[acammer]

Hey guys - I thought I'd drop a little follow up in here. I was able to complete a bit of dyno testing a few days ago to get a feel for what the 9" diff is costing for horsepower. It's fairly commonly accepted that the larger hypoid offset of a 9" (which increases the tooth engagement) both results in increased strength and decreased efficiency. Most sources suggest 1-3% loss over more conventional ring and pinion setups. In general more gear ratio will consume some efficiency as well.

There is very little testing out there which actually documents the losses, so I was keen to do my own test. The combination has not changed since dyno'd at GPI, with exception that I'm running the complete Vararam, rather than just the open lid used on all our tests.

Pull #2 is the best comparison, Vararam with no filter. 538whp vs the 551whp at GPI would put the loss at 2.4%. That assumes the dynos read very close, there could be some variance there as well, and we probably have a touch more intake restriction now than when tested at GPI. As expected, the 9" is consuming a bit of power. But, for a stick shift car, getting the strength to handle the abuse, along with the aggressive starting line ratio and better rpm out the back all contributes to better performance and reliability.

For those curious on the other pulls:
#1 - Vararam with filter
#2 - Vararam no filter
#3 - open throttle body, iat reading about 25*F too high (helping keep the fueling right)
#4 - open throttle body, iat reading accurately (too rich)
#5 - turbo guard with velocity stack.

All tests were at about *84 ambient, 185* on water temp, and 200-220* oil temp. A more substantial cool down and hero pull might have made a couple more.