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Old 10-27-2017, 12:22 AM   #1
scottyjones16
 
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What gear Ratio is best?

My 10 A6 has a Paxton 1500 blower making 600rwhp. I run a 3400 PAS yank converter. I currently bought a entire rear diff with 3.91 gears and a eaton trutrac. My tire size is 325/45/18 hoosier dot slick which is 30in tall. Before I install the new diff, do I need to drop to a 3.70 gear? The car is a Sunday driver and i usually race 8th mile tracks. Thanks
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Old 10-27-2017, 12:28 AM   #2
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What made you choose the Eaton may I ask?
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Old 10-27-2017, 02:17 AM   #3
scottyjones16
 
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My factory diff clutches were fried, I found that diff used so it came with it when I bought it.
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Old 10-27-2017, 09:10 AM   #4
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What does the car trap now? Shift point rpm? I would pick a gear based on getting the best 1/8th mile performance, if that's what you typically run.

A 3.91 goes to 4th at 92mph on a 30" tire with 3.91s, shifting at 6200rpm. I would guess with 600whp and good tires you're running the 1/8th mile out closer to 97-100mph.

The 3.73 would go 97mph in third at 6200rpm. If you twist it up to 6500rpm that's 102mph. Gives you a 2,000rpm cruising rpm in 6th at 70mph, which is livable, and definitely a little better than 3.91s. That would seem like your best gear option to me.

I don't know how the car is hooking now, but with that aggressive 4.027 first gear in a 6L80 you don't need a crazy final drive ratio (that's what the converter is for) to get off the line, even with a heavy beast like this. Even a 3.55 might be a consideration, if you're trying to tone it down some for cruising around, still crosses the 1/8th around 6,000rpm.

If you're going quarter mile racing then leave the 3.91 in, that'll help with the big end. Also, in the calcs above, there is no compensation for converter slip, so you might find it runs a little slower at a given rpm.
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Old 10-27-2017, 10:15 AM   #5
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Most F/I peeps on here prefer a lower gear to keep from blowin the rear tires off at the line and make it up at the top end...18" tire will work for a while but like above if your plan is straight line with the 3.91's then your best bet is a Carlyle 15" conversion cause the bigger sidewall is need with a taller gear....good luck and keep us posted!
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Old 10-27-2017, 01:47 PM   #6
scottyjones16
 
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Thanks for the help guys, I also have a set of 3.45s laying around lol. Decisions, decisions!! I will keep y’all posted as soon as I figure it out. Thanks again!!!
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Old 10-27-2017, 03:35 PM   #7
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3.45s for me on my blower. 3.91s are too much (worse in an auto)

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