Author Topic: Rearend gear ratio advice wanted  (Read 3539 times)

Offline Dunc the Funk

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Rearend gear ratio advice wanted
« on: December 30, 2021, 09:56:32 AM »
We have run 7.5s in the quarter at around 180mph this season & we are wanting to go a bit quicker next season.
We have a powerglide, 3.7:1 gears & Hoosier 33x15x15 tyres. We are now crossing the finish at near 8000rpm, which is our current rev limit.
I'm thinking of going to 3.5:1 or even 3.4:1 rather than winding the motor higher.
What gears/rpm do you folks use and what diameter tyres?

Offline e-drag

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Re: Rearend gear ratio advice wanted
« Reply #1 on: January 04, 2022, 09:30:06 AM »
What you are stating does not compute:

http://www.wallaceracing.com/calcrgr.php

This calculator is pretty reliable.  I would probably run a 4.11 gear if I were you.  Again, do the math.  I would be looking at your converter!

Offline masracingtd1167

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Re: Rearend gear ratio advice wanted
« Reply #2 on: January 06, 2022, 06:22:04 AM »
I would also look at the converter .  A 3.70 gear with that tire should be fine for your combo !

Offline Dunc the Funk

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Re: Rearend gear ratio advice wanted
« Reply #3 on: January 06, 2022, 12:32:26 PM »
First season we ran a 4.29:1 & it was WAY too low, very violent launches. If the track was not good we had to leave in top. 3.7:1 does work well, our 60' is now down in the 1.05 sec range. I think either the converter is slipping a lot or the rev counter is telling lies.

Offline masracingtd1167

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Re: Rearend gear ratio advice wanted
« Reply #4 on: January 07, 2022, 10:18:29 AM »
A1.05 is a decnt 60 ft but it seems like your finish line rpm dosent jive with your mph ! Might be worth talking to your converter guy about this !

Offline Supercat

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Re: Rearend gear ratio advice wanted
« Reply #5 on: January 07, 2022, 10:44:43 AM »
I run 3.89 gears and same Hoosier as OP.
We run through the lights at about 7800rpm at 192mph 7.2-- ET. 1.01 - 1.05 60 ft.. 3800 to 4200 launch rpm.
Check the converter as others had mentioned

Offline Dunc the Funk

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Re: Rearend gear ratio advice wanted
« Reply #6 on: January 10, 2022, 01:42:26 PM »
Thanks for responses. We don't have a data logger so I'm getting the rpm figure from onboard camera. I have reviewed the footage & it's actually only about 7500 rpm on a 178 mph run.

Using a slip calculator eg TCIs gives a figure of 11.8% (assuming no tire growth). Supercat's numbers indicate only 2.57%. At launch our motor always goes straight to 5000rpm so I guess our converter is looser? What is a normal range of slip?

Offline Frontenginedragsters

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Re: Rearend gear ratio advice wanted
« Reply #7 on: January 10, 2022, 04:20:26 PM »
Many years ago ATI would tell us that 5% slip was a good convertor.
This was years before data recorders became common.
He used to figure it out with the Moroso slide rule.
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Offline Supercat

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Re: Rearend gear ratio advice wanted
« Reply #8 on: January 11, 2022, 02:54:14 PM »
At the finish line my car,  just ahead of the rear tire, is about 6 inches off the ground. So tire growth would be 4 to 6 inches anyway.  How does that change your numbers?