What does RWTQ actually show?

I see a lot of people posting dyno charts and claiming RWHP and RWTQ. It's not just people, but also the people who run the dynos using the term.

Correct me if I'm wrong here but actual rear wheel torque should be a very high number. It should be crank torque multiplied by the gearing and divided by the radius of the rear tire in feet. My BMW that makes 240 crank, if dynoed in 5th gear(1:1), should put down 703.2 rear wheel torque after I multiply the crank torque by the gearing(2.93) and use my standard 24" tall tires.

So on the dyno we're seeing a RWTQ number that is close to the expected crank torque number. I assume that the dyno is doing (RWHP*5252)/RPM. Is that what's going on? If so, that number doesn't really mean anything does it?

 the crossing point on a graph of torque and Horsepower is the 5252 RPM like you post, but Torque falls fast above that mark and HP goes on up. really most have reached highest torque by 3500 to 4500 rpm . My BB Chevy makes most torque by 3500 rpm but most Power at 7000. 

  Horsepower will take you OVER the Mountain  But Torque will MOVE the Mountain. 

In reply to GTXVette :

HP is how FAST you hit something...TORQUE is how FAR you drag it.

I am not a dyno operator, but if they effectively know the gear ratio, trans gear ratio of the gear the pull was made in, and tire diametee it’s pretty simple for the computer to work backwards to figure out how much of the engine’s torque output is making it to the ground with simple division.

They dyno software takes your engine RPM and the drum RPM and calculates out the driveline gear ratio.

TQ in general is REALLY confusing when thinking about engines, because TQ has no time component. I can apply 2000 ft lbs of torque by hanging on a 10 ft bar, but me and my 10 ft bar will not get your honda down the 1/4 mile in 10 seconds. As you state, gearing vastly changes torque, but does not affect HP.

When thinking about engines, HP (or kW, or whatever unit you like) is what you are looking for (at whatever RPM you are looking to optimize for), and TQ is kinda the marketing term for HP an engine makes at speed less than 5252 rpm.

I realize a lot also has to do with the Dyno. Some measure torque and and calculate HP, others measure HP and calculate torque. 

Horsepower will take you OVER the Mountain  But Torque will MOVE the Mountain. 

HP is how FAST you hit something...TORQUE is how FAR you drag it.

I've always hated these analogies and others like them. It's just... wrong.

Its all in the math.  Using wheel torque is a method of finding out engine torque, and the difference is in frictional/drivetrain losses.  The gear ratio is factored and removed from the equation.

I thought all dyno runs were done in 1:1 gear, typically fourth or fifth in a manual trans, third in an old four or three speed auto.

Am I wrong, what trans doesn't have a 1:1 ratio in it somewhere.  Should be the gear with the least loss therefore the most accurate with one less variable to compuguess.

"rear wheel torque" is the torque measured at the rear wheels, divided by the gear ratio in the drivetrain.  So it's showing you crank torque minus the drivetrain losses.

To look at the two main dyno types I've used, an inertial dyno like a DynoJet measures power directly, by monitoring the speed of a rotating drum of known mass.  It also uses a spark lead pickup to measure engine RPM and then calculates torque using those two values.  OTOH, a DynaPack is bolted directly to the hub and measures torque using a load cell.  It also measures wheel speed (so that it can calculate power) and the user configures the rear end ratio in the software so that it can generate a torque curve against engine RPM.  In principle the DynaPack doesn't need a spark pickup although I think they usually have one anyway, I'm not sure why.

codrus said:

"rear wheel torque" is the torque measured at the rear wheels, divided by the gear ratio in the drivetrain.  So it's showing you crank torque minus the drivetrain losses.

To look at the two main dyno types I've used, an inertial dyno like a DynoJet measures power directly, by monitoring the speed of a rotating drum of known mass.  It also uses a spark lead pickup to measure engine RPM and then calculates torque using those two values.  OTOH, a DynaPack is bolted directly to the hub and measures torque using a load cell.  It also measures wheel speed (so that it can calculate power) and the user configures the rear end ratio in the software so that it can generate a torque curve against engine RPM.  In principle the DynaPack doesn't need a spark pickup although I think they usually have one anyway, I'm not sure why.

