratios. I don't know why you'd do that, but you could.

 

[Steve M]

"Horsepower is measured on a Dynojet style inertia dyno, not torque. "



I'm not trying to be argumentive. If I can learn something from all this, I'm all ears. But I've always been under the impression that HP is a calculated expression of the rate at which torque is applied. Torque and speed are measured and HP is the result.



"The printout shows rear wheel horsepower. If you also measure engine RPM, it can calculate the flywheel torque. It's flywheel because that's where the RPM measurement is being taken. "



If you crunch some numbers, you'll see that HP will follow right through the driveline to the rear wheels. So rear wheel HP will be the same as flywheel HP relative to engine rpm in any gear minus driveline friction. But torque multiplied through gear reduction will increase as rear wheel speed decreases with the same engine rpm. So at 70mph, my 4. 10 rear ratio should be putting about 14% more torque to the pavement than the same truck with a 3. 55 rear end. I'd like to see that result on a dyno printout. Maybe I'm the one who's looking at it backward. I could take HP, rpm and road speed and calculate torque at a given speed and gear, but it would be neat to be able to run on a dyno at a given speed in different gears to see how much torque is applied in each gear at the same speed. Maybe I'm thinking of a water brake type dyno to do something like that.

Is an enertia type dyno capable of constant speed or is it accerleration only?

 

[Gary - K7GLD]

"The printout shows rear wheel horsepower. If you also measure engine RPM, it can calculate the flywheel torque. It's flywheel because that's where the RPM measurement is being taken. "



I also respectfully disagree with the analysis above that claims that *HP* will be graphed as a value of the power sampled at the rear wheel vs engine RPM -while using the SAME TWO base events, HP and RPM, will NOW reveal torque at some OTHER distant point in the drivetrain.



Logic would seem to dictate that BOTH HP and torque are occuring at the same point - the dyno wheels, and using readings obtained at THAT point vs measured engine RPM are giving both HP *and* torque as applied at the rear wheels, NOT at other various points in the drivetrain...



And Steve, the Dynojet (as *I* understand it!) purely measures the TIME it takes to accellerate a weighted roller to a given speed, then calculates the HP it takes to accomplish it, and again the formula using known engine RPM vs rear wheel HP will reveal the torque also applied - so no, if it was merely spun at a continuous single speed, no meaningful data would be available - that's where a brake type dyno is valuable, it will provide an adjustable load on the wheels at whatever speed desired, and then the force applied to the dyno drum can be measured and displayed.



Sure would like some more dyno experts to weigh in here - us amateurs are about exhausted... .

 

[KLockliear]

Originally posted by Steve M

I'm not trying to be argumentive. If I can learn something from all this, I'm all ears. But I've always been under the impression that HP is a calculated expression of the rate at which torque is applied. Torque and speed are measured and HP is the result.



If you crunch some numbers, you'll see that HP will follow right through the driveline to the rear wheels. So rear wheel HP will be the same as flywheel HP relative to engine rpm in any gear minus driveline friction. But torque multiplied through gear reduction will increase as rear wheel speed decreases with the same engine rpm. So at 70mph, my 4. 10 rear ratio should be putting about 14% more torque to the pavement than the same truck with a 3. 55 rear end. I'd like to see that result on a dyno printout.



Is an enertia type dyno capable of constant speed or is it accerleration only?

With a brake dyno, you are correct from what I know. With an inertial dyno, HP is recorded, TQ is calculated. This is why without RPM pickup, the dyno can still deliver a HP vs. speed graph. With RPM, you can then have RPM and TQ graphed.



I agree that the 4. 10 geared truck will have more rear wheel tq than a 3. 54 geared truck... but we are trying to get as much of the variables out of the equation as possible. RPM is measured at the engine, so the calculation of TQ = (HP * 5252 ) / RPM is only true to the engine. As I mentioned before, you COULD measure wheel RPM and get that figure, but what good does it do you? Also, the dyno limits are usually 1200ft-lbs on the early ones, 1500ft-lbs on some and 2000ft-lbs on the rest. With some trucks making 600+tq at the engine, multiply that by a 4. 10 gear and you'll have more than the standard dyno is designed to graph.



The beauty of the inertial dyno is that the load is a constant (not user adjustable) so it is the same every time you get on. It's an acceleration run only (for the most part - there are a few load style out there as well), which gives a repeatable, consistent graph each time.

Originally posted by Gary - KJ6Q

"The printout shows rear wheel horsepower. If you also measure engine RPM, it can calculate the flywheel torque. It's flywheel because that's where the RPM measurement is being taken. "



I also respectfully disagree with the analysis above that claims that *HP* will be graphed as a value of the power sampled at the rear wheel vs engine RPM -while using the SAME TWO base events, HP and RPM, will NOW reveal torque at some OTHER distant point in the drivetrain.



