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3:9 diff ratio 

 

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 Post subject: 3:9 diff ratio
Posted: Fri May 11, 2007 7:27 pm 
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Thinking of changing my 3:45 diff to a 3:9 ratio...anyone think its a good idea? I know i will get way better take offs but im just worried about up high when say cruising at 100 on the highway and it will be revving its guts out and chewing the fuel..
anyone got 3:9's?? how much improvment in take of did you get and how does your car drive when driving on highway?

Cheers,

 

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Posted: Fri May 11, 2007 7:35 pm 
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Going by others, your motor will sit on around 2600rpm+ @ 100km/h.

Not real practical in my opinion.
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Posted: Fri May 11, 2007 7:40 pm 
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thats fairly high hey, i sit on 2000rpm at the moment at 100...
do you lose any top end power with 3:9's or gain?

 

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Posted: Fri May 11, 2007 7:53 pm 
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Sorry i just saw you had an auto, so will be doing a little less on the highway than what i said, maybe 2400rpm+ @ 100km/h.

3.7 & 3.9 gears combined with an auto arnt tooooo much of a sacrifice on highway. But with a manual box and its final drive ratio i wouldnt go higher than 3.45's
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Posted: Fri May 11, 2007 8:16 pm 
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you will find on the highway you will probably get better mileage, because the motor is more within the torque range, therefore not working as hard, even though it is doing a couple hundred more rpm.

Diff ratio will not affect your power, but it will increase the torque multiplication at the wheels, which means it will get up and go a lot quicker when you put your foot into it.

You will need to use a speedo corrector wired in behind the cluster to correct the speedo.
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Posted: Fri May 11, 2007 9:12 pm 
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Thanks for the advice 66 coupe exactly what i wanted to hear!!!

I'll defenently go the 3:9's...

Along with a 2500 stallie i have waiting to be installed should go hard of the line!

 

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Posted: Fri May 11, 2007 11:52 pm 
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66 coupe wrote:
you will find on the highway you will probably get better mileage, because the motor is more within the torque range, therefore not working as hard, even though it is doing a couple hundred more rpm.



i would question that logical severely....the part load torque curve is a lot different to the full load curve....and if thats the case then why even use OD just keep it in a higher gear and youll get better economy..

try it on your trip computer at 100 in 3rd then 4th (auto) and see the difference

 

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Posted: Sat May 12, 2007 10:30 am 
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gogetta wrote:
66 coupe wrote:
you will find on the highway you will probably get better mileage, because the motor is more within the torque range, therefore not working as hard, even though it is doing a couple hundred more rpm.



i would question that logical severely....the part load torque curve is a lot different to the full load curve....and if thats the case then why even use OD just keep it in a higher gear and youll get better economy..

try it on your trip computer at 100 in 3rd then 4th (auto) and see the difference


Because the i6's are MAP based, and combined with a cam = less vacuum, they will use less fuel with lower diff gears, because the engine load is 'less' when cruising at a higher rpm etc than with tall gears at a low rpm.

I also said he will probably get more milage, i did not say that he will be guaranteed more mileage...

AU1 XR6, if you find the trans goes into limp mode when stalling up on the line with the 2500 converter, let me know ;)
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Posted: Sat May 12, 2007 1:05 pm 
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No worries 66 coupe, will do...will it go into limp home mode with a Chiptorque chip installed cause ive been looking into getting one?

 

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