What does RWTQ actually show?

RWTQ is calculated with the ratios in mind because they don't matter.

Chassis dynos (used to measure RW numbers) don't actually measure torque.  They calculate how quickly the oomph coming from the tires accelerates a known rotational mass (the drums).  This measures HP and then you calculate TQ based on the formula: hp = (tq x rpm)/5250.  From there you can estimate how much torque is lost in the driveline based on guesses and experience to get an estimate of what it probably makes at the crank.  Chassis dynos are basically like using 1/4 mile trap speeds and ETs to estimate, (where the known resistance is vehicle weight) but they do it with an actual direct measurement and avoid things like reaction time and driver error.

Engine dynos measure torque using some form of resistance (brake) to figure out how much brake it needs to apply to overcome engine torque.  They then calculate hp with the same formula.

So in most circles, if they dyno'd the engine they say "the engine makes 550 hp at the crank" and can then venture a guess as to how much that is at the wheels.  Its easier and cheaper to dyno once its in the vehicle, so the same combo on a chassis dyno they might say "it makes 475 hp at the wheels"

Its just different ways of measuring.  Chassis dynos measure HP and calculate TQ.  Engine dynos measure TQ and calculate HP.

So, like has been stated, you either measure it at the engine and guess how much is actually getting to the asphalt, or you measure it directly at the tires and know for sure.  Engine dynos make big pretty numbers.  Chassis dynos give actual numbers after all the friction factors have taken their toll.

Or, like I do it, I do dyno simulations using software, so all of my engines make 950 hp.

alfadriver said:

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.

Yup.  In your standard front engine, RWD setup, the 1:1 gear just directly couples the input and output shafts with no gear teeth involved.  No gears means the minimum drivetrain losses but also the maximum gearbox strength.  It'll also be the most consistent between cars -- doing it in 2nd is silly because different cars have different 2nd gear ratios, so will see different loss effects.

Plus, if you're dynoing a turbo car, you want to dyno in a higher gear in order to spool the turbo at as low an RPM as possible.  If you dyno it in 2nd and it doesn't spool up til 4500 RPM, whereas on the street in 5th it'll spool at 3500, then you've got a couple of cells that you didn't tune properly.

Curtis said:

Chassis dynos (used to measure RW numbers) don't actually measure torque. 

Inertial chassis dynos don't measure torque, but there are chassis dynos that do.  DynaPacks, for example.

I've come up with an interesting method of explaining torque vs, horsepower, and how they affect the performance of a vehicle.  I welcome the hive's thoughts on this:

Since horsepower is the measure of the ability to do actual work (to move an actual thing an actual distance in an actual time), time is an important factor in understanding the relationship between it and torque. Torque is the actual force the engine applies, and horsepower is how rapidly it can apply that force. A low torque engine with high RPM applies a smaller force, but more often, so it can have a higher horsepower rating. An engine that turns slower can't apply the torque as quickly, so it needs to have more of it to achieve the same horsepower. 

Curtis said:

RWTQ is calculated with the ratios in mind because they don't matter.

Chassis dynos (used to measure RW numbers) don't actually measure torque.  They calculate how quickly the oomph coming from the tires accelerates a known rotational mass (the drums). 

Not all chassis dyno run on inertia alone.  Just most of the ones people here are going to see and put their car on.  

And if you wanted to do more effective tuning, and not just a WOT (or partial throttle run), you should go and find a dyno that is capable of running steady state- thus making a direct measurement of tractive effort.

codrus said:

alfadriver said:

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.

 

Yup.  In your standard front engine, RWD setup, the 1:1 gear just directly couples the input and output shafts with no gear teeth involved.  No gears means the minimum drivetrain losses but also the maximum gearbox strength.  It'll also be the most consistent between cars -- doing it in 2nd is silly because different cars have different 2nd gear ratios, so will see different loss effects.

 

Plus, if you're dynoing a turbo car, you want to dyno in a higher gear in order to spool the turbo at as low an RPM as possible.  If you dyno it in 2nd and it doesn't spool up til 4500 RPM, whereas on the street in 5th it'll spool at 3500, then you've got a couple of cells that you didn't tune properly.

 

If you have an engine controller that limits power in the lower gears  and you want to do WOT tuning then yes, you really do need to test in a higher gear.  I don't know what testing you'd be doing on a chassis dyno where you care about what another car has for gear ratios but if you do then you need to take final drive ratios, tires (size, pressure, manufacturer and model) and lubricating oils all into account as well.

Your comments about a turbo car are only relevant if you're using an inertia dyno and if you're tuning on an inertia dyno you're leaving a whole bunch of cells untuned anyway.  With an eddy current dyno you can control the ramp time for power pulls so even the slowest spooling turbo has time to build boost.  You can also do steady state testing so that you can do multiple loads at any RPM.  One of the other advantages of an eddy current dyno is that you don't risk burning up the clutch on lighter lower powered vehicles trying to get the big drum to turn.

Once upon a time my city DPW had different dump trucks.  Two had 90hp and two had 140 hp.   When loaded with sand the 90 hp truck would be called upon to pull the 140hp out of the sand bank.

Why was this ?

 The 90's had more torque at a lower rpm.

In reply to iceracer :

The real reason is that it had more power at a lower rpm. 

Just to reinforce what some others have said, doing the pull in a 1:1 gear is often preferred, but depending on the redline, gear ratio etc. the wheel speed gets very high. When I dynoed my Supercoupe in fourth gear (1:1) the speedo read over 140 mph and I was staring at a brick wall 6 feet in front of the hood. You can only hope the guy who strapped the car down did a good job.

Will said:

Just to reinforce what some others have said, doing the pull in a 1:1 gear is often preferred, but depending on the redline, gear ratio etc. the wheel speed gets very high. When I dynoed my Supercoupe in fourth gear (1:1) the speedo read over 140 mph and I was staring at a brick wall 6 feet in front of the hood. You can only hope the guy who strapped the car down did a good job.

And it is also possible in some cars to hit the speed limiter before you hit the rev limiter!

iceracer said:

Once upon a time my city DPW had different dump trucks.  Two had 90hp and two had 140 hp.   When loaded with sand the 90 hp truck would be called upon to pull the 140hp out of the sand bank.

Why was this ?

 The 90's had more torque at a lower rpm.

What that really means is that the 140s weren't geared appropriately for their engine and weight.  So they couldn't put enough power to the ground at very low speed to start moving, while the 90s either had a lower 1st gear or or more low rpm torque, either of which allows more torque to the wheels at very low speeds to get the truck moving. 

In reply to rslifkin :

Being old medium/heavy duty city trucks I bet the 90hp had such low gears that they were on the redline at something like 50mph in top gear. 

In reply to buzzboy :

Actually not true.

Torque can be measured, Horsepower is derived or calculated from measurements.

In reply to clshore :

Actually, not true.

In order to directly measure horsepower, you just have to have a machine that allows you to measure how long it takes an engine to lift 100 pounds 32 feet. 

O.K. my $.02 worth. Horsepower is torque over time. Torque is how much weight can be moved. Horsepower is how fast. Big rig engines from my youth had small horsepower number and high torque numbers. They could haul many tons but to follow one from a stop is very slow. Modern small displacement engines need to run in the higher RPM range. Large displacement engines can do the same thing at lower RPM. Example: In the 80's I had a Honda 350CC cycle. I would rev it up to 5000 RPM to get it moving. My brother came down on his Harley it redlined at 5000RPM.