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?? 3.73 or 3.89 ??

This is a discussion on ?? 3.73 or 3.89 ?? within the Drivetrain forums, part of the LSx Technical Help Section category; ordering 9"rear. need advice on gear ratio in A4 car? I have 3.73 now with a 25"ish tire they are ...

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    ?? 3.73 or 3.89 ??

    ordering 9"rear. need advice on gear ratio in A4 car? I have 3.73 now with a 25"ish tire they are great but switching to the 9" due to heads cam mod, dont want to break stock rear. thinking about 3.89 but worried about traction problems.
    anyone running or have thoughts

    thanks

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    Moderator 35th-ANV-SS's Avatar
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    I don't think going to a 3.89 gear is that much of a difference from a 3.73.

    What size cam are you running? What tires are you going to be running?

    I do know that the 3.89 has a special cut to it, which tends to make it noisier than other standard gears like 3.42/3.73/4.10/4.11. At least that is what a Moser dealer/installer told me when I was choosing my gear ratio.

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    Senior Member TLS_Addict's Avatar
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    I had 3.90s in my old Camaro and loved them. Now that was a th350 so not as steep of a 1st gear and didnt have overdrive but the gears were fine for that. I also ran a little taller tire than the 275/40/17s. What tire are you running? If its taller the 3.89 might be enough to off-set the taller tires so it will act like you are running 3.73s and not actually the 3.89s.

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    tire size is a 285/40/17... cam is a : 233/239, .595"/.603"......
    I drive the car about 150-200 miles a week. My 3.73 in the stock rear are loud (wine at 55-65) so I'm not sure I could handle louder gear noise after sinking $$ into it, my 85 had a 9" with 4.11 and made no noise but had duel glass-paks dumped before the rear-end so I couldnt hear much anyway.
    thanks for all the help just trying to get it right the first time.

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    I am running 3.73's in our 12 bolt both on the street and at the track. A little deeper gear, such as the 3.89 you are contemplating, should be fine.

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    Junior Member need4fun2469's Avatar
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    I'm running 3.89's in my 9" with no noise at all. It all depends on how good the shop is that sets up the gears.

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    Quote Originally Posted by need4fun2469 View Post
    I'm running 3.89's in my 9" with no noise at all. It all depends on how good the shop is that sets up the gears.
    My statement came from Bob at EPP, now Brute Speed. He installs Moser 9" rears for a living. Figured he knew what he was talking about.

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    What is your goal with a gear swap?

    Where's your peak HP? Around 6200-6300? From a track standpoint with no converter slippage, 3.89's would be good up to 129 and 3.73's 134 trapping at 6500 rpm. I don't think a stock cube HCI car is going to get there with that cam unless you spray it.

    Sent from my DROID2 using Tapatalk

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    Thanks for the advice, Iím happy with the 3.73 in the car now so I stuck with 3.73 parts on the way, if I change my mind I can always swap out the center section and stuff a deeper gear in it.

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