 

Maybe it needs a spark pickup for autos or to detect other slippage?   It wouldn't be the first time a clutch was overexerted for the first time after some major mods, like a supercharger and stock worn clutch.

akylekoz said:

I thought all dyno runs were done in 1:1 gear, typically fourth or fifth in a manual trans, third in an old four or three speed auto.

Am I wrong, what trans doesn't have a 1:1 ratio in it somewhere.  Should be the gear with the least loss therefore the most accurate with one less variable to compuguess.

 

Plenty of FWD cars don't have 1:1 ratio, old ACVWs don't and neither do S2000s just to name a few. When my BMW was dynoed they used 4th because they didn't want to spin the dyno as fast as my 2.93 diff would spin it. 

"rear wheel torque" is the torque measured at the rear wheels, divided by the gear ratio in the drivetrain.  So it's showing you crank torque minus the drivetrain losses.

My only problem with that statement is that there are no driveline losses of torque, only driveline gains. HP is lost to heat but not torque.

GTXVette said:

 the crossing point on a graph of torque and Horsepower is the 5252 RPM like you post, but Torque falls fast above that mark and HP goes on up. really most have reached highest torque by 3500 to 4500 rpm . My BB Chevy makes most torque by 3500 rpm but most Power at 7000. 

  Horsepower will take you OVER the Mountain  But Torque will MOVE the Mountain. 

That calculation is a quirk of the units. It only works for power in hp and torque in lb-ft.  When you go to watts and N-m, the equation changes. 

(and the generalities of what torque and HP do at that point are generally not correct, too)

As for the original question, moving your car down the road is a hp game.  Torque is used because it can be a direct measurement.  Power pretty much always is calculated- even with the rotational mass based system- it's a calculation based on the rate of acceleration of the rotating mass.

The reason all of those cute messages about tq and how it wins races works is because how it translates to power.  Whoever has the most accessible power will the faster one, so a broad and high torque curve (if you have to use it all) can be better than a peaky and higher torque curve because of how it translates to your ability to put power to the ground.

In reply to akylekoz :

Buzzboy said it all but I will add that the 3 dyno shops I have experience with all run in 3rd gear to keep roller speed down. I have been in the shop when someone did a 5th gear run on the dynojet and the rollers churning the air sounded like a 747 was landing in the building. 

By spark lead, do you mean actually hooking up a lead to the spark plug and/or engine?

I personally cringe when people say their car pulled xxx wheel torque on a dyno. As previously stated, the actual amount of torque at the wheel is dependent on what gear the trans is in. Even the most anemic fwd econobox could typically produce 2000 ft-lbs or more at the tire in first gear.

freetors said:

I personally cringe when people say their car pulled xxx wheel torque on a dyno. As previously stated, the actual amount of torque at the wheel is dependent on what gear the trans is in. Even the most anemic fwd econobox could typically produce 2000 ft-lbs or more at the tire in first gear.

Makes one think about all these crazy torque numbers with electric vehicles.

Sonolin said:

By spark lead, do you mean actually hooking up a lead to the spark plug and/or engine?

Yes. Like a timing light clip. That is the tach pickup for the dyno

alfadriver said:

As for the original question, moving your car down the road is a hp game.  Torque is used because it can be a direct measurement.  Power pretty much always is calculated- even with the rotational mass based system- it's a calculation based on the rate of acceleration of the rotating mass.

The reason all of those cute messages about tq and how it wins races works is because how it translates to power.  Whoever has the most accessible power will the faster one, so a broad and high torque curve (if you have to use it all) can be better than a peaky and higher torque curve because of how it translates to your ability to put power to the ground.

