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Old 07-09-2003, 05:08 PM   #1
Scooby Specialist
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2012 Non-Subaru

Default Is loss of HP at the wheels a straight loss or is it a proportional loss?

I know a WRX's crank hp is supposed to be 227, and actual wheel hp is approximately 60 hp less. Stated another way, there is an approximate 26% loss of hp at the wheels.

I was wondering if that loss is linear or proportional. For example, if by adding a catless uppipe and a reflash my whp increases from 167 to 197, is it appropriate to assume that my crank hp has increased by 30 hp (linear), or would it have increased by around 40 hp (proportional)?
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Old 07-09-2003, 05:10 PM   #2
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to my mind, it's both.

some amount of friction is unavoidable. that represents a flat loss.

other sources of friction will have a greater loss as the transmitted torque goes up. that represents a percentage.

as to what kind of #s we're talking here, or a formula, i haven't enough experience to hazard a guess.

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Old 07-09-2003, 06:34 PM   #3

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16 WRX (white)
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It's a fairly steady number of around 60-75hp or so.
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Old 07-09-2003, 06:37 PM   #4
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2002 WRX Wgn 292whp
Alex Performance


It's both, kinda. It's a constant plus an a small incremental amount as the power goes up and everything in the driveline distorts a little, thus requiring more power to perform the same function. So if you make say 250 whp, it does not mean that you crank hp is 30% more and will continue in the same fashion as your power goes up. From what I've seen it's about 40 hp lost in our AWD system, more on cars with higher horsepower. Use of synthetic lubricats lowers the power loss.

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Old 07-09-2003, 07:07 PM   #5
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2002 Impreza WRX

Default complex mix

Losses from the crank shaft power number is a mix of both.

There are certain losses that are essentially fixed at any given rpm. Examples will be, fluid shearing losses in bearing greases, and transmission oil getting thrown around the inside of the transmission, bearing losses in the wheel bearings, CVjoints and so on.

The losses in the gear meshes are however proportional to power transmitted. In common helical gears and especially in the hypoid gears of a differential there is a significant friction loss due to sliding between the gear teeth. In a perfect world the contact point in a gear mesh would be a pure rolling contact point as the gear teeth meet at the pitch line. In the real world the gears are not perfect and they do not mesh exactly as theory would predict. Gear lash is never perfect and under high loads the gear mesh changes. The gear teeth deform slightly under high load and sliding friction goes up. I've seen numbers for friction losses as high as 12% of the power transmitted in hypoid differentials. The numbers for simple gear meshes in a manual transmission are less the number that comes to mind is about 8% but would have to verify that.

Bottom line you have a mix of fixed friction sources that will not change significantly at higher power levels and a handful of friction sources that change quite a bit depending on power transmitted, temperature of the gears and gear lube etc.

At typical power levels you won't be off by much if you pick a fixed % number that matches well with dyno numbers. At high power levels your losses are going to go up due to deformation of key componets in the drive train and increased friction in the transmission and differentails.

For stock power levels you can figure about 75% of the flywheel power (more or less) will get to the wheels or about 170 hp to the wheels for a strong stock wrx. at higher power levels increase the losses a bit to about 30% of the flywheel number.

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Old 07-09-2003, 09:28 PM   #6
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As others have said, there are both propotional and fixed losses in the system. We beat this topic to death about a month ago in FFI; you can probably find it with a search and read all of those explanations and opinions.
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