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TexasTodd
Dodge Dakota
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2/12/2004
17:09:51
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 Subject: RWHP-affected by gears?
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Would changing your gears, like a 3.55, to a 3.92, change the RWHP at all?
I know it does nothing for the true engine HP, but what about how we get our HP, from a chasis dyno?
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eddy
Dodge Dakota
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2/12/2004
17:21:59
| RE: RWHP-affected by gears?
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Changing the gears changes how the engine torque is multiplied at the wheels. Does not change actual output of the engine.
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ZenDak
Dodge Dakota
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2/12/2004
17:37:37
| RE: RWHP-affected by gears?
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I would think you would show higher hp and tq figures,,but obviously doesn't improve engine number. In a nut shell I believe changing your gears would close the gap between crank hp, and rwhp.
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Neil
Dodge Dakota
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2/13/2004
05:36:18
| RE: RWHP-affected by gears?
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It's nopt going to close any gap. As said above it does not change the actual numbers.
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pdak
Dodge Dakota
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2/13/2004
08:18:20
| RE: RWHP-affected by gears?
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The rear wheel horsepower will not change. The only change is that the engine will be functioning/operating in the sweet spot of the power band and will "appear" to have picked up some power by the old butt dyno. Check out any dyno graph and you will see that most engines have ther best torgue in the mid RPM ranges with peak horsepower occuring usually above that point.
Pdak
2002 4.7 CC Ztubed tonneau
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.boB
Dodge Dakota
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2/13/2004
13:23:10
| RE: RWHP-affected by gears?
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Power application to the pavement is measured at the rear wheels as torque. HP is calculated from the measured torque. Different gear ratios apply torque differently. Higher ratio numbers provide better torque application. Just like a longer handle on your wrench applies more torque for the same effort. So higher gear ratio numbers will provide higher chassis dyno numbers. That's why tests are usually done with the transmission in whatever gear is 1:1, to eliminate that variable.
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TexasTodd
Dodge Dakota
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2/13/2004
14:33:28
| RE: RWHP-affected by gears?
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Thanks .boB,
I forgot about that. Although I don't think they all do in reality. I,ve seen a few graphs of going thru the gears, but I'm sure most do one gear.
On my 4.7, 5spd, 3.55 LSD,
3rd gear is a 1:1 with the rpm and speedo, exactly 1:1.
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WipLash
R/T
2/20/2004
23:45:15
| RE: RWHP-affected by gears?
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Actually, there is a slight loss in HP the higher the gear ratio is. With a direct hook up (1:1) you have no friction loss. Everytime you add a tooth or a gear to increase the ratio you add one more element of friction to create drag. I design material handeling equipment for a living. I use Dodge, Boston, and Summitoma gear reducers on all my machines. They all have tables that show the loss through the gears. A 5:1 gear reducer is over 85% efficient. A 20:1 reducer is only 78% efficient. A 60:1 reducer is only 67% efficient. I'm looking at a Dodge Gear Reducer manual and those are the numbers listed for a 175 series gear reducer. So, while the truck will be faster due to better torque multiplication, more energy will be wasted in the gears to make that torque.
These are Hypothetical numbers, but it will give you an idea what I'm talking about.
An 8.25 with 3.23's may be 90% efficient. Lets assume the numbers for the engine at 240HP and 295lf/lb torque. Then you would get at the rear wheels:
240hp x 0.90 = 216RWHP
295ft/lb x 3.23 x .90 = 875.6ft/lb at the axle.
Now lets do a 9.25 with 3.92's and assume .80 efficiency.
240hp x 0.80 = 192RWHP
295ft/lb x 3.92 x .8 = 925ft/lb at the axle.
These numbers are exagurated a little for example purposes. The actual differnce in efficiency between a 3.55 and a 3.92 gear would be less than 2% probably on a 9.25 rear end. The difference betwen the 8.25 and the 9.25 is a little more.
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