wrd1972

iTrader: (12)

What is the parasitic loss of the 9 inch rearend compared to the 10 bolt?
What is the parasitic loss of 4.11's compared to 3.42's?
What is the parasitic loss of a 3" chromeoly DS compared to a 3" aluminum DS?

I am trying to determine the "magic percentage number" to best guess my FWHP. I have a 9" rear with 4.11's, Detroit truetrac and the Strange chromeoly DS and heavy 285/40/17 tires on 17x10 wheels.

wrd1972

iTrader: (12)

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Weezzer

1. The % lost will basically be due to the increase of the moment of inertia due to the larger size - 9" diameter vs 7.625 " diameter ring gear. It also probably has bigger bearings due to larger diameter axle shafts - which will result in a slightly larger friction force along with an increase in the moment of inertial for all the larger bearings as well as any larger axles.
Its like the analogy of the person who buys bigger brake rotors with an aluminum hat, the disks weight are less than the smaller cast iron disks - but you will lose HP on the dyno due to the increase in power needed to overcome the increase of the moment of inertial of the larger disk.
2. Loss due to taller gear: Probably gain a little as its easier to turn the gear, kinda like when you put your ten speed bike into a taller gear to go up a hill.
3. Assuming ID and OD are the same for the DS's, then the lighter one will have slightly less force required to spin it.

Basically, in my opinion, these listed comparisons are pretty much negligible (you probably gain more performance just by having an almost empty gas tank as opposed to a full one - I belive gas weighs close to 6.2 lbs to the gallon).
One would need to run this thru finite element analysis for the total picture... (I think - lots of brewskys tonight!!!!)

Its pretty much swag.... (Army term for scientific wild *** guess)....

cam

iTrader: (9)

Pinion location on the 9" adds friction over other diffs like the 10 bolt or 8.8

3-5% rwhp is typically the generally accepted amount of parasitic loss from what I have seen. One guy posted somewhere here that he did dyno higher through his 9" than his stocker but I saw no proof of it. I did see another post where some guy went from 390+ rwhp down to 360 odd from going to a 9". That guy was pissed

wrd1972

iTrader: (12)

3-5% on top of the amount already robbed by the 10 bolt?

cam

iTrader: (9)

Yes. No differential spins without some sort of loss so you have to compare one to another. A 9" robs 3-5% more power than a stock 7.5" ten bolt in most cases. Sometimes more, sometimes less, but thats a good ball park estimate of what you will lose