Yes, I fully understand that torque is kind of a silly measurement for engines. HP is a great measurement of an engine because it is a measurement of power. HP is also a great measurement because it isn't affected by the gearing or RPM. This allows us to compare two engines that make way different amounts of crank torque but the same HP. Just because the diesel makes more torque it's going to need to be geared higher to go as fast as the gas engine so the gearing will remove the torque advantage.

codrus said:

"rear wheel torque" is the torque measured at the rear wheels, divided by the gear ratio in the drivetrain.  So it's showing you crank torque minus the drivetrain losses.

This is the answer to the initial question. It's exactly the same distinction as between wheel HP and crank HP; with and without drivetrain losses deducted, respectively.

I wonder whether it would help clear up the torque/HP confusion for some folks (probably just add more mayhem) to go ahead and finish the torque/speed calculations to give pounds of thrust at road speed for each gear. "Pushing the car forward *this* hard" seems relatively intuitive.

The reason for using the gear with the 1:1 ratio is that it's usually the one with the least amount of frictional loss.  If your goal is bragging rights then that's important.  If you're using it as a tuning/testing tool (which is really what it is) then it's really not important.  One could argue that the higher the driveline losses are the more variable they're likely to be but if I'm testing something that's that critical and looking for changes that small I use an engine dyno.

I use the highest gear I can that keeps the equivalent road speed below 100 MPH.  That's easier on the tires, ears and my dyno.  I've done some messing around with testing in different gears and I've seen changes in peak numbers ranging from 0 to 10%.  I haven't done any kind of analysis to try and figure out why it makes a bigger difference on some vehicle than others.

buzzboy said:

My only problem with that statement is that there are no driveline losses of torque, only driveline gains. HP is lost to heat but not torque.

HP is nothing more than torque and RPM...If HP is lost to heat, torque must be as well.  If there are no driveline losses of torque, there would be no driveline losses of HP either.

You're confusing two completely independent concepts as being the same thing.  One is torque multiplication of gearing, the other is gear transmission efficiencty.  While the term RWTQ could technically be applied to either, in the common vernacular it is in reference to the latter...Probably because it is the direct relation to RWHP, which is also (not just coincidentally) in reference to gear transmission efficiency.

The one and only time I've had a car on a dyno he insisted in running it in 2nd gear.  The operator (an old timer and VERY experienced) said that higher gears give higher results so people that like to brag about their "number" want a 1:1 pull but people who just want to tune and test their rides are better off with the lower speed on the equipment.

Anyone remember that mustang tire fail dyno video where it comes apart at 130 mph?  Sketchy.

buzzboy said:

alfadriver said:

As for the original question, moving your car down the road is a hp game.  Torque is used because it can be a direct measurement.  Power pretty much always is calculated- even with the rotational mass based system- it's a calculation based on the rate of acceleration of the rotating mass.

The reason all of those cute messages about tq and how it wins races works is because how it translates to power.  Whoever has the most accessible power will the faster one, so a broad and high torque curve (if you have to use it all) can be better than a peaky and higher torque curve because of how it translates to your ability to put power to the ground.

Yes, I fully understand that torque is kind of a silly measurement for engines. HP is a great measurement of an engine because it is a measurement of power. HP is also a great measurement because it isn't affected by the gearing or RPM. This allows us to compare two engines that make way different amounts of crank torque but the same HP. Just because the diesel makes more torque it's going to need to be geared higher to go as fast as the gas engine so the gearing will remove the torque advantage.

Well, remember that you can directly measure torque, but you can't directly measure power.  

The key, like you point out, is knowing what torque you are talking about, whereas power is just lost as you go from the engine to the wheels. 

KyAllroad (Jeremy) said:

The one and only time I've had a car on a dyno he insisted in running it in 2nd gear.  The operator (an old timer and VERY experienced) said that higher gears give higher results so people that like to brag about their "number" want a 1:1 pull but people who just want to tune and test their rides are better off with the lower speed on the equipment.

Anyone remember that mustang tire fail dyno video where it comes apart at 130 mph?  Sketchy.

Balance the strength of the dyno vs. the strength of the drivetrain.  Some powertrains limit torque in lower gears due to durability concerns.  If I had to choose a low gear for high output testing, I'd stop at 3rd.