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3.9 LSD into EF, Speed gears? 

 

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 Post subject: 3.9 LSD into EF, Speed gears?
Posted: Wed Aug 01, 2007 10:10 pm 
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I got a EF GLi and i want to put a 3.9 LSD in it that i have arranged. Problem is that i wont be able to get a speedo gear big enough.

Here is what it says on the calculator.
Speedo gear- Brown 20
Tyres - 205/65 R15
Diff - 3.08 SS

i calculated it so that it is within half a KM and this is what i got.

Speedo Gear - Black 23
Tyres - 205/85 R16
Diff - 3.9LSD

And its just as much out with

Speedo Gear - Black 23
Tyres 205/65 R17
Diff - 3.9LSD

Whats going on??? What profile tyres and diff gear to have the 3.9 in?

 

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Posted: Wed Aug 01, 2007 11:08 pm 
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dude why are ya worried....just get a speedo corrector to fix the speedo signal to your dash... putting in big diff gears doesnt hurt your shift points or anything...asuming your auto. just change the speed signal to the dash not the ecu and you will be fine.

 

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Posted: Wed Aug 01, 2007 11:11 pm 
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ebfairmont3.9 wrote:
dude why are ya worried....just get a speedo corrector to fix the speedo signal to your dash... putting in big diff gears doesnt hurt your shift points or anything...asuming your auto. just change the speed signal to the dash not the ecu and you will be fine.


Tell me more. LOL

I want the 3.9 in so ill have to work this out.

Ill be going manual probably at the same time.

 

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Posted: Thu Aug 02, 2007 1:29 am 
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ebfairmont3.9 wrote:
dude why are ya worried....just get a speedo corrector to fix the speedo signal to your dash... putting in big diff gears doesnt hurt your shift points or anything...asuming your auto. just change the speed signal to the dash not the ecu and you will be fine.


thats not right because the circuit goes sender->dash->ECU so if you use a speedo corrector then the ECU will hold gears for way too long before changing....u will need 2 speedo correctors (i think)

 

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Posted: Thu Aug 02, 2007 1:40 am 
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speedo gear for 3.9's should be green and i'm not sure of the number!

 

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Posted: Thu Aug 02, 2007 7:48 am 
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Or a police spec dash! =)

 

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Posted: Thu Aug 02, 2007 8:26 am 
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From what ive heard, the ecu shifts gears based on road speed, so if you hook up the speedo corrector to the speedo input wire going to the cluster, then the speedo output wire will get the adjusted road speed signal to the ecu... all good !!!

Heres a doco i did on installing a jaycar speedo corrector if anyones unsure how to install.....

http://www.trueblueford.com/discussion/ ... php?t=3187

(you may have to be a member to view it?)
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Posted: Thu Aug 02, 2007 1:37 pm 
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just had a look at them and it looks easy enough to install. Ill see what the manual box has in it first and how that goes.

 

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Posted: Thu Aug 02, 2007 9:33 pm 
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black 23 tooth is the biggest available gear but as has been mentioned, just fit a jaycar corrector. i have also heard that you may need to fit 2 although unsure on this. i have 3.9's in mine with black speedo gear and jaycar speedo corrector fitted. i don't have any trouble with hitting the limiter or gear changes but thats prolly more due to the fact that i'm running aftermarket ecu.

speedo correctors from jaycar come in kit form so you'll need to be handy with a soldering iron or know someone who is.

 

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Posted: Fri Aug 03, 2007 10:54 pm 
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FordFairmont wrote:
From what ive heard, the ecu shifts gears based on road speed, so if you hook up the speedo corrector to the speedo input wire going to the cluster, then the speedo output wire will get the adjusted road speed signal to the ecu... all good !!!

Heres a doco i did on installing a jaycar speedo corrector if anyones unsure how to install.....

http://www.trueblueford.com/discussion/ ... php?t=3187

(you may have to be a member to view it?)

You don't want the ECU to get the adjusted speedo signal, thats what makes it shift wrong. You need to supply the dash the right signal, so you see the right speed, but then modify it BACK to being wrong for the ECU.

 

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Posted: Fri Aug 03, 2007 11:13 pm 
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http://www.shiftkits.com.au/
give them a call, they do a dash only speed correcter.
Do the auto as well and ya shift points are rooted
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Posted: Sat Aug 04, 2007 2:04 pm 
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SO i want the Dash Cluster to get the modified signal but the ECU to get the standard signal?

 

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Posted: Sat Aug 04, 2007 2:12 pm 
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ive been thinking about what steady ed said and i still cant understand how it works.

I thought the ecu had to see the adjusted road speed from the speedo corrector so it changed gears earlier instead of giving it a higher speed signal (thats out from the diff change) from the cluster, i guess its not that hard to experiment with, just have to buy 2 correctors
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Posted: Sat Aug 04, 2007 3:03 pm 
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Okay, I'll explain it again.
Stock diff, stock ECU, stock speedo gear.
ECU shifts 1st >2nd at 80km/h under WOT.
You put 3.9s in, stock ecu, stock speedo gear.
Speedo is wrong, ECU will still shift at 80km/h under WOT, but your actual speed will be only say 60.
NOW, if you modify the signal, so the ECU sees the CORRECT speed (eg. 60km/h at 5000rpm as opposed to 80), it will be waiting for 80km/h to come so it can shift to 2nd.
But it will never get there, it'll bounce off the limiter a few times before it figures it out or you back off and force a shift.

 

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Posted: Sat Aug 04, 2007 3:15 pm 
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Ah makes sense now, after trawling throught countless threads about this issue, what you just said was the best response and the most clear.
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