Author Topic: Diff gearing for TH700  (Read 2108 times)

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Offline Stevo

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Diff gearing for TH700
« on: November 09, 2015, 10:40:08 PM »
Hi guys

I finally bit the bullet and sold my old engine ready for a 383 crate motor upgrade.

The 1st question is what diff ratio would you recommend for a th700 box, i need it snappy around town and good for weekend highway cruising.  the diff in it now is a 3.08.

2nd question, is what torque converter would you recommend,brand,stall,lockup,and why.

im sure there is a early c3 out there with the same upgrades 383 450hp,th700 box with that perfect diff ratio and torque converter setup.

Thanks again i appreciate any advise.
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Offline sirfixalot

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Re: Diff gearing for TH700
« Reply #1 on: November 10, 2015, 08:52:16 AM »
Turbo 700 has a lower 1st gear than i assume original t350  so will give you plenty of go off the line at the other end t700 has 0.7  to  1 ratio. I have a 3.55 ratio & does 2k rpm   @ 100KPH. 3.08 ratio engine would be literally idling at 100kph. Would also be determined by what revs the cam in motor works at?

Offline Bfit

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Re: Diff gearing for TH700
« Reply #2 on: November 10, 2015, 10:58:10 AM »
Why  change the diff  before trying the set up you  have in the car.
fit the 700 and see how it goes, you may find it will do the job for you.

The diff change is an easy thing to do later if required.
lower RPM on the highway will save you money.Having said that, I run 4.11 in the rear end with a .6 overdrive in top gear and get 18MPG highway runs
Bfit
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Offline Stevo

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Re: Diff gearing for TH700
« Reply #3 on: November 10, 2015, 12:58:06 PM »
THanks for that.

Im now tossing up between 3.73 and 3.55.
But now leaning to 3.73 due to tall tyres 27.74 inch and .70 over drive.
The calcs are below:

3.08 = 1645 RPM @ 100kph or 1776 @ 110
3.55 = 1896 RPM @ 100kph or 2047 @ 110
3.73 = 1992 RPM @ 100kph or 2151 @ 110
4.11 = 2195 RPM @ 100kph or 2370 @ 110

As you can see with the calc`s the 3.08 will labour the motor to an early death or have i got it all wrong? :omg:.
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Offline Bfit

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Re: Diff gearing for TH700
« Reply #4 on: November 10, 2015, 01:37:55 PM »
I think the RPM`s would actually be a little lower than  those  quoted.
How ever a 3.73 is a good compromise , make sure you use a good  diff  shop  these can howl if not set up  100% correctly.
a 3.73 would give you around 2200 at 110kph
If the engine you put in has enough grunt  it will cope with the 3.08,  How did the car drive with the 3 speed ?
the turbo 700 has lower gearing in  1st and  2nd
3.06 1.62 1.00 0.70 ( 700 )
2.52 1.52 1.00   (T 350 )
 I would give the 3.08 a go first up.
 A common diff ratio  in a commodore with an 4L60E  ( which has the same ratio`s as  700 )  is a 2.92
Bfit

PS  shiftworks does a kit  for the shifter, that will keep the swap legal for rego preposes.
http://www.shiftworks.com/pdf/Shiftworks-catalog-12-sm.pdf
Freedom's just another word for nothin' left to lose
George Orwell 1984