The difference inefficiency is because of the difference in the strength.
The 9" has greater strength because it has a larger area of engagement
between the ring gear and the pinion because the pinion is located lower
down on the ring gear but the trade off is a slight increase in friction
causing a slight decrease in efficiency.
The 12 bolt has less engagement between the ring gear and pinion so it
offers less resistance but less engagement means less strength.
The 9" also offers an internal rear-pinion support for the gear end of
the pinion to limit gear deflection under high torque loads which
detracts slightly from efficiency.
Everything I've read shows a 3.5% efficiency difference between the 12
bolt and the 9".
John Thornton
John Burk wrote:
> Last July Tony Prerra posted that he had done a chassis dyno comparison
> between a QC , 9" & a 12 bolt dropout . He found the CG was the worst , the
> 9" was 6 hp better & the dropout was a total of 10 hp better .
>
> I asked for more details and he wrote back :
>
> "Engine was an XO GMC , all ratios were about 2.47:1 , 6200 rpm in high gear
> , 140 deg , having the dry sump on the rear on or off made little difference
> , oil temperature made "some" difference , gears were "shot pened" , Synergy
> nascar qualifying oil "
>
> I assume these were done on the same day . With conventional oil it would
> seem the differences would be greater and a change from cold to hot would be
> noticeable .
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