Rwhp vs tire size on a dyno - Great Lakes 4x4. The largest offroad forum in the Midwest

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Old June 6th, 2012, 11:50 AM   #1
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Default Rwhp vs tire size on a dyno

I am being told that if you take 300hp truck at the wheels with 35 I'n tire. And then switch to a 44 In tire that your rear wheel Hp will be the same only ur torque numbers will change.. Is this true?
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Old June 6th, 2012, 11:53 AM   #2
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I don't claim to be fully understanding in this area but I would think if you plugged in the correct formula it should compensate accordingly in the results
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Old June 6th, 2012, 12:08 PM   #3
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SInce there is a difference in HP at the crank to the rear wheels (RWHP always being less), then I would think if you add a larger load to the drivetrain, HP will go down.

BUt I also agree with the above, that load can be compenstaed for accordingly.
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Old June 6th, 2012, 12:21 PM   #4
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The difference between HP at the crank and HP at the wheels is drivetrain loses. I would not expect large tires to significantly change drivetrain loses, so Yes, it's true.
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Old June 6th, 2012, 12:29 PM   #5
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The difference between HP at the crank and HP at the wheels is drivetrain loses. I would not expect large tires to significantly change drivetrain loses, so Yes, it's true.

Now a larger tire size with the same axle ratio can reduce your ability to deliver the power/torque at the wheels. So while the engine may be producing it, the dyno won't be seeing it.

One of purposes of a chassis dyno over an engine dyno.
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Old June 6th, 2012, 12:47 PM   #6
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Now a larger tire size with the same axle ratio can reduce your ability to deliver the power/torque at the wheels. So while the engine may be producing it, the dyno won't be seeing it.

One of purposes of a chassis dyno over an engine dyno.
Turning the bigger tire may require more horsepower, but the horsepower avalible to do it shouldn't change.
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Old June 6th, 2012, 12:50 PM   #7
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Most dynos used are inertia based, and would show a lower number, due to the higher rotating mass/inertia of the larger tires.

You can end up with similar differences depending on what gear you select, also.

Plenty of variables will mess with the numbers. Correction numbers in the dyno, locked vs unlocked converter, gear selection, dyno type, etc.

Horsepower is just a math calculation of torque, so if torque is less, horsepower will also be proportionally less at that engine speed.
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Old June 6th, 2012, 12:54 PM   #8
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Why do i read dyno runs from turbo/supercharged 4.0's, with 35's and larger only putting down like 170 to the wheels, when the engine "should" be making 300 at the crank. That is much more than the typical 15-20 percent loss typical in a street car.
I think spinning tires that weigh 100lbs or more each does show lower numbers than you would see in a car. There are a lot of variables that would need to be factored by the dyno operator.
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Old June 6th, 2012, 12:56 PM   #9
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Quote:
Originally Posted by brewmenn View Post
Turning the bigger tire may require more horsepower, but the horsepower avalible to do it shouldn't change.
Available at the engine, no, but at the wheels, yes.

From actual losses, you lose power in the converter, the transmission, transfer case, rear axle, and tire sidewall friction loss.

From inertia, you 'lose' horsepower with most increases in drivetrain mass.


On the dyno, most of the time heavier and larger tires 'lose' horsepower, because dynos like a dynojet calculate power based on how fast it can accelerate the known weight of the rollers. Adding more inertia to the system slows it down, fooling the computer into thinking you have less power.

On the street, that affect is seen in the real world, heavy tires = slower acceleration. Its one of the few points where you really can sometimes see a difference.

On the dyno, you can see differences between smaller and larger axles. Axles like a Ford 9" have higher parasitic loss, but you rarely see any real effect of this in the real world.
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Old June 6th, 2012, 01:00 PM   #10
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Single vs double rollers on the dyno, and roller diameter also make a difference. As do running those that you remove the wheels and bolt the axle flanges to the dynos.
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Old June 6th, 2012, 01:09 PM   #11
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The tire style can make a big difference too even if they are the same size. Years back i dynoed my Mustang with ET Streets, and then swapped to BFG Drag radials. It made like 380hp with the ET streets, and around 430 with the drag radials.
Friction and weight i guess.

For a truck/Jeep im sure that a street tire will show a higher number than an aggressive mud tire of the same size and even weight.
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Old June 6th, 2012, 01:59 PM   #12
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You don't race a dyno I always hear.



Other than that, I got nuthin. All I care is that the vehicle gets me home.
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Old June 6th, 2012, 02:35 PM   #13
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Is like to dyno my YJ some time, but will do it with stock rollers on it. :)
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Old June 6th, 2012, 02:37 PM   #14
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torque is what is actually measured. that with speed is calculated to give HP. change wheel dia. and torque will increse/ decrease and so will the speed change but inverse to torque values. the formula stays the same so the HP will remain the same. Many years ago I had a Mustang chasis dynoed. We couldnt get enough traction to get proper torque and speed numbers. I put 3 guys in the trunk and it went up with each guy in. Just couldnt fit anymore in and didnt have better tires. all we really did was produce alot of tire smoke
so to answer your question YES the HP will stay the same (if traction sttayes the same) the torque will go down. thats why we change gears when going to bigger tires

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Old June 6th, 2012, 02:46 PM   #15
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Torque x RPM / 5252 = HP. I mainly work with Load Cell's and Non Contact Torque meters, We do have a Chassis Dyno but I don't do anything with it besides move it when it gets in my way..

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Old June 6th, 2012, 03:00 PM   #16
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Thanks Spence1986
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Old June 7th, 2012, 08:37 AM   #17
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Interesting...
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Old June 7th, 2012, 09:41 AM   #18
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How does tire weight factor in?
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Old June 7th, 2012, 10:08 AM   #19
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less torque for bigger & heavier tire/rim combo's. why do you think competitive racers want the lightest combo possible?
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Old June 7th, 2012, 10:24 AM   #20
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No nothing about the tires changes HP except the ability to maintain enough traction on the rollers
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