Axle ratios vs dyno numbers?

[Gary - K7GLD]

I'm curious as to the effect of different rearend ratios in dyno readings - all other factors being equal...



IF a truck with a 3:55 ratio is dynoed against an IDENTICAL truck with 4:10's - both manual trannies and both running in direct gear - what would the resulting power and torque numbers show - which would generate the bigger numbers, and by how much - anyone know?

 

[DavidGardner]

Good question. All being equal and both in direct drive i. e. 4th gear should be the same. If the gear ratio in the differential does make a difference then would tire size also make a difference?

 

[klenger]

Good question. I assume that the dyno operator has to enter the gearing and tire size information into the dyno (or post process the data) to compensate.

 

[Steve M]

I was puzzled by this a while back. The dyno actually calculates engine HP/Torque using speed of the rollers and engine rpm. Tire size and ratio are all automatically figured in with comparison of the two.

I've always thought it would be more useful to have a print out of actual rear wheel torque from 0-redline. Calculated HP and torque are great for bragging but how that torque is applied to the ground is really what matters.

Theoretically, if your two motors both produce 600ft/lbs of torque, the one with 4. 10s is going to put 14% more torque to the rear wheels in any gear.

600 X 4. 10 = 2460ft/lbs.

600 X 3. 73 = 2238ft/lbs.

600 X 3. 55 = 2130ft/lbs.



With 3. 55s you can run 70mph and still be in the power band of the 3rd gen. With 4. 10s you'll be running out of breath.



70 X 4. 10 X 336 / 30. 5 = 3161rpm

70 X 3. 55 X 336 / 30. 5 = 2737rpm



So with the 4. 10 you'll have to run in O/D to do 70mph. So knock 31% off for O/D.



600 X 4. 10 X . 69 = 1697ft/lbs.



So with 3. 55s you have the ability to put more torque to the rear wheels at 70 than with 4. 10s at 70. But the 4. 10s will get you out of the hole quicker and seem to better match the powerband in O/D for highway speeds 55-70mph with the automatic and 3. 55s seem better suited for manuals with their lower O/D gear.



55 X 4. 10 X . 69 X 336 / 30. 5 =1714rpm

55 X 3. 55 X . 75 X 336 / 30. 5 =1613rpm



It's complicated and everyone has their opinion on which is best. My personal favorite of all I've driven or owned is a 6spd/3. 55 combo. The gears come up slow and relaxed and it's very easy to drive towing or empty.

 

[Gary - K7GLD]

Thank Steve - pretty much as I suspected:



"Theoretically, if your two motors both produce 600ft/lbs of torque, the one with 4. 10s is going to put 14% more torque to the rear wheels in any gear. "



It makes sense to me - but things that make sense to me frequently get me in a HEAP of trouble!

 

[Steve M]

Gary, when you ran your truck on the dyno, did the printout give any info about actual torque at the rear wheels? What they've done is taken the actual torque and devided it by the ratios and tire diameter formula to calculate what your truck's engine would produce at the flywheel if it were on an engine stand. Such as... ,

2573. 75 / 3. 55 = 725ft/lbs

Crude but something to that effect.

The amount of torque at the tires is mind boggling with a bombed Cummins in 4-LO and 1st gear. It's a wonder the drive shafts hold on.

 

[lmills]

When I dynoed my truck at the fall brawl, I dynoed considerably lower than other 6 speed trucks with 3. 73's. I have the 6 speed with 4. 10's and the HO in my 03. Of course I had a dually also. But some of the numbers were off as much as 50 horsepower.

 

[KLockliear]

We are also kinda talking about this over at: https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=89362

 

[Gary - K7GLD]

"What they've done is taken the actual torque and devided it by the ratios and tire diameter formula to calculate what your truck's engine would produce at the flywheel if it were on an engine stand. "



UMMMmm - niether of the 2 places I have dynoed MY truck provided any "flywheel HP/torque" numbers, but only what they presented as REAR WHEEL power numbers - it's then up to the customer to make their best guess as to power train losses if they wanna estimate actual FLYWHEEL power number...



Maybe I missed something somewhere... ?

 

[KLockliear]

Since the RPM is being measured at the engine, not the rear wheel, the calculation will give flywheel tq.

 

[Steve M]

It's called "at the rear wheels" because that's where the torque is being measured on the drums. It would be way too inconvenient to remove the motor to measure power "at the flywheel" so the next best thing is to read motor rpm and measure torque at the wheels and calculate what that would amount to at the flywheel. Since torque is measured at the rear wheels, driveline losses should be counted into the calculation. You're actual flywheel torque should be a tad more than the calculation shows. If you had a 1:1 ratio in your transmission which is 3rd in our automatics or 5th in the 6spds and a 1:1 ratio rear end, with 12" loaded radius tires, then you would be real close to reading actual flywheel HP/torque minus driveline friction. But you wouldn't move much. My 460ft/lbs of torque wouldn't do much work at all without torque multiplication through gear reduction.

 

[Gary - K7GLD]

HMMmmmm - SOMETHING isn't ringing true here - and the above 2 posts seem to contradict each other...



SO, in plain english, are all these dyno printouts we see giving HP and torque #'s as measured at the rear wheels, OR are those printouts meant to show what the HP/torque is supposed to be at the flywheel...



ALL discussions *I* have been involved in as far as dyno readings are concerned, have been based on the notion that those fancy graphs were displaying REAR WHEEL numbers - and for THAT matter, dyno numbers I have seen on stock trucks typically show numbers signifiicantly LESS than advertised HP/torque - but fairly close if appropriate drivetrain losses are then factored in...



I'm confused...



(NOT and unnatural state... )

 

[lmills]

The only way to get the brake horsepower is to pull the motor and set it on a dyno stand. That will give you the flywheel HP/TQ. When they set it on a dyno and spin the rollers, it is measuring the rear wheel HP/TQ.

 

[Gary - K7GLD]

"When they set it on a dyno and spin the rollers, it is measuring the rear wheel HP/TQ. "



That's EXACTLY the way I have always understood it...

 

[KLockliear]

The interial dynos measure HP, then calculate torque. The calculation is based on the RPM given... which comes from the ENGINE, not the drive wheels.



They use the rear wheel HP and engine RPM to calculate a TQ value, which would be a FW tq value minus drivetrain loss.

 

[fbaurley]

So if two identical trucks are dynoed ( assuming everything equal except) one has 4:10s and one has 3:55 The torque should be greater on the truck with the 4:10s . Thr Rpm would be greater on the truck with 3:55s (this is all at rear wheels)The Horsepower should be equal because horsepower equals (torque X Rpm) / a number. So when you increase one you will decrease the other , but when multiplied the effect is the same. Kinda like 3X4 =12 or 6X2 =12. It seems to sound reasonable.

 

[Steve M]

HP is measured at the rear wheels? Let's see... 2900rpm on a 30. 5" diameter tire would be over 300mph. That's pretty impressive for a stock truck...

It's calculated using engine rpm and rear wheel torque devided by gear ratio.

 

[KLockliear]

Originally posted by Steve M

HP is measured at the rear wheels? Let's see... 2900rpm on a 30. 5" diameter tire would be over 300mph. That's pretty impressive for a stock truck...

It's calculated using engine rpm and rear wheel torque devided by gear ratio.

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



2900rpm on 30. 5" diameter = 300mph? How do you figure this? Even in OD, at 2900rpm with a 33" tall tire, my truck only went 108mph.

 

[KLockliear]

Originally posted by fbaurley

So if two identical trucks are dynoed ( assuming everything equal except) one has 4:10s and one has 3:55 The torque should be greater on the truck with the 4:10s .

The rear wheel torque would be higher, but because the HP would be lower, the computation would show lower tq as well. This is because with the 4. 10's, the truck may go (hypothetically here) 95mph in 5th gear at redline, where the 3. 55 geared truck will go say 115mph. The delta speed is different, which is why the computation works either way.

 

[lmills]

Originally posted by Steve M

HP is measured at the rear wheels? Let's see... 2900rpm on a 30. 5" diameter tire would be over 300mph. That's pretty impressive for a stock truck...

It's calculated using engine rpm and rear wheel torque devided by gear ratio.

So what your saying then is your truck will go 300 mph? How do you figure the dyno is running 300mph? Unless you are checking the BHP with a dyno bolted to the flywheel, you are only getting the HP at the rear wheels.