Logic would seem to dictate that BOTH HP and torque are occuring at the same point - the dyno wheels, and using readings obtained at THAT point vs measured engine RPM are giving both HP *and* torque as applied at the rear wheels, NOT at other various points in the drivetrain...



And Steve, the Dynojet (as *I* understand it!) purely measures the TIME it takes to accellerate a weighted roller to a given speed, then calculates the HP it takes to accomplish it, and again the formula using known engine RPM vs rear wheel HP will reveal the torque also applied - so no, if it was merely spun at a continuous single speed, no meaningful data would be available - that's where a brake type dyno is valuable, it will provide an adjustable load on the wheels at whatever speed desired, and then the force applied to the dyno drum can be measured and displayed.

Well Gary, I'm not going to go into the long version of this (they don't pay me to tech support the Dynojet dyno anymore ), BUT the Dynojet will measure rear wheel horsepower. The simple equation is HP = Mass * Acceleration. We know the mass (inertia of the dyno drums) and we measure the acceleration (change in speed over time). With these known variables, the math can be done to display HP vs speed or time. These are the variables collected.



While both HP & TQ are occuring at the same point (dyno drums) it is also occuring at another given point (flywheel). Look back to my calculation of TQ above... it REQUIRES RPM. In the case of our dyno runs, we measure it at the engine/flywheel. So this TQ value is representative of what the engine is making, since this is where the RPM is recorded from. Make sense?

 

[Gary - K7GLD]

Thanks for your patience with us "students" Keith!



I'm curious about this last statement tho':



"1200ft-lbs on the early ones, 1500ft-lbs on some and 2000ft-lbs on the rest. With some trucks making 600+tq at the engine, multiply that by a 4. 10 gear and you'll have more than the standard dyno is designed to graph. "



Are we to understand from that statement - especially the last part, that if a flywheel torque of 600 ft. lbs. exists, and the power is routed thru a 4. 10 differential, there would be approximately *1600 ft. lbs. * of torque to the pavement?



Seems that would MAYBE be true IF the outer tire diameter was only 12 inches - but would vary coniderably depending on tire diameter...



This may be a bit of a clue to some earlier issues...

 

[KLockliear]

Hmmm, maybe my calculator is messed up



600 * 4. 10 = 2460 for me.

 

[klenger]

Ft lbs at the tire is not dependent on tire size, but is dependent of gearing between the engine and rear axle. Ft Lbs is the the product of Feet * Lbs and is the same whether you have 2 lbs at 1 foot or 1 lb at 2 ft. They are both 2 ft-lbs. Actual force to the ground in lbs IS dependent on tire size, which is why bigger tires are slower off the line.

 

[Gary - K7GLD]

"600 * 4. 10 = 2460 for me. "





AW shucks, GOT me on that one - must be the "old" math!

 

[Gary - K7GLD]

"Actual force to the ground in lbs IS dependent on tire size, which is why bigger tires are slower off the line. "



And THAT is exactly the point I was working my way towards - tire size and gear ratio are only TWO of the complicating variables between the flywheel and dyno wheels that, in my view, make it totally impractical and maybe impossible to accurately determine FLYWHEEL torque based purely upon force applied to the dyno wheels vs engine rpm.



A mere inch difference in outer tire diameter might make a significant difference in the calculation - and truth is, I don't remember a dyno operator EVER measuring tire size or checking differential ratio before making a dyno run, which would SEEM to be absolutely NECESSARY if an actual translation of rear wheel vs flywheel torque was to be made - maybe something else I missed



ON the other hand, using a tire diameter of 32 inches, and a gear ratio of 3. 55, a 600 ft. lb. engine SHOULD register somewhere in the vicinity of 1640 ft. lbs. at the rear wheels (of course, I've already publicly displayed how good MY math is... )- and that is certainly more than the 725 or so MY truck registered - so I surrender, and Kieth wins...

 

[KLockliear]

Remember that with an RPM pickup at the engine, the computer now has a calculated gear ratio of RPM per MPH. This means that for every so many RPM change, the MPH changes 1 mph. That's the overall drive ratio the dyno calculates. It will do this for any/every gear the truck is run in.

 

[Gary - K7GLD]

Well, I guess the GOOD news from all this is, again using calculations of gear ratio and tire size, without figuring drivetrain losses, I *should* have about 1980 lbs. of torque to the pavement at 1900 RPM - or about 65 MPH in my truck at full snort...



Shucks, THAT sounds LOTS better than 725 ft. lbs anyway!