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3.9 diff gears and auto box speed limiter 

 

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 Post subject: 3.9 diff gears and auto box speed limiter
Posted: Tue Aug 03, 2010 9:19 pm 
Oompa Loompa
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I am looking to get my diff reconditioned and while its apart i was going to get a set of 3.9 gears from a r31 skyline installed and a 4wd systems lokka. i know that there is no speedo gear that suits 3.9s so i was going to use a corrector kit from jaycar (CAT. NO. KC5435) for the dash and connect the computer straight to the speed sender. Problem is that i think the speed limiter will cut in at around 140km/h before i can finish a quarter mile.

The car has mild mods including twin throttle bodies with twin 3" intakes and pods, 977b cam, 2.5" mandrel bend exhaust one straight through muffler, DIY variable shift kit, etc.

i want to know if anyone has run 3.9 diff gears with a auto box and how they corrected the speedo. If the auto falcons with 3.45 diff gears were limited to 180 as well and if i put the speedo gears in for a 3.45 diff if it would bring the limiter to 160.

Thanks
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 Post subject: Re: 3.9 diff gears and auto box speed limiter
Posted: Tue Aug 03, 2010 9:28 pm 
Getting Side Ways
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could you tune it out? or just take the gear out when you take it to the 1/4 just use a gps with a speed reading.

or do autos need speed sensors to shift?

 

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 Post subject: Re: 3.9 diff gears and auto box speed limiter
Posted: Wed Aug 04, 2010 7:03 am 
Getting Side Ways
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I have run 3.9's in an Auto V8.

I fixed the speedo like this.

1 USe and XD transducer on the box with a black gear( cant remember the number of teeth), this made the speedo read accurately,
2 use a shift kits speedo corrector on the speedo input to the auto controller corrected to read it had 3.45 gears.

I dont know weather this would work on a 6, as they dont have a seperate auto controller, but it deffinitely worked on the V8. As for the speed limiter, mine has been removed with the installation of and aftermarket ECU.

Good luck
Brett

 

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 Post subject: Re: 3.9 diff gears and auto box speed limiter
Posted: Thu Aug 05, 2010 8:15 pm 
Oompa Loompa
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Location: Brisbane
QLD, Australia

DormanAU wrote:
could you tune it out? or just take the gear out when you take it to the 1/4 just use a gps with a speed reading.

or do autos need speed sensors to shift?


I would still like to keep the speed limiter to prevent gearbox/drive shaft failure. I think the gearbox output speed is used for shifting.


TerroristGHIA wrote:
I have run 3.9's in an Auto V8.

I fixed the speedo like this.

1 USe and XD transducer on the box with a black gear( cant remember the number of teeth), this made the speedo read accurately,
2 use a shift kits speedo corrector on the speedo input to the auto controller corrected to read it had 3.45 gears.

I dont know weather this would work on a 6, as they dont have a seperate auto controller, but it deffinitely worked on the V8. As for the speed limiter, mine has been removed with the installation of and aftermarket ECU.

Good luck
Brett


Can anyone tell me if the computer is different on falcons with 3.45 diff gears to 3.08 diff gears or if it is only the gearbox speed sender gears that change.

Has anyone used a Jacyar speedo corrector or any other speedo correctors on a 6cyl falcon and how did they connect them?

Is the speed limiter on falcons with factory autos and 3.45 diff gears set to 180 ?
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 Post subject: Re: 3.9 diff gears and auto box speed limiter
Posted: Thu Aug 05, 2010 8:36 pm 
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TriggerHappy wrote:

I would still like to keep the speed limiter to prevent gearbox/drive shaft failure. I think the gearbox output speed is used for shifting.


the problem is, its not the road speed that contributes to the 4 speed auto failing, its the tailshaft speed/rpm..
so if you speed limiter is cutting in at aprox 140kmph (road speed with 3.9's)? that means your tail shaft speed is at the equivilent 180kmph, so even if you keep the speed limiter up at 180kmph road speed you tail shaft would be spinning at around 220+ kmph..

unfortunatly with the super low diff gears there is always the risk of extension housing failure at high speeds